Insect adaptations to cold and changing environments, Sømme (1999), and Bale (2002). These studies have led to the discovery of more and more elements that contribute to insect cold

Insect adaptations to cold and changingenvironments1

H.V. Danks

Biological Survey of Canada (Terrestrial Arthropods), Canadian Museum of Nature, P.O. Box3443, Station D, Ottawa, Ontario, Canada K1P 6P4 (e-mail: [email protected])

Danks23Abstract—A review of insect adaptations for resistance to cold and for life-cycle timing revealsthe complexity of the adaptations and their relationships to features of the environment. Coldhardiness is a complex and dynamic state that differs widely among species. Surviving cold de-pends on habitat choice, relationships with ice and water, and synthesis of a variety of cryo-protectant molecules. Many aspects are time-dependent and are integrated with other factors suchas taxonomic affinity, resource availability, natural enemies, and diapause. Timing adaptations re-flect the fact that all environments change over many different time frames, from days to thou-sands of years. Environments differ in severity and in the extent, nature, variability, andpredictability of change, as well as in how reliably cues indicate probable conditions in the fu-ture. These differences are reflected by a wide range of insect life-cycle systems, life-cycle de-lays, levels of responsiveness to various environmental signals, genetic systems, and circadianresponses. In particular, the degree of environmental change, its predictability on different timeframes, and whether it can be monitored effectively dictate the balance between fixed and flexi-ble timing responses. These same environmental features have to be characterized to understandcold hardiness, but this has not yet been done. Therefore, the following key questions must beanswered in order to put cold hardiness into the necessary ecological context: How much do con-ditions change? How consistent is the change? How reliable are environmental signals?

Résumé—La présente rétrospective des adaptations pour la résistance au froid et l’ajustementtemporel des cycles biologiques illustre la complexité des adaptations et leurs relations aux ca-ractéristiques du milieu. La résistance au froid est un état dynamique et complexe qui varieconsidérablement d’une espèce à l’autre. La survie au froid dépend du choix de l’habitat, des re-lations avec la glace et l’eau et de la synthèse d’une gamme de molécules cryoprotectrices. Plu-sieurs des aspects sont reliés au temps et sont aussi intégrés à d’autres facteurs, tels que l’affinitétaxonomique, la disponibilité des ressources, les ennemis naturels et la diapause. Les adaptationsd’ajustement temporel sont reliées au fait que tous les milieux changent à des échelles temporel-les multiples, allant de jours à plusieurs millénaires. Les milieux différent entre eux quant à la ri-gueur et à l’étendue, la nature, la variabilité et la prévisibilité du changement; de plus, les indicesdes conditions futures probables y sont plus ou moins fiables. Ces différences se reflètent dansles systèmes de cycles biologiques des insectes, les délais dans les cycles biologiques, les degrésde réaction aux divers signaux environnementaux, les systèmes génétiques et les réponses circa-diennes. En particulier, le degré de changement environnemental, sa prévisibilité aux différenteséchelles temporelles et la possibilité d’en faire un suivi efficace déterminent l’équilibre entre unajustement temporel fixe et un flexible. Ces mêmes caractéristiques de l’environnement doiventêtre définies de manière à pouvoir comprendre la résistance au froid, mais ce n’est pas encorefait. Pour replacer la résistance au froid dans son contexte écologique obligé, les questions essen-tielles à poser sont donc : quelle est l’importance du changement de conditions? ce changementest-il uniforme? les signaux de l’environnement sont-ils fiables?

[Traduit par la Rédaction]

Can. Entomol. 138: 1–23 (2006) © 2006 Entomological Society of Canada

1

Received 6 October 2004. Accepted 8 June 2005.

1Presented as part of “Adaptations and Constraints: a Symposium in Honour of Richard Ring” at the JointAnnual General Meeting of the Entomological Societies of Canada and British Columbia, 3 November2003, Kelowna, British Columbia.

Introduction

Insects display a remarkable range of adapta-tions to cold and changing environments. Themany selective pressures and combinations oftraits make for complex systems of adaptation.In particular, temperate insects face two sorts ofchallenges. First, they must cope with the coldof winter (as well as hot dry summers in typicalcontinental climates with cold winters). Second,they must time their life cycles not only so thatresistant stages are present to cope with sea-sonal severity, but also so that developmentaland reproductive stages coincide with suitableconditions of temperature, moisture, resourceavailability, food quality, and other potentialconstraints.

Here I review the suites of adaptations for re-sistance to the cold of winter and for the timingof the life cycle and their relationships to envi-ronmental changes on seasonal and other tem-poral scales. Viewing these aspects togetherreveals instructive parallels. In particular, life-cycle adaptations accord with the extent andconsistency of environmental changes and howreliably future conditions can be predicted fromavailable signals about environmental quality.The onset and extent of cold hardiness mustalso accord with these same features of the en-vironment, but cold hardiness has not yet beenexamined sufficiently in an ecological context.

Responses to cold

Cold hardinessInsect cold hardiness has been studied in in-

creasing detail over the years; sample reviewsare those by Cannon and Block (1988), Dumanet al. (1991), Lee and Denlinger (1991), Storeyand Storey (1992), Danks (1996, 2000b,2005a), Sømme (1999), and Bale (2002). Thesestudies have led to the discovery of more andmore elements that contribute to insect coldhardiness, as summarized in Table 1.

The conditions of cold actually experienceddepend on region and microhabitat, which aremodified by insect movement and habitatchoice, and on the effects of cold temperatures,which are modified chiefly by physiologicaland biochemical means. During cold, temperatewinters, a few species remain active beneath oreven above the snow (e.g., Aitchison 1979,1989; Soszynska and Durska 2002), but mostspecies are inactive. Potential challenges in-clude the mechanical impact of external ice

(water expands by about 4% on freezing), dis-ruption of biochemical processes, and damageto cells and membranes. Beyond the effects ofhabitat, three core factors have proven to be es-pecially significant: ice crystallization (con-trolled by nucleators), water relations(controlled by water status and availability),and molecules used for cryoprotection.

Habitat effectsOften overlooked because of the interest in

biochemical mechanisms is the importance ofclimate and weather, coupled with the choice ofmicrohabitat (Table 1). Most species chooseprotected sites beneath litter, soil, or snow.Such substances, especially when air is trappedwithin them, provide good insulation againstlow ambient temperatures, so that below thesnow temperatures seldom fall far below freez-ing, even in cool temperate winters (Danks1991a). Sheltered microhabitats also reduce therisk of ice damage or ice inoculation. In partic-ular, they slow the rate of temperature change.This is potentially significant because somespecies survive freezing, or at least survive ingreater numbers, only after very slow cooling(fractions of degrees per minute) (Miller 1978;Shimada and Riihimaa 1988; Bale et al. 1989).The choice of overwintering sites is thereforevery important. Although anecdotal evidencesuggests that sites are carefully chosen on thebasis of light, temperature, moisture, touch,gravity, and other factors (Danks 1991a), fewquantitative experiments have been done. Rela-tively small-scale dispersal to more favourablesites is a common strategy or partial strategy(e.g., Knülle 2003). However, the alternative —long-distance displacement or migration awayfrom areas with cold winters — is relativelyrare among insects, especially in cool temperateregions with large tracts of similar terrain, andtherefore is not considered further here.

Ice and nucleationThe two main types of cold hardiness cus-

tomarily recognized differ in whether or notwater in the insect freezes. Freezing-resistantspecies supercool, the body fluids remainingunfrozen even well below 0 °C, whereasfreezing-tolerant species survive the formationof extracellular ice within the body. These cate-gories are somewhat artificial: for example,some species die at temperatures above 0 °C,some species that supercool are killed at tem-peratures above the supercooling point, and

© 2006 Entomological Society of Canada

2 Can. Entomol. Vol. 138, 2006

populations may differ in the supercoolingpoint and in the level of injury at different tem-peratures above the supercooling point (e.g.,Knight et al. 1986; Bale 1987; Turnock et al.1990, 1998; Turnock and Boivin 1997). Never-theless, the two customary categories serve tohighlight the major significance of ice forma-tion.

Water assembles into a hexagonal crystalconfiguration during ice formation; it freezeseasily when a nucleus is available to maintainthat configuration. Once a single ice crystalforms, it can nucleate others. In the absence ofsuitable nuclei, ice crystals develop when thewater molecules are moving slowly enough tomaintain the configuration for a finite time.Movement is slow enough only at low tempera-tures, and in small droplets of pure water suchspontaneous or homogeneous nucleation doesnot take place until the temperature falls toabout –40 °C (Vali 1995; Matsumoto et al.2002). Freezing-resistant species apparentlyhave ways to mask internal heterogeneousnucleators, and other adaptations, notably re-duced water content, limit homogeneous nucle-ation (Zachariassen et al. 2004; see also Wilsonet al. 2003 and review by Danks 2005a).

Insects can reduce the chance of freezing byavoiding external nucleators such as dust parti-cles, surface ice crystals, food fragments, andice-nucleating bacteria (see Strong-Gundersonet al. 1989; Lee et al. 1995; Lee and Costanzo1998). In some species, overwintering cocoonssignificantly protect against inoculation by ice(e.g., Sakagami et al. 1985; review by Danks

2004b). Special, tightly applied winter cocoonsof some chironomid midges are believed togive mechanical protection against surroundingice in pond substrates (see review by Danks1971). The structure of the cuticle also influ-ences resistance to ice inoculation (Kelty andLee 2000).

To remain unfrozen, insects must eliminateor mask nucleators in the gut and tissues. How-ever, if a supercooled insect begins to freeze,ice will spread very rapidly and thus danger-ously because the temperature is now well be-low 0 °C. Therefore, in contrast to mostfreezing-resistant insects, which eliminatenucleators, most (but not all) freezing-tolerantinsects manufacture ice-nucleator proteins forthe winter (Zachariassen and Hammel 1976; re-view by Duman 2001). These nucleators ensurethat freezing will take place at relatively high,and hence less dangerous, subfreezing tempera-tures. Several species survive after freezing byinoculation but not after freezing at lower tem-peratures in the absence of surface ice (e.g.,Tanno 1977; Fields and McNeil 1986; reviewby Duman et al. 1991); such species may lackice-nucleating proteins. Nevertheless, a fewspecies can survive freezing after extensivesupercooling (Ring 1982, for a pythid beetle).Moreover, some species remain supercooledeven at –60 °C, a temperature well below thehomogeneous nucleation point of water (Ring1981, 1982, 1983; Ring and Tesar 1981; Miller1982; Miller and Werner 1987).

A few species are known that survive coldtemperatures either by supercooling or by


Danks 3

Element Sample features Main effects on cold hardiness

Region Climate, weather Temperature and its variation, freezingrate

Microhabitat Choice, site features, modification Insulation, protection against inoculativefreezing, freezing rate, effects of ice

Surface phenomena Cocoons, cuticle Ice inoculation, external mechanicaleffects

Crystallization Freezing resistance, freezing tolerance Absence or presence of iceExternal nucleators, internal nucleators

(cf. feeding), ice-nucleating proteinsIce formation

Water relations Status, availability, dehydration Ice amount, “unfreezable” water, watermanagement and solute interactions,supercooling by dehydration

Cryoprotectants Solutes, antifreeze proteins, shockproteins

Inhibition or modification of ice orinjury

Other molecules Enzymes, antioxidants Modification of function or injury

Table 1. Elements of insect cold hardiness.

freezing tolerance, perhaps depending on condi-tions in a given year (Horwath and Duman1984; Duman et al. 1991). Some freezing-tolerant species include individuals that reducetheir supercooling points after being frozen andthawed. Consequently, their subsequent re-freezing is less likely, but these individuals aremore likely to die if they are frozen again (Baleet al. 2000, 2001; Brown et al. 2004). Bale(2002) suggested that this strategy might reflectthe greater costs of repeated freezing and thaw-ing compared with a single freezing.

Water relationsThe behaviour of water and ice within an in-

sect depends on water status and availability.Water can be in one of a number of states, in-cluding water in matrix molecules (a very smallpercentage), water closely associated with bio-logical structures and surfaces, and storage wa-ter not closely associated with the cells (Danks2002a). Ice is another potential state infreezing-tolerant species. Typically, water in aninsect is in dynamic balance among the variousstates, depending on the condition of the insectand the nature of the environment. Water boundmore or less tightly to cellular constituents hasbeen termed “unfrozen water” or “osmoticallyinactive water” (also, less usefully, “unfreezablewater” or “bound water”). Wolfe et al. (2002)pointed out that the amount of unfrozen watermay exceed the potentially “unfreezable”amount because equilibrium is not reached overnormal time frames at low temperatures; seeBlock (2002, 2003) for further discussion. Un-frozen water comprises 10%–30% of the totalwater in different species (Storey and Storey1988; Block 1996), and typically the proportionthat will not freeze increases in preparation forwinter (Storey et al. 1981; Storey and Storey1988).

Under normal conditions, water is managedinternally by solute or ion transport acrossmembranes (water follows by solvent drag).Higher concentrations of solutes (see below) af-fect the behaviour of water in solutions. At ex-treme solute concentrations, achieved especiallyby drying, viscous rubbers and glasses are pro-duced in which molecular kinetics are ex-tremely slow, preventing physical and chemicalinteractions including nucleation (Danks2000a).

Water balance is used as a means of remain-ing unfrozen when surrounded by ice duringwinter by some springtails (Holmstrup and

Sømme 1998; Worland et al. 1998) and otherinvertebrates (Holmstrup 1992; Holmstrup andZachariassen 1996; Sømme and Birkemoe1997; Holmstrup et al. 2002). In these species,water is lost from the animal to external ice, aloss that is favoured because ice has a lowervapour pressure than water at a given tempera-ture. Thus dehydrated, with corresponding re-ductions in melting point and supercoolingpoint, the organism cannot freeze at the giventemperature, provided its cuticle is permeableenough to transfer water out relatively rapidly.Of course, such a mechanism requires an abilityto survive drying, which causes stresses similarto those created by internal ice formation. Pro-tection against drying comes especially fromcertain solutes such as trehalose (Ring andDanks 1998; Danks 2000a). In insects, suchsolutes have most commonly been referred toas cryoprotectants (Ring and Danks 1994,1998).

Cryoprotectants and other adjustmentsSeveral kinds of cryoprotectant molecules

that may increase in winter in cold-hardy spe-cies mediate effects on internal water and ice.Solutes of low molecular weight (especiallyglycerol, sorbitol, and other polyhydric alco-hols, but also trehalose and other sugars and afew other substances; see review by Lee 1991)reduce the melting point and hence the freezingpoint by osmotic effects (for other roles, see be-low). Such effects depend on the number of sol-ute molecules, such that numerous smallmolecules are most efficient. Many speciesmanufacture more than one kind of these sol-utes at the same time (Miller and Smith 1975;Morissey and Baust 1976; Ring 1977; and seeLee 1991, Table 2.1).

Antifreeze proteins, or thermal hysteresisproteins (see review by Duman 2001 for insectsand by Fletcher et al. 2001 for fishes), are largemolecules that inhibit ice formation by bindingin specific orientations to the ice surface at theice–water interface or by binding to and reduc-ing the efficiency of nucleating sites (e.g., Wil-son and Leader 1995; Madura et al. 2000;Daley et al. 2002; Wilson et al. 2003; Wathenet al. 2003). Glycerol and other small solutesenhance these actions of antifreeze proteins, ap-parently by disabling nucleators (Li et al. 1998;Duman 2002; Duman and Serianni 2002).

Recrystallization inhibitors prevent poten-tially injurious ice recrystallization in frozentissues. Such growth or change of form of ice



crystals is a common occurrence as tempera-tures change, especially in the presence of sol-utes (which make available more interveningwater molecules to facilitate transfer betweenice crystals). Antifreeze proteins and other largemolecules can serve as recrystallization inhibi-tors, and smaller specific recrystallization in-hibitors that are effective at very lowconcentrations have been reported from a nem-atode (Ramløv et al. 1996). Such phenomenahave been little studied in insects.

Shock proteins, or stress proteins (Morimotoet al. 1994; Goto and Kimura 1998; Denlingeret al. 2001), appear very rapidly in many spe-cies in response to marked temperature shockby heat or cold, but for the most part their rolesin cold hardiness or in the very rapid cold-hardening responses described in some specieshave not been determined (see review by Danks2005a). Other molecules manufactured or mod-ified for cold hardiness include enzymes, whichmay be altered to change the temperature atwhich they are activated or damaged, and anti-oxidants, which serve to repair or protect cellsfrom some of the products of freezing damage(e.g., Joanisse and Storey 1996, 1998).

Despite much work on cryoprotectants, muchremains to be understood, given the multiplemodes of protective action of these substancesand their interactions. For example, some spe-cies that are very cold hardy lack the usualcryoprotectants (Ring 1981, 1983; Storey et al.1993).

Adaptive changes in cell membranes also ap-pear to be associated with cold hardiness. In thefly Chymomyza costata (Zetterstedt) (Diptera:Drosophilidae), significant changes in phospho-lipid composition are associated with freezingtolerance (Košt’ál et al. 2003).

Degradation of mitochondria has beenassociated with cold hardiness (Kukal et al.1989, for the high Arctic moth Gynaephoragroenlandica (Wocke) (Lepidoptera: Lyman-triidae)). The amount of mitochondrial DNA inoverwintering individuals is thereby reduced,but in both G. groenlandica and Eurostasolidaginis (Fitch) (Diptera: Tephritidae), stableRNAs are stored over winter, allowing mito-chondrial proteins to be restored rapidly whentemperatures rise (Levin et al. 2003).

Cold hardiness and timeMany aspects of cold hardiness are time-

dependent (Table 2). These temporal effects in-fluence the effectiveness of adaptations. For

example, whether or not freezing occurs duringsupercooling depends on the temperature andon the duration of the supercooled state, be-cause nucleation is time-dependent and any nu-cleation event will precipitate freezing of theinsect. Once the insect is frozen, temperatureand time also determine the likelihood and rateof recrystallization. Cryoprotectants serve tomitigate these rearrangements as well as protectagainst the freezing process. Water balance andthe dynamics of internal water status and avail-ability are also time-dependent. Moreover,cryoprotectants and other molecules accumulatebefore winter, but in some species they are alsoadjusted during winter (Baust and Lee 1982;Baust and Nishino 1991). Such changes occurin response to environmental conditions includ-ing low temperatures (Baust 1982; Layne andKuharsky 2000), photoperiod (e.g., Meier andZettel 1997), and dehydration (e.g., Layne andKuharsky 2000), but some may simply accordwith developmental status or the passage oftime. Year-round cold hardiness is known in afew species from polar climates (e.g., Kukal etal. 1988; Sinclair and Chown 2003).

The fact that the environmental effects oftemperature and the status and responses ofmost overwintering insects change over time inthese ways greatly complicates the assessmentof cold hardiness. Cooling rate and, separately,warming rate influence survival (Baust andNishino 1991). Survival of even freezing-tolerant species is both temperature-dependentand time-dependent (e.g., Danks 1971 for highArctic midge larvae). Moreover, the fact that inat least some species site seeking and cryo-protectant synthesis are triggered by differentenvironmental cues and are controlled by hor-mones (e.g., Danks 1996, p. 391) and that mul-tiple cryoprotectant substances can be adjustedduring winter suggests that cold hardiness isbest regarded as a dynamic programme linkedto environmental conditions. These ongoingchanges are not consistent with the older con-cept of cold hardiness as some sort of static,“hardened” state for winter protection. Conse-quently, interpreting the many time-dependentelements of cold hardiness requires detailed as-sessments of microhabitat temperatures in na-ture over relatively long periods.

Integration of cold hardiness with otherfactors

Sheltering from or withstanding cold is notthe only requirement for survival in cold


Danks 5

environments. Many other features are inte-grated with insect adaptations to cold (Table 3).The fauna of cold environments is dominatedby groups that evolved in cold conditions andhabitats, at a variety of taxonomic levels. Thus,the order Diptera among the insects, the familyChironomidae among the Diptera, and the ge-nus Spilogona among the Muscidae are verywell represented in Arctic faunas. They consti-tute 61%, 46%, and 80% of the high Arcticinsects, flies, and muscids, respectively, com-pared with only 24%, 7%, and 24% of thesetaxa in Canada as a whole (Danks 1992b,1993). Only groups with coordinated adapta-tions persist at the highest latitudes. Such re-lated species may be especially helpful inunderstanding how particular features of coldhardiness arose. For example, chironomidmidges, the dominant family in the high Arctic,are supposed to have evolved in seasonallyfrozen habitats (Brundin 1966), and freezingtolerance is especially widespread in this family(Danks 1971).

Microhabitat and the behaviour associatedwith its choice determine much more thanexposure to cold; they also determine (at particu-lar times of year) moisture availability, develop-mental temperature, and exposure to naturalenemies. If seasonal microhabitat change is notfeasible, winter and summer needs may con-flict. For example, the sites most sheltered fromcold are slowest to warm up in spring, and con-sequently Arctic species that develop early tendto overwinter in cold, exposed places (review byDanks 2000b). Species in cold sites also con-serve energy (e.g., Parry 1986; and see below).

Cocoons and cuticular structures that mightenhance cold hardiness serve both active andinactive individuals, providing heat gain, cam-ouflage, water retention, and protection againstdisturbance or attack by natural enemies (Danks2002b, 2004b). The thick walls of cocoons pro-tect against many adverse conditions simulta-neously, but such a feature must be balancedwith the cocoon’s other uses because the con-struction of shelters has a measurable cost (e.g.,



Element Temporal effects

Region SeasonalityLife cyclesControl of development

Microhabitat Habitat selectionWinter versus spring demands

Surface phenomena SupercoolingCrystallization Supercooling

RecrystallizationWater relations Availability

StatusCryoprotectants and other molecules Acclimation in fall

Modulation in winter

Table 2. Key temporal effects influencing insect cold hardiness.

Element Relevant sample features in addition to cold hardiness

Region Phylogeny, evolutionary pathway, faunal compositionMicrohabitat Humidity, temperature, natural enemiesSurface phenomena Heat gain, camouflage, water loss, natural enemies, mechanical strengthWater relations Desiccation tolerance and resistance, including water acquisition, and conservation

by both cuticular and molecular meansEnergy relations Energy storage, respiration, metabolism, development, reproductionTiming Life cycles, including diapause or quiescence responding to seasonal elements that

include resource availability, dryness, and food quality

Table 3. Some other elements related to insect cold hardiness (for additional details see Table 2 in Danks1996).

Stevens et al. 1999, 2000). However, the dura-bility and effectiveness of cocoons for multiplepurposes, such as water permeability and resis-tance to mechanical damage, predators, and iceinoculation, has not been measured (Danks2004b). Protective structures around the eggsmay play similar roles (cf. Danks 2002b; Riverset al. 2002).

Securing and retaining enough water is oneof the greatest challenges for terrestrial insects,and many adaptations are known. These includemaintaining high basic water content, survivingdrying, drinking, eating wet food, absorbing at-mospheric water, metabolizing water from foodor stored reserves, and limiting water loss byecological, mechanical, and physiological–biochemical means (Danks 2000a). Somemechanisms are not available to individuals thatare immobile in winter. Moreover, storage, re-duction, or change in status of an organism’swater during winter changes its availability toparticipate in freezing, desiccation, fluid andionic balance, excretion, transport, and otherprocesses. For example, water content (as as-sessed by drying) usually drops in winter, com-monly from about 75% to 60%, as the amountof “unfreezable” water increases. This 15%change represents the “loss” of fully half of thewater in the body (Hadley 1994, p. 31) and co-incides with increased cold hardiness. Reducedcuticular permeability in winter (e.g., by in-creased lipid layers: Danks 1987, p. 23; Yoderet al. 1992; Nelson and Lee 2004) reflects theneed for additional desiccation resistance dur-ing inactivity, but all such changes must be in-tegrated with demands at other times of theyear.

Energy budgeting throughout the year is co-ordinated with winter survival. Energy demandsfor summer feeding, development, reproduc-tion, and flight, for example, are balanced withother needs for energy. Those needs pertain notjust to cold hardiness (e.g., the manufacture ofcryoprotectants) but also to requirements to sur-vive inactivity in the winter shelter and to sur-vive in fall and spring when it is warm enoughto metabolize significant reserves (Irwin andLee 2000, 2003). Reserves typically are storedas large amounts of fat (see review by Danks1987). At the same time, reserves may be con-verted to glycerol and other cryoprotectants(Storey and Storey 1988). Respiration and coldhardiness are linked in some habitats becauseorganisms encased in ice in spring respire

anaerobically (Conradi-Larsen and Sømme1973a, 1973b; Sømme 1974a, 1974b; Meidell1983).

Metabolism is completely suppressed duringanhydrobiosis, a condition known in chirono-mids (Hinton 1960), springtails (Greenslade1981), and other small invertebrates (Barrett1991; Sømme 1996; Ricci 2001). Anhydrobiosisallows these species to withstand very low tem-peratures and even immersion in liquid nitro-gen, as well as extreme drying. Anhydrobiosisrequires specific energy-dependent preparation(see review by Danks 2000a) and so, as alreadynoted for cold hardiness, is not simply a formof static resistance.

In some species, cold hardiness does not ap-pear to be linked with diapause (e.g., Danks1987, p. 41; van der Woude and Verhoef 1988),but a linkage between the two is now known inan increasing number of other species (Pullinand Bale 1989; Denlinger 1991; Hodková andHodek 1994, 2004; Pullin 1994; Watanabe andTanaka 1998a, 1998b, 1999), and several spe-cies must even be in diapause before they candevelop cold hardiness (e.g., Šlachta et al.2002). Diapause puparia of Sarcophagacrassipalpis Macquart (Diptera: Sarcophagidae)are more resistant to inoculative freezing thannon-diapause puparia (Kelty and Lee 2000), in-dependent of the surface lipids. Vernon andVannier (2002) have even proposed from lim-ited evidence that freezing tolerance occursonly in individuals in diapause. Whether or notcold hardiness and diapause are linked, the ma-jor environmental cues that induce them, photo-period and temperature, are similar. Of course,diapause and other life-cycle programmes mightbe linked to an inability to be active or to findor assimilate food during cold winters at leastas much as to dangerously low temperatures.Other aspects of life-cycle timing are consid-ered in the next section.

Responses to change

Changing environmentsAll environments change over many different

time frames (Table 4). This wide range con-trasts with a current preoccupation among sci-entists and the general public with change overintermediate periods (e.g., 50 years) that mightbe tied to human activity. Climatic norms in-clude characteristic levels of regular and irregu-lar variation as well as average values. For


Danks 7

example, mean July temperature is not exactlythe same from one year to the next, the mini-mum temperature on a given date usually dif-fers from the long-term average, and annualextremes can differ widely from year to year.

Key elements of climates (Danks 1999) ac-cording to the time frames shown in Table 4 in-clude severity (persistent conditions that limitlife, such as low temperatures or limited heatfor growth, and very low winter temperatures),variability (changes in temperature or other po-tential constraints from year to year), seasonal-ity (changes on an annual time frame), andunpredictability (short-term changes within agiven year). The potential complexity of thesedifferent patterns makes it difficult to describechanges in detail. Organisms would be expectedto respond to trends, which are directionalchanges in significant features over many years,including changes in extremes as well as meanvalues. Nevertheless, because organisms areadapted to “normal” patterns of change on vari-ous time frames and their responses are compli-cated by relationships with habitat and otherbiota, as explained below, the results of a giventrend may not be conspicuous.

The effects of current variability and of anypotential future trends therefore differ accord-ing to the species and the time frame. At thelongest time frame, major astronomical, topo-graphical, and latitudinal features (global rota-tional shifts, orogeny and continental drift,differential solar heating) dictate regional cli-mates. Marked variability from year to yearpunishes fixed life-cycle patterns such as emer-gence at a given time of year, which exposes in-dividuals to unsuitable conditions in cold years.Seasonality restricts the time at which develop-ment, activity, and reproduction, each of whichhas different requirements, can be successful.These impacts drive seasonal life cycles, eventhough (as already noted) the need to survivewhile inactive, especially during cold winters,has attracted particular study. Unpredictablechanges within the year force adaptations forflexibility. Changes on a daily time frame dic-tate patterns of activity. Habitat is especiallyimportant because it governs immediate physi-cal and biotic effects.

The adaptations required depend also onwhether resources, especially temperature, areclose to the limits for life. For example, a rapidchange from 8 °C to 2 °C in the high Arcticwould be common and would inhibit the activ-ity of species there. The same 6 °C change



Tim

efr

ame

No.

ofye

ars

Sam

ple

patt

ern

ofco

ndit

ions

Exp

lana

tion

Sam

ple

resp

onse

Lon

g-te

rm10

00s

Cli

mat

icno

rms,

clim

ate

tren

ds,

seve

rity

Lon

g-te

rmav

erag

eof

all

cond

itio

ns,

long

-ter

mtr

ends

Gen

etic

syst

ems

Sup

ra-a

nnua

l2

to10

0sV

aria

bili

tyan

dch

ange

Dif

fere

nces

amon

gye

ars,

mid

-ter

mtr

ends

Pat

tern

sof

gene

tic

vari

atio

nA

nnua

l1

Sea

sona

lity

Pat

tern

sw

ithi

nea

chye

arL

ife-

cycl

eco

ntro

lsS

ub-a

nnua

l<

1D

egre

eof

unpr

edic

tabi

lity

Dif

fere

nces

wit

hin

year

sF

lexi

ble

deve

lopm

ent

Dai

ly0.

0027

Am

plit

ude,

vari

abil

ity,

unpr

edic

tabi

lity

Die

lpa

tter

nsof

ligh

t,te

mpe

ratu

re,

and

moi

stur

edr

iven

bych

ange

sin

sola

rel

evat

ion

Cir

cadi

anpa

tter

nof

acti

viti

es

Tab

le4.

Tim

efr

ames

ofcl

imat

ech

ange

.

from 28 °C to 22 °C would have far less impactin the temperate region, where a rapid changeof the same relative magnitude with respect tofreezing (28 °C to 7 °C) would be unusual.

The significance of change depends too onwhether environmental signals can reliably pre-dict future environments. For example, shortdays precede the onset of the cold temperaturesof the winter season, but there is no reliable sig-nal to predict the onset of several years ofdrought. Climates, habitats, and environmentalsignals therefore interact to govern the re-sponses of insects to changes on various scales.These responses comprise basic life-cycle sys-tems, various life-cycle delays and how they arecontrolled in response to environmental signals,as well as long-term genetic and short-term cir-cadian responses.

Life-cycle systemsThe core elements of insect life cycles are

duration, complexity, and whether basic devel-opmental responses are active or passive (Ta-ble 5).

When habitats are favourable for long peri-ods, especially during summer, abbreviated lifecycles are possible. Species of several taxacharacterized by rapid development and smallsize, such as mites, parasitic Hymenoptera, andaphids, can complete a generation in as little asa few days at high temperatures (e.g., Campbelland Mackauer 1975; Tillman and Powell 1991;Sabelis and Janssen 1994) and are multivoltine.In cool temperate regions, where seasonal pat-terns are reliable but resources do not alwaysallow multiple generations, many species arestrictly univoltine (Danks and Foottit 1989;Danks 2002a). Harsh or intermittently favour-able habitats may prolong life cycles over sev-eral seasons, especially in large species (seereview by Danks 1992a). Severity includes notonly short, cool, or unpredictable summers, butalso patchy or unreliable food; food of lowquality, such as wood (Danks 1992a); and thepresence of natural enemies that would exactsevere losses on populations that appeared ev-ery year, as suggested for periodical cicadas(e.g., Lloyd and Dybas 1966; Williams and Si-mon 1995; Itô 1998) and some moths (e.g.,Wipking and Mengelkoch 1994; Lafontaine andWood 1997).

When environments are variable or unpre-dictable, different individuals may have life cy-cles of different durations. In some species,individuals from even the same cohort emerge


Danks 9

Ele

men

tA

lter

nativ

efe

atur

esD

escr

ipti

onTy

pica

lcl

imat

eor

habi

tat

Dur

atio

nof

life

cycl

eD

ays

Mul

tivol

tine

life

cycl

eP

rolo

nged

favo

urab

leO

neye

arS

tric

tly

univ

olti

neR

elia

bly

seas

onal

Yea

rsS

low

life

cycl

e,sp

read

acro

ssm

ore

than

one

seas

onS

ever

ean

din

term

itte

ntly

favo

urab

leV

aria

ble

Indi

vidu

als

have

life

cycl

esof

diff

eren

tdu

rati

ons

Var

iabl

ean

dun

pred

icta

ble

Num

ber

ofav

aila

ble

life

-cyc

lero

utes

One

Fix

edor

sim

ple

life

cycl

eP

redi

ctab

leM

any

Mul

tipl

eal

tern

ativ

epa

thw

ays,

com

plex

life

cycl

esV

aria

ble

orun

pred

icta

ble

Def

ault

deve

lopm

enta

lre

spon

seA

ctiv

ede

velo

pmen

tD

evel

opm

ent

unle

sssi

gnal

led

othe

rwis

eS

uita

ble,

oren

viro

nmen

tal

sign

als

reli

able

Pass

ive

dela

yD

elay

unle

sssi

gnal

led

othe

rwis

e(“

obli

gate

diap

ause

”,et

c.)

Sev

ere

and

unpr

edic

tabl

e,or

envi

ronm

enta

lsi

gnal

sun

reli

able

Tab

le5.

Cor

eel

emen

tsof

life

-cyc

lesy

stem

sin

inse

cts

that

resp

ond

toth

ena

ture

ofch

ange

incl

imat

esor

habi

tats

.

in different seasons (cohort splitting: e.g.,Townsend and Pritchard 1998; Aoki 1999).More commonly, individuals from unpredict-able habitats are spread out within and betweenseasons, typically through variable and delayeddevelopment, sometimes in multiple stages (seebelow). The means by which such delays arecontrolled are considered in later subsections.

Predictable habitats, whether or not they arestrongly seasonal, allow simple or fixed life cy-cles. Individuals of species in predictable habi-tats typically follow the same fixed life cycle —for example, developing in summer and enter-ing diapause in winter. However, in many spe-cies from less predictable habitats, differentindividuals follow different routes among avariety of alternative life-cycle pathways. Dif-ferent routes — including slow or rapid devel-opment, diapause or non-diapause, andquiescence or its absence — are availablethrough genetic polymorphism (see below) orenvironmental responses. Sample sets of path-ways are figured by Danks (1987, 1991b,1994). For example, the stored-product miteLepidoglyphus destructor (Schrank) (Acarina:Glycyphagidae) varies greatly in genetic pro-pensity for both the onset and duration ofdiapause in a specialized hypopal stage (Knülle1987, 1991). High-quality food reduces the in-cidence of diapause in intermediate genotypes,and temperature and humidity change its dura-tion. After diapause ends, development stopsunless humidity is high. These genetic andenvironmental responses result in life-cyclepathways ranging very widely from direct de-velopment (i.e., without a hypopal stage) tovery long delays. Even in species in which thepopulation as a whole reproduces continuously,there may be cryptic dormant individuals(Spieth 2002).

Finally, the core structure of life cycles de-pends on whether development or delay is thedefault condition (Danks 2001, 2002a). Whenthe default is active, development proceeds un-less delays are specifically signalled. Such re-sponses accord with habitats that are suitablefor activity or with habitats that (even if inter-mittently unsuitable) supply environmental sig-nals that reliably indicate when conditions willdeteriorate, allowing development to be ad-justed adaptively. Under such circumstances, apassive response would cause safe opportunitiesfor development to be missed.

When the default is passive, developmentstops and does not continue unless signalled to

resume by changed conditions. Passive responsesare prevalent in species from more severe andunpredictable habitats or from habitats whereenvironmental signals do not reliably indicatefuture suitability. Such responses, includingfixed diapause and very narrow requirementsfor development, are conservative. Active re-sponses in difficult habitats would probablyexpose vulnerable individuals to dangerousconditions.

The existence of passive systems has led tomany difficulties in understanding life cycles.This is especially true of so-called “obligate”diapause, which often has been seen as an inev-itable programmed arrest of development. How-ever, many such diapauses simply end as soonas there is a required change in cues, typicallyin photoperiod (for examples, see Danks 1987,Table 23) but also in temperature (Worland etal. 2000) or food (Glitho et al. 1996). Passivedefaults permit very conservative, slow, or com-plex life cycles, especially when there are mul-tiple requirements for development to resume.Passive or active defaults can be used to governnot only diapause, but also other seasonal de-velopmental responses such as wing morph andinstar number (Danks 2002a).

Life-cycle delaysInsect life cycles are controlled especially by

delaying development. Delays take four forms(Table 6): directly limited growth; reducedgrowth rates under indirect control by cues; di-rectly arrested development when conditionsare unsuitable (quiescence); and programmedsuppression of development (diapause). Theseadjustments serve to avoid adversity, exploit fa-vourable conditions, or enforce univoltinism.

Ordinary environmental variations govern therate of growth directly — for example, throughthe influence of temperature on the rate of me-tabolism. However, many species also modifythe rate of growth by using environmental con-ditions as cues or indirect signals, notablyphotoperiod (for examples in 80 species of in-sects see Danks 1987, Table 33) and density(Danks 1987, Table 36). Growth or develop-ment can be adjusted continuously or succes-sively in response to these signals, allowingdevelopment to be optimized despite seasonalconditions that change within and betweenyears. For example, some species slow develop-ment by increasing the number of instars in re-sponse to photoperiod (Danks 1987, Table 35).In several different taxa, growth-rate responses




Danks 11

Ele

men

tF

eatu

res

oral

tern

ativ

efe

atur

esD

escr

ipti

onTy

pica

lcl

imat

eor

habi

tat

Dir

ectl

ym

odif

ied

grow

thra

teS

low

ergr

owth

atlo

wer

tem

pera

ture

sO

rdin

ary

and

seas

onal

vari

atio

nsIn

dire

ctly

mod

ifie

dgr

owth

rate

Cue

def

fect

sS

low

er(o

rm

ore

rapi

d)gr

owth

acco

rdin

gto

phot

oper

iod,

etc.

Ong

oing

adju

stm

ents

tova

riab

leha

bita

tsC

onti

nuou

s,pl

asti

cre

spon

ses

Ong

oing

resp

onse

tocu

es,

vari

able

num

ber

ofin

star

s,st

atio

nary

mou

lts

Com

plex

seas

onal

patt

erns

and

vari

atio

nQ

uies

cenc

eC

ompl

ete

arre

stth

roug

hdi

rect

acti

onof

asi

ngle

fact

or,

usua

lly

tem

pera

ture

Sea

son

wit

hli

mit

edad

vers

ity

Dia

paus

eP

rogr

amm

edre

spon

se,

but

sing

le,

lim

ited

Dev

elop

men

tbu

tno

tac

tivit

ysu

ppre

ssed

Mod

erat

ely

adve

rse

cond

itio

ns

Sin

gle,

fixe

d,re

lativ

ely

shor

tdu

rati

onD

evel

opm

ent

and

activ

ity

supp

ress

eddu

ring

asi

ngle

adve

rse

seas

on;

diap

ause

ends

ata

stan

dard

rate

Pre

dict

ably

seas

onal

Sta

ged

Sev

eral

com

pone

nts

ina

sing

ledi

apau

seS

easo

nal

patt

erns

and

vari

atio

n(r

espo

nses

allo

won

goin

gco

ntro

l)R

e-in

duct

ion

Suc

cess

ive

entr

ies

into

equi

vale

ntdi

apau

ses

inth

esa

me

stag

eS

easo

nal

patt

erns

and

vari

atio

n(r

espo

nses

allo

won

goin

gco

ntro

l)A

djac

ent

diap

ause

sS

ucce

ssiv

ede

lays

imm

edia

tely

afte

ron

ean

othe

rS

easo

nal

patt

erns

and

vari

atio

n(r

espo

nses

allo

won

goin

gco

ntro

l)M

ulti

ple

diap

ause

sL

ife-

cycl

epr

ogra

mm

ew

ith

diap

ause

sat

inte

rval

sS

easo

nal

patt

erns

and

vari

atio

n(r

espo

nses

allo

won

goin

gco

ntro

l)C

ompl

exdi

apau

se-e

ndin

gre

spon

seM

ulti

ple

requ

irem

ents

;di

apau

seca

nen

dat

anac

cele

rate

dra

teS

easo

nal

patt

erns

and

vari

atio

n(r

espo

nses

allo

won

goin

gco

ntro

l)L

ong

orve

rylo

ngdu

rati

onP

rolo

nged

diap

ause

for

mor

eth

anon

ese

ason

,du

rati

onus

uall

yva

riab

leU

nrel

iabl

e

Tab

le6.

Type

sof

dela

ysin

inse

ctli

fecy

cles

that

resp

ond

toth

ena

ture

ofch

ange

incl

imat

esor

habi

tats

.

to photoperiod reverse direction during devel-opment (e.g., Ryan 1975; Norling 1976;Khaldey 1977; Tanaka 1983).

When conditions are not especially variableor adverse, advance preparations for cold hardi-ness or energy conservation are not necessary.In such circumstances, development may stopsimply by directly controlled quiescence. Nev-ertheless, especially in temperate regions, thelife cycles of most species include at least oneof the programmed delays known as diapause.

Diapauses structure life cycles in many ways,ranging — according to environment — from asingle limited suppression of development tomultiple and long-term responses (Table 6). Thetrend shown in Table 6 is that a more complexor longer-lasting diapause is correlated withgreater seasonal variation or less predictablehabitats and allows more precise seasonal coin-cidence or programming. For example, a singlediapause can include several component stages(e.g., Danks 1987; Lavenseau and Hilal 1990);adult diapause can be re-induced (e.g., Hodekand Hodková 1992); several diapauses can fol-low one another within a stage (Ingrisch 1986;Niva and Becker 1998) or between stages (e.g.,Nishizuka et al. 1998); multiple diapauses canintervene at different stages throughout the lifecycle (e.g., Danks 1991b); and diapause-endingsignals can act in more than one way (e.g.,Hodek 1981, 1983; Košt’ál et al. 2000). There-fore, diapause intensity is highly variable(Masaki 2002).

Diapause can even last for two or more sea-sons, a trait that is strongly correlated with un-reliable conditions. Most species withprolonged diapause, although belonging to di-verse taxa, live in unstable habitats such as theArctic or deserts or feed on erratic resourcessuch as the cone crops of evergreen trees(Danks 1987, Table 27; Danks 1992a, Table 5;Hanski 1988).

Responsiveness to environmental signalsA critical element of adaptations to changing

environments is how insects respond to thepresence and reliability of environmental sig-nals, not just the environmental conditionsthemselves. Insects use one or a combination ofphotoperiod, temperature, thermoperiod, mois-ture, food, and other factors to assess the cur-rent or future suitability of habitats fordevelopment or reproduction. The best cues arereliable, frequent, and easily recognized (Ta-ble 7). Therefore, photoperiod is normally the

most useful external factor because it is highlyinformative about seasonal time, can be moni-tored daily, and is easily visible in most habi-tats. Thermoperiod shares some of the samefeatures. However, which cues can be used by aparticular species depends on the species’ habi-tat and its life cycle. For example, it would bedifficult for insects concealed in opaque sub-strates to detect photoperiod reliably, even withvery low thresholds for light detection. The sig-nal given by temperature is too noisy for pre-cise use in many places and is most useful inhabitats buffered against daily variations. Fordetails of the seasonal correlations, variability,and other features of various cues, see Danks(1987, ch. 7).

Although environmental signals indicate sea-sonal time, photoperiod and many other factorsare not themselves significant selective forcesbut simply serve as a proxy for climate andother ultimate factors. Depending on the spe-cies, its habitat, and its cues, the degree of cor-relation of environmental signals with theultimate selective forces might be different aftera change in climate.

Whatever the cue or cues used to monitor en-vironments, several patterns of response can beidentified (Table 8). Patterns of sensitivity in-clude direct regulation only (without any use ofindirect signals or token stimuli) and fixed de-lays in development that are insensitive to cues.Most common is the integration of multiple sig-nals to govern complex life cycles that includediapause and adjustments of growth (seeabove). It is significant that even when the reli-able, frequent, and recognizable signals fromphotoperiod are available, most species inte-grate information from temperature or food tomodify this photoperiodic control (e.g.,Goehring and Oberhauser 2002; Luker et al.2002). Evidently, even a small contribution ofadditional evidence from other cues aboutlikely future environmental changes has adap-tive value (Danks 2002a).

Many species respond to cues in a simplequalitative manner: their responses differ eitherside of an absolute threshold such as a criticalphotoperiod or differ according to the presenceor absence of a cue such as food or rainfall.However, in some species the level of the cuesets a quantitative response. In such species, theactual photoperiod or temperature can deter-mine the rate of growth (e.g., Wassmer andPage 1993), the incidence of diapause (Kimura1990), and diapause intensity during induction



(Nakamura and Numata 2000; Kalushkov et al.2001) or during diapause (Nakamura andNumata 2000). Moreover, in a few specieschanging photoperiods have different effectsfrom stationary ones on the duration ofdiapause or the rate of development (Canardand Grimal 1993; Košt’ál and Hodek 1997;Kemp 2000).

The stages at which cues are monitored andat which responses take place vary among spe-cies; the range of possibilities is shown in thelower part of Table 8. Stages sensitive todiapause-inducing cues are more commonly ac-tive (larvae, adults) than passive (eggs, pupae),apparently because active stages have better-developed sense organs and greater exposure tothe environment, favouring monitoring (Danks1987, p. 80). Normally, diapause is governed bythe stage that immediately precedes it, appar-ently because environmental information ismore reliable the closer it is to the event beingpredicted. Nevertheless, a common pattern adds

sensitivity in preceding stages (allowing envi-ronments to be monitored for a longer period),even though the most recent information isgiven more weight. Sensitivity in the precedinggeneration is not uncommon (see reviews byDanks 1987, Table 14; Mousseau and Dingle1991); for example, maternal conditions influ-ence egg diapause. However, very early com-mitment to developmental delays is relativelyrare, although there are some spectacular casessuch as the silkworm Bombyx mori (L.)(Lepidoptera: Bombycidae), in which eggdiapause is induced in the eggs of the precedinggeneration.

Most species have only one dormant stageper generation. Stages most often dormant arelarvae, especially during developmental pausessuch as the prepupal stage, and adults, espe-cially in the pre-reproductive stage. Also preva-lent are stages in a given taxon thatcharacteristically are resistant to adverse condi-tions. Finally, dormant stages correspond with


Danks 13

Element Description Sample factors and properties

Reliable Correlated with seasonal position andpredictability of seasonal change

Photoperiod, thermoperiod reliable; food oftenreliable; other factors mostly unreliable

Frequent Available for regular monitoring Temperature, moisture, photoperiod, thermoperiodfrequent; other factors mostly intermittent orvariable

Recognizable Sensors available and seasonal rateof change high

Sensors available for most factors but mostfactors with low rates of change

Table 7. Features of environmental signals used by insects (after Danks 2001).

Element Alternative features Description

Type of sensitivity Direct regulation only Temperature-controlled growth,quiescence

Fixed pattern “Obligate” diapauseIndirect, multiple factors Diapause and related responses

Type of response to cues Qualitative, fixed Presence/absence or absolutethresholds

Quantitative Actual value or duration of cueaffects response; responses maychange through stage

Stage of sensitivity andresponse

One instar sensitive Signals monitored in only oneinstar

Many instars or stages sensitive Long-term integration ofenvironmental signals

Response soon after sensitive stage More or less immediate responseResponse long after sensitive stage Response only after a delay, even

in next generation

Table 8. Types of responsiveness to environmental signals by insects (for further details see Danks 1987).

other timing requirements such as the availabil-ity of food plants at the correct stage or theneed for synchronous emergence of reproduc-tive adults. All of these adaptations provideboth precision to ensure survival and reproduc-tion and flexibility to cope with change. As aresult of this complexity, there is an enormousrange of timing possibilities both among andwithin species. Consequently, environmentalchanges can create unexpected effects both di-rectly and through the signals that govern de-velopment.

Genetic systemsEnvironmental responses, whereby a given

genotype allows a range of plastic reactions todifferent environments, act in concert with ge-netic variation (Table 9). All organisms showindividual variation. Traits that are critical tosurvival may be canalized so that important ad-aptations will not be lost through variationaway from the mean (e.g., Stearns and Kawecki1994). By the same token, canalization limitsadaptation to change. On the other hand, exten-sive variation makes it possible to produce indi-viduals adapted to variable habitats. Suchvariation tends to be genetic rather than envi-ronmental in two circumstances: first, whenhabitats — or at least different regional habitats— are stable, and second, when habitats changeand the changes cannot be predicted from envi-ronmental cues, preventing adequate plastic ad-justments. A wide range of variation thenspreads developmental or other adaptive typesthrough time. For example, staggered egg hatch

or asynchronous development is characteristicof species from habitats with unpredictable sup-plies of water (Hynes and Hynes 1975; Camp-bell 1986; Moreira and Peckarsky 1994; Zwick1996), streams with unpredictable spates(Courtney 1991), areas with late winter storms(Neal et al. 1997), and so on. In many speciesthe range of this genetic variation is not contin-uous, but is subdivided into two or more dis-crete sets (which has the advantage ofsynchronizing the population within each set,even if the sets do not cover the full potentialrange of variation). Such polymorphism pro-duces a set of different genotypes within a co-hort to cope with different unpredictablecircumstances. Common examples are bimodalemergence (e.g., Biron et al. 1999; review byDanks 1987, Table 27) and prolonged diapause(see above).

Such bet-hedging or risk-spreading adapta-tions have been recognized in several settingsas responses to unpredictability (see Frank andSlatkin 1990 for sample theory). Moreover, be-yond striking examples of polymorphism, gen-eral variation is preserved in all species by anumber of mechanisms, suggesting that all hab-itats — even those that seem stable and favour-able — have some level of unexpected risk.Many temperate species include occasional in-dividuals that vary greatly from the mean — forexample, in size, developmental time, diapauseincidence and duration, dispersal, and resis-tance to physical or chemical adversity — andclearly they result from adaptations rather thanfrom accidents (Danks 1983). Part of the



ElementAlternative

features Description Typical climate or habitat

Patterns of geneticvariation

Limited Little variation, normally distributed;canalization

Many

Extensive Wide and retained variation VariablePolymorphism Different morphs in one cohort Unpredictable habitats

without reliableenvironmental signals

Parthenogenesis Obligate Reproduction always parthenogenetic Severe and variable overthe long term (withother factors)

Dominant Occasional sexual reproduction SevereCyclic Parthenogenesis and sexual

reproduction alternate seasonallyAlternating habitats

Intermittent Occasional parthenogenesis, butusually reproduce sexually

Unknown

Table 9. Genetic systems in insects that respond to the nature of change in climates or habitats.

preserved variation undoubtedly arises simplybecause environments are heterogeneous, so noone genotype suits them all: interactions be-tween genotype and environment help to main-tain variation (Iriarte and Hasson 2000; Jia etal. 2000). Variation is also retained because notall genes, especially in diploid and polyploidorganisms, are exposed to selection in anygiven generation. A pool of dormant individu-als also shelters some genotypes from selection(Hairston and De Stasio 1988). In addition, spe-cific genetic mechanisms exist to retain varia-tion. Consequently, it is very difficult toeliminate diapause completely in most specieseven by very stringent selection against it overmany generations (Danks 1987, Table 30).

In summary, variation tends to be retainedbecause it is adaptive in real environments,which are characterized by change and by dif-ferences in both the nature of the change itselfand the availability of environmental signals topredict the change.

Parthenogenesis is more prevalent in Arcticenvironments than elsewhere (Downes 1965;Danks 1981), apparently because it bufferschange. In such severe habitats the relativelyrapid selection to current circumstances thatwould follow recombination would remove ge-notypes that are best adapted to the long-termseverity. In some Arctic species, as in manyplants, parthenogenesis is dominant but not uni-versal. Systems in which parthenogenesis domi-nates may retain the advantages of sexualreproduction in allowing recombination butconstrain its potential for dangerous change.Cyclical or annual parthenogenesis allows ex-ploitation of different habitats that becomeavailable alternately throughout the year. Inmany common species of aphids, for example,ephemeral host plants are rapidly exploited byparthenogenetic viviparae in summer, but theoverwintering egg stage is produced throughsexual reproduction. Finally, rare parthenogene-sis occurs even in species that normally repro-duce sexually (e.g., Grodhaus 1971 forunfertilized chironomid eggs), although its sig-nificance is not known.

Circadian responsesThe timing of daily activity depends on bio-

logical features: visual predators are active byday, for example, whereas many species sensi-tive to desiccation are active only at night.

In parallel with most seasonal responses, cir-cadian timing is controlled by levels of light,

temperature, and moisture, by internal dailyrhythms, or by internal programmes that can bemodified by external environments. Circadianrhythms are typical of stable temperate regions.However, in the high Arctic, daily photoperiod(whether or not it can be detected from thedaily changes in light intensity during 24 h day-light) is not a useful indicator of what is impor-tant for insect existence in places wheretemperature is often limiting (Danks and Oliver1972). Instead, direct responses to temperaturegovern the daily activity of most species there(see review by Danks 1981, pp. 271–272;Lankinen and Riihimaa 1997). Unfortunately,most of the recent work on circadian responseshas focussed on timing mechanisms rather thanon ecological relevance (Danks 2003, 2005b).

Conclusions

Insects survive in seasonal climates by adap-tations to resist adversity, including inactivitywhen appropriate, as well as by adaptations toensure that activity or reproduction coincidewith suitable conditions. Withstanding cold re-quires complex adaptations, and different spe-cies use different elements among a wide rangeof possibilities (Table 1). Comparable complex-ity and diversity characterize the means bywhich life cycles are timed to cope with envi-ronmental changes on a range of scales (Ta-ble 5). Moreover, cold hardiness and life-cycletiming interact. For example, many high Arcticspecies overwinter in exposed places, wherethey require extreme cold hardiness, becausethese places warm up earliest in spring, a majorbenefit when summers are very short and cool.In some species, adaptations such as cold hardi-ness and diapause are clearly linked.

However, we do not yet know why differentspecies have different modes of cold hardiness.For example, different species supercool to dif-ferent degrees or freeze through inoculation byexternal ice, through freezing of the gut con-tents, or through haemolymph nucleators. Onlyrecently has a start been made towards under-standing such differences by considering theevolution of freezing tolerance (Vernon andVannier 2002; Voituron et al. 2002) and thetypes of cold hardiness. Nevertheless, “classifi-cations” of cold hardiness (Bale 1993, 1996;Sinclair 1999; Nedv�d 2000), although theydocument the fact that cold temperatures can beharmful in the absence of freezing, are otherwiseof limited value because of the multifactorial


Danks 15

nature of the adaptations (see Danks 1987, ch.2, for classifications of diapause).

Care is required in developing generaliza-tions because one element can serve in manydifferent ways. For example, in one or anothercircumstance or species, certain key molecules(cryoprotectants) can have one or more effectsamong multiple possibilities: they appear to fa-vour supercooling or disable nucleators, resistor protect against desiccation, protect frozenmembranes or other cell constituents directly,modify the freezing process, mitigate cellulardamage from ice crystallization or re-crystallization while tissues are frozen or whilethey are thawing, and repair damage after thaw-ing. In the same way, developmental delays canserve multiple purposes: they may conserve en-ergy, protect against adversity, synchronize thefeeding stage with food resources, optimize thetiming of reproduction, synchronize individualswith one another, prevent a risky generation latein the year, or assist in further life-cycle pro-gramming by allowing the environment to bemonitored for a longer period (Danks 2002a).Moreover, there is more than one way toachieve the same end: selection is insensitive toexactly how a functional requisite is met. Con-sequently, successful species may have differentcryoprotectants or different diapause stages notonly to serve different ends but also becausethey have different evolutionary histories. Dif-ferent species in the same place can havewidely divergent strategies (e.g., Worland andBlock 2003). Therefore, multiple simultaneousexperimental approaches will be needed toidentify the components of such strategies. Forboth cold hardiness and life cycles there is par-ticular interest in how the patterns of variation(including genetic components of prolongeddiapause, for example) are maintained in re-sponse to long-term patterns of environmentalvariation.

However, cold-hardiness adaptations havescarcely been analyzed in the context ofmicroenvironments or environmental changebecause of an emphasis on summer conditions,such as mean summer temperature or durationof the growing season (e.g., Hodkinson et al.1998; Danks 2004a). The major reason for thisneglect is that, with limited exceptions (e.g.,Sinclair 2001), winter habitats have not beenadequately studied (Danks 1978, 1991a,2000b). Information on life cycles shows thathabitat features are the key to understanding re-sponses to change, but comparable insights are

not yet possible for cold hardiness because wedo not know the conditions experienced byoverwintering insects, even in species for whichcryoprotectant profiles are known. Without de-tailed knowledge of microhabitat conditions wecannot predict the likely effects of environmen-tal changes on winter survival, especially forthe time frames of intermediate duration thatare of particular current interest. Such knowl-edge would also inform experimental work,suggesting appropriate cooling rates, tempera-ture patterns to test survival or energy use, andpreconditioning requirements, for example(Danks 2005a).

The parallels between cold hardiness andlife-cycle control — the complexity of adapta-tions, the multiplicity of roles, and the influ-ence of key elements of the habitat — confirmthat both sets of adaptations are driven by thesame kinds of environmental elements. Conse-quently, we now need to ask for cold hardinesstoo: How much do the conditions experiencedchange on various temporal and spatial scales?How consistent is the change on given (espe-cially annual) time frames? And how reliablydo the available environmental signals forecastcoming changes? To answer these questions,more detailed knowledge about environments,not just about insects, is required.

Acknowledgements

This paper was prepared as a basis for anopening presentation for the symposium “Adap-tations and Constraints: a Symposium in Hon-our of Richard Ring”, recognizing the manycontributions of Professor Ring to the study ofinsect adaptations in cold and changing envi-ronments. I thank N. Winchester and R.Bennett, who organized the symposium, for in-viting me to present it.

References

Aitchison, C.W. 1979. Winter-active subnivean in-vertebrates in Southern Canada. I. Collembola; II.Coleoptera; III. Acari; IV. Diptera andHymenoptera. Pedobiologia, 19: 113–120; 121–128; 153–160; 176–182.

Aitchison, C.W. 1989. The ecology of winter-activecollembolans and spiders. Aquilo Ser Zoologica,24: 83–89.

Aoki, T. 1999. Larval development, emergence andseasonal regulation in Asiagomphus pryeri (Selys)(Odonata: Gomphidae). Hydrobiologia, 394: 179–192.



Bale, J.S. 1987. Insect cold hardiness: freezing andsupercooling — an ecophysiological perspective.Journal of Insect Physiology, 33: 899–908.

Bale, J.S. 1993. Classes of insect cold hardiness.Functional Ecology, 7: 751–753.

Bale, J.S. 1996. Insect cold hardiness: a matter oflife and death. European Journal of Entomology,93: 369–382.

Bale, J.S. 2002. Insects and low temperatures: frommolecular biology to distributions and abundance.Philosophical Transactions of the Royal Societyof London, B: Biological Sciences, 357: 849–862.

Bale, J.S., Hansen, T.N., and Baust, J.G. 1989. Ef-fect of cooling rate on the survival of larvae,pupariation, and adult emergence of the gall flyEurosta solidaginis. Cryobiology, 26: 285–289.

Bale, J.S., Block, W., and Worland, M.R. 2000.Thermal tolerance and acclimation response of thesub-Antarctic beetle Hydromedion sparsutum. Po-lar Biology, 23: 77–84.

Bale, J.S., Worland, M.R., and Block, W. 2001. Ef-fects of summer frost exposures on the cold toler-ance strategy of a sub-Antarctic beetle. Journal ofInsect Physiology, 47: 1161–1167.

Barrett, J. 1991. Anhydrobiotic nematodes. Agricul-tural Zoology Reviews, 4: 161–176.

Baust, J.G. 1982. Environmental triggers to coldhardening. Comparative Biochemistry and Physi-ology A, 73: 563–570.

Baust, J.G., and Lee, R.E., Jr. 1982. Environmentaltriggers to cryoprotectant modulation in separatepopulations of the gall fly Eurosta solidaginis(Fitch). Journal of Insect Physiology, 28: 431–436.

Baust, J.G., and Nishino, M. 1991. Freezing toler-ance in the goldenrod gall fly (Eurostasolidaginis). In Insects at low temperature. Editedby R.E. Lee, Jr. and D.L. Denlinger. Chapman andHall, New York. pp. 260–275.

Biron, D.-G., Brunel, E., Nenon, J.-P., Coderre, D.,and Boivin, G. 1999. Etude préliminaire dudéterminisme génétique contrôlant l’expression del’émergence bimodale de Delia radicum (Diptera:Anthomyiidae). Annales de la Société Entomo-logique de France, 35(Suppl.): 104–108.

Block, W. 1996. Cold or drought — the lesser of twoevils for terrestrial arthropods? European Journalof Entomology, 93: 325–339.

Block, W. 2002. Interactions of water, ice nucleatorsand desiccation in invertebrate cold survival. Eu-ropean Journal of Entomology, 99: 259–266.

Block, W. 2003. Water or ice? The challenge for in-vertebrate cold survival. Science Progress, 86: 77–101.

Brown, C.L., Bale, J.S., and Walters, K.F.A. 2004.Freezing induces a loss of freeze tolerance in anoverwintering insect. Proceedings of the Royal Soci-ety Biological Sciences Series B, 271: 1507–1511.

Brundin, L. 1966. Transantarctic relationships andtheir significance, as evidenced by chironomidmidges, with a monograph of the subfamilies

Podonominae and Aphroteniinae and the AustralHeptagyiae. Kungliga Svenska Vetenskapsak-ademiens Handlingar, 11: 7–472.

Campbell, A., and Mackauer, M. 1975. Thermal con-stants for development of the pea aphid(Homoptera: Aphididae) and some of its parasites.The Canadian Entomologist, 107: 419–423.

Campbell, I.C. 1986. Life histories of some Austra-lian siphlonurid and oligoneuriid mayflies (Insecta:Ephemeroptera). Australian Journal of Marine andFreshwater Research, 37: 261–288.

Canard, M., and Grimal, A. 1993. Multiple action ofphotoperiod on diapause in the green lacewingMallada picteti (McLachlan) (NeuropteraChrysopidae). Bollettino dell’Istituto di Entomo-logia “Guido Grandi” della Universita degli Studidi Bologna, 47: 233–245.

Cannon, R.J.C., and Block, W. 1988. Cold toleranceof microarthropods. Biological Reviews of theCambridge Philosophical Society, 63: 23–77.

Conradi-Larsen, E.-M., and Sømme, L. 1973a.Anaerobiosis in the overwintering beetle, Pelophilaborealis. Nature (London), 245: 388–390.

Conradi-Larsen, E.-M., and Sømme, L. 1973b. Theoverwintering of Pelophila borealis Payk. II. Aer-obic and anaerobic metabolism. Norsk Entomo-logisk Tidsskrift, 20: 325–332.

Courtney, G.W. 1991. Life history patterns of Nearc-tic mountain midges (Diptera: Deuterophlebiidae).Journal of the North American Benthological So-ciety, 10: 177–197.

Daley, M.E., Spyracopoulos, L., Jia, Z., Davies, P.L.,and Sykes, B.D. 2002. Structure and dynamics ofa β-helical antifreeze protein. Biochemistry, 41:5515–5525.

Danks, H.V. 1971. Overwintering of some north tem-perate and arctic Chironomidae. II. Chironomidbiology. The Canadian Entomologist, 103: 1875–1910.

Danks, H.V. 1978. Modes of seasonal adaptation inthe insects. I. Winter survival. The Canadian En-tomologist, 110: 1167–1205.

Danks, H.V. 1981. Arctic arthropods. A review ofsystematics and ecology with particular referenceto North American fauna. Entomological Societyof Canada, Ottawa, Ontario.

Danks, H.V. 1983. Extreme individuals in naturalpopulations. Bulletin of the Entomological Soci-ety of America, 29: 41–46.

Danks, H.V. 1987. Insect dormancy: an ecologicalperspective. Biological Survey of Canada (Terres-trial Arthropods), Ottawa, Ontario.

Danks, H.V. 1991a. Winter habitats and ecologicaladaptations for winter survival. In Insects at lowtemperature. Edited by R.E. Lee, Jr. and D.L.Denlinger. Chapman and Hall, New York. pp.231–259.

Danks, H.V. 1991b. Life-cycle pathways and theanalysis of complex life cycles in insects. The Ca-nadian Entomologist, 123: 23–40.


Danks 17

Danks, H.V. 1992a. Long life cycles in insects. TheCanadian Entomologist, 124: 167–187.

Danks, H.V. 1992b. Arctic insects as indicators ofenvironmental change. Arctic, 45: 159–166.

Danks, H.V. 1993. Patterns of diversity in the Cana-dian insect fauna. In Systematics and entomology:diversity, distribution, adaptation and application.Edited by G.E. Ball and H.V. Danks. Memoirs ofthe Entomological Society of Canada, 165: 51–74.

Danks, H.V. 1994. Diversity and integration of life-cycle controls in insects. In Insect life-cycle poly-morphism: theory, evolution and ecological conse-quences for seasonality and diapause control.Edited by H.V. Danks. Series Entomologica 52.Kluwer Academic Publishers, Dordrecht. pp. 5–40.

Danks, H.V. 1996. The wider integration of studieson insect cold-hardiness. European Journal of En-tomology, 93: 383–403.

Danks, H.V. 1999. Life cycles in polar arthropods —flexible or programmed? European Journal of En-tomology, 96: 83–102.

Danks, H.V. 2000a. Dehydration in dormant insects.Journal of Insect Physiology, 46: 837–852.

Danks, H.V. 2000b. Insect cold hardiness: a Cana-dian perspective. CryoLetters, 21: 297–308.

Danks, H.V. 2001. The nature of dormancy re-sponses in insects. Acta Societatis ZoologicaeBohemicae, 65: 169–179.

Danks, H.V. 2002a. The range of insect dormancyresponses. European Journal of Entomology, 99:127–142.

Danks, H.V. 2002b. Modification of adverse condi-tions by insects. Oikos, 99: 10–24.

Danks, H.V. 2003. Studying insect photoperiodismand rhythmicity: components, approaches and les-sons. European Journal of Entomology, 100: 209–221.

Danks, H.V. 2004a. Seasonal adaptations in arcticinsects. Integrative and Comparative Biology, 44:85–94.

Danks, H.V. 2004b. The roles of insect cocoons incold conditions. European Journal of Entomology,101: 433–437.

Danks, H.V. 2005a. Key themes in the study of sea-sonal adaptations in insects. I. Patterns of coldhardiness. Applied Entomology and Zoology, 40:199–211.

Danks, H.V. 2005b. How similar are daily and sea-sonal biological clocks. Journal of Insect Physiol-ogy, 51: 609–619.

Danks, H.V., and Foottit, R.G. 1989. Insects of theboreal zone of Canada. The Canadian Entomolo-gist, 121: 626–677.

Danks, H.V., and Oliver, D.R. 1972. Diel periodicitiesof emergence of some high arctic Chironomidae(Diptera). The Canadian Entomologist, 104: 903–916.

Denlinger, D.L. 1991. Relationship between coldhardiness and diapause. In Insects at low

temperature. Edited by R.E. Lee, Jr. and D.L.Denlinger. Chapman and Hall, New York. pp.174–198.

Denlinger, D.L., Rinehart, J.P., and Yocum, G.D.2001. Stress proteins: a role in insect diapause? InInsect timing: circadian rhythmicity to seasonal-ity. Edited by D.L. Denlinger, J.M. Giebultowicz,and D.S. Saunders. Elsevier, Amsterdam. pp.155–171.

Downes, J.A. 1965. Adaptations of insects in thearctic. Annual Review of Entomology, 10: 257–274.

Duman, J.G. 2001. Antifreeze and ice nucleator pro-teins in terrestrial arthropods. Annual Review ofPhysiology, 63: 327–357.

Duman, J.G. 2002. The inhibition of ice nucleatorsby insect antifreeze proteins is enhanced by glyc-erol and citrate. Journal of Comparative Physiol-ogy B: Biochemical, Systemic, and EnvironmentalPhysiology, 172: 163–168.

Duman, J.G., and Serianni, A.S. 2002. The role ofendogenous antifreeze protein enhancers in thehemolymph thermal hysteresis activity of the bee-tle Dendroides canadensis. Journal of InsectPhysiology, 48: 103–111.

Duman, J.G., Wu, D.W., Xu, L., Tursman, D., andOlsen, T.M. 1991. Adaptations of insects tosubzero temperatures. Quarterly Review of Biol-ogy, 66: 387–410.

Fields, P.G., and McNeil, J.N. 1986. Possible dualcold-hardiness strategies in Cisseps fulvicollis(Lepidoptera: Arctiidae). The Canadian Entomol-ogist, 118: 1309–1311.

Fletcher, G.L., Hew, C.Y., and Davies, P.L. 2001.Antifreeze proteins of teleost fishes. Annual Re-view of Physiology, 63: 359–390.

Frank, S.A., and Slatkin, M. 1990. Evolution in avariable environment. American Naturalist, 136:244–260.

Glitho, I.A., Lenga, A., Pierre, D., and Huignard, J.1996. Changes in the responsiveness during twophases of diapause termination in Bruchidiusatrolineatus Pic (Coleoptera: Bruchidae). Journalof Insect Physiology, 42: 953–960.

Goehring, L., and Oberhauser, K.S. 2002. Effects ofphotoperiod, temperature, and host plant age oninduction of reproductive diapause and develop-ment time in Danaus plexippus. Ecological Ento-mology, 27: 674–685.

Goto, S.G., and Kimura, M.T. 1998. Heat- and cold-shock responses and temperature adaptations insubtropical and temperate species of Drosophila.Journal of Insect Physiology, 44: 1233–1239.

Greenslade, P. 1981. Survival of Collembola in aridenvironments: observations in south Australia andthe Sudan. Journal of Arid Environments, 4: 219–228.

Grodhaus, G. 1971. Sporadic parthogenesis in threespecies of Chironomus (Diptera). The CanadianEntomologist, 103: 338–340.



Hadley, N.F. 1994. Water relations of terrestrial ar-thropods. Academic Press, New York.

Hairston, N.G., Jr., and De Stasio, B.T., Jr. 1988.Rate of evolution slowed by a dormant propagulepool. Nature (London), 336: 239–242.

Hanski, I. 1988. Four kinds of extra long diapause ininsects: a review of theory and observations.Annales Zoologici Fennici, 25: 37–53.

Hinton, H.E. 1960. A fly larva that tolerates dehydra-tion and temperatures from –270 °C to +102 °C.Nature (London), 188: 336–337.

Hodek, I. 1981. Le rôle des signaux del’environnement et des processus endogènes dansla régulation de la reproduction par la diapauseimaginale. Bulletin de la Société Zoologique deFrance, 106: 317–325.

Hodek, I. 1983. Role of environmental factors andendogenous mechanisms in the seasonality of re-production in insects diapausing as adults. InDiapause and life cycle strategies in insects.Edited by V.K. Brown and I. Hodek. SeriesEntomologica. Vol. 23. Dr. W. Junk, The Hague.pp. 9–33.

Hodek, I., and Hodková, M. 1992. Regulation ofpostdiapause reproduction by recurrent photo-periodic response. In Advances in regulation ofinsect reproduction. Edited by B. Bennettová, I.Gelbi�, and T. Soldán. Institute of Entomology,Czech Academy of Sciences, Ceske Budejovice.pp. 119–124.

Hodkinson, I.D., Webb, H.R., Bale, J.S., Block, W.,Coulson, S.J., and Strathdee, A.T. 1998. Globalchange and arctic ecosystems: conclusions andpredictions from experiments with terrestrial in-vertebrates on Spitsbergen. Arctic and Alpine Re-search, 30: 306–313.

Hodková, M., and Hodek, I. 1994. Control ofdiapause and supercooling by the retrocerebralcomplex in Pyrrhocoris apterus. EntomologiaExperimentalis et Applicata, 70: 237–245.

Hodková, M., and Hodek, I. 2004. Photoperiod,diapause and cold-hardiness. European Journal ofEntomology, 101: 445–458.

Holmstrup, M. 1992. Cold hardiness strategy in co-coons of the lumbricid earthworm Dendrobaenaoctaedra (Savigny). Comparative Biochemistryand Physiology A, 102: 49–54.

Holmstrup, M., and Sømme, L. 1998. Dehydrationand cold hardiness in the Arctic collembolanOnychiurus arcticus Tullberg 1876. Journal ofComparative Physiology B: Biochemical, Sys-temic, and Environmental Physiology, 168: 197–203.

Holmstrup, M., and Zachariassen, K.E. 1996. Physi-ology of cold hardiness in earthworms. Compara-tive Biochemistry and Physiology A, 115: 91–101.

Holmstrup, M., Bayley, M., and Ramløv, H. 2002.Supercool or dehydrate? An experimental analysisof overwintering strategies in small permeable

arctic invertebrates. Proceedings of the NationalAcademy of Sciences of the United States ofAmerica, 99: 5716–5720.

Horwath, K.L., and Duman, J.G. 1984. Yearly varia-tions in the overwintering mechanisms of thecold-hardy beetle Dendroides canadensis. Physio-logical Zoology, 57: 40–45.

Hynes, H.B.N., and Hynes, M.E. 1975. The life his-tory of many of the stoneflies (Plecoptera) ofsoutheastern mainland Australia. Australian Jour-nal of Marine and Freshwater Research, 26: 113–153.

Ingrisch, S. 1986. The plurennial life cycles of theEuropean Tettigoniidae (Insecta: Orthoptera). I.The effect of temperature on embryonic develop-ment and hatching. Oecologia (Berlin), 70: 606–616.

Iriarte, P.F., and Hasson, E.A. 2000. The role of theuse of different host plants in the maintenance ofthe inversion polymorphism in the cactophilicDrosophila buzzatii. Evolution, 54: 1295–1302.

Irwin, J.T., and Lee, R.E., Jr. 2000. Mild winter tem-peratures reduce survival and potential fecundityof the goldenrod gall fly, Eurosta solidaginis(Diptera: Tephritidae). Journal of Insect Physiol-ogy, 46: 655–661.

Irwin, J.T., and Lee, R.E., Jr. 2003. Cold wintermicroenvironments conserve energy and improveoverwintering survival and potential fecundity ofthe goldenrod gall fly, Eurosta solidaginis. Oikos,100: 71–78.

Itô, Y. 1998. Role of escape from predators in peri-odical cicada (Homoptera: Cicadidae) cycles. An-nals of the Entomological Society of America, 91:493–496.

Jia, F.-Y., Greenfield, M.D., and Collins, R.D. 2000.Genetic variance of sexually selected traits inwaxmoths: maintenance by genotype × environ-ment interaction. Evolution, 54: 953–967.

Joanisse, D.R., and Storey, K.B. 1996. Oxidativestress and antioxidants in overwintering larvae ofcold-hardy goldenrod gall insects. Journal of Ex-perimental Biology, 199: 1483–1491.

Joanisse, D.R., and Storey, K.B. 1998. Oxidativestress and antioxidants in stress and recovery ofcold-hardy insects. Insect Biochemistry and Mo-lecular Biology, 28: 23–30.

Kalushkov, P., Hodková, M., Nedv�d, O., andHodek, I. 2001. Effect of thermoperiod ondiapause intensity in Pyrrhocoris apterus(Heteroptera Pyrrhocoridae). Journal of InsectPhysiology, 47: 55–61.

Kelty, J.D., and Lee, R.E., Jr. 2000. Diapausing pu-pae of the flesh fly Sarcophaga crassipalpis(Diptera: Sarcophagidae) are more resistant toinoculative freezing than non-diapausing pupae.Physiological Entomology, 25: 120–126.

Kemp, D.J. 2000. The basis of life-history plasticityin the tropical butterfly Hypolimnas bolina (L.)


Danks 19

(Lepidoptera: Nymphalidae). Australian Journalof Zoology, 48: 67–78.

Khaldey, Ye. L. 1977. The biology, phenology andphotoperiodic reaction of Forficula tomis(Dermaptera, Forficulidae.) Entomological Re-view, 56(4): 6–12. Translation of Entomo-logicheskoye Obozreniye, 56: 721–730.

Kimura, M.T. 1990. Quantitative response tophotoperiod during reproductive diapause in theDrosophila auraria species-complex. Journal ofInsect Physiology, 36: 147–152.

Knight, J.D., Bale, J.S., Franks, F., Mathias, S.F.,and Baust, J.G. 1986. Insect cold hardiness:supercooling points and pre-freeze mortality.CryoLetters, 7: 194–203.

Knülle, W. 1987. Genetic variability and ecologicaladaptability of hypopus formation in a stored prod-uct mite. Experimental and Applied Acarology, 3:21–32.

Knülle, W. 1991. Genetic and environmental deter-minants of hypopus duration in the stored-productmite Lepidoglyphus destructor. Experimental andApplied Acarology, 10: 231–258.

Knülle, W. 2003. Interaction between genetic and in-ductive factors controlling the expression of dis-persal and dormancy morphs in dimorphicastigmatic mites. Evolution, 57: 828–838.

Košt’ál, V., and Hodek, I. 1997. Photoperiodism andcontrol of summer diapause in the Mediterraneantiger moth Cymbalophora pudica. Journal of In-sect Physiology, 43: 767–777.

Košt’ál, V., Shimada, K., and Hayakawa, Y. 2000.Induction and development of winter larvaldiapause in a drosophilid fly, Chymomyza costata.Journal of Insect Physiology, 46: 417–428.

Košt’ál, V., Berková, P., and Simek, P. 2003. Re-modelling of membrane phospholipids duringtransition to diapause and cold-acclimation in thelarvae of Chymomyza costata (Drosophilidae).Comparative Biochemistry and Physiology B,135: 407–419.

Kukal, O., Serianni, A.S., and Duman, J.G. 1988.Glycerol metabolism in a freeze-tolerant arctic in-sect: an in vivo 13C NMR study. Journal of Com-parative Physiology B: Biochemical, Systemic,and Environmental Physiology, 158: 175–183.

Kukal, O., Duman, J.G., and Serianni, A.S. 1989.Cold-induced mitochondrial degradation andcryoprotectant synthesis in freeze-tolerant arcticcaterpillars. Journal of Comparative PhysiologyB: Biochemical, Systemic, and EnvironmentalPhysiology, 158: 661–671.

Lafontaine, J.D., and Wood, D.M. 1997. Butterfliesand moths (Lepidoptera) of the Yukon. In Insectsof the Yukon. Edited by H.V. Danks and J.A.Downes. Biological Survey of Canada (TerrestrialArthropods), Ottawa, Ontario. pp. 723–785.

Lankinen, P., and Riihimaa, A. 1997. Effects of tem-perature on weak circadian eclosion rhythmicity

in Chymomyza costata (Diptera: Drosophilidae).Journal of Insect Physiology, 43: 251–260.

Lavenseau, L., and Hilal, A. 1990. Regulation des cy-cles saisonniers chez la sesamie (lepidoptère,Noctuidae). In Régulation des cycles saisonnierschez les invertébrés. Edited by P. Ferron, J.Missonnier, and B. Mauchamp. Institut National dela Recherche Agronomique, Paris. pp. 243–246.

Layne, J.R., Jr., and Kuharsky, D.K. 2000. Trig-gering of cryoprotectant synthesis in the woollybear caterpillar (Pyrrharctia isabella Lepidoptera:Arctiidae). Journal of Experimental Zoology, 286:367–371.

Lee, R.E., Jr. 1991. Principles of insect low tempera-ture tolerance. In Insects at low temperature.Edited by R.E. Lee, Jr. and D.L. Denlinger. Chap-man and Hall, New York. pp. 17–46.

Lee, R.E., Jr., and Costanzo, J.P. 1998. Biologicalice nucleation and ice distribution in cold-hardyectothermic animals. Annual Review of Physiol-ogy, 60: 55–72.

Lee, R.E., Jr., and Denlinger, D.L. (Editors). 1991.Insects at low temperature. Chapman and Hall,New York.

Lee, R.E. Jr., Warren, G.J., and Gusta, L.V. (Edi-tors). 1995. Biological ice nucleation and its ap-plications. American Phytopathological Society,St. Paul, Minnesota.

Levin, D.B., Danks, H.V., and Barber, S.A. 2003.Variations in mitochondrial DNA and gene tran-scription in freezing-tolerant larvae of Eurostasolidaginis (Diptera: Tephritidae) and Gynaephoragroenlandica (Lepidoptera: Lymantriidae). InsectMolecular Biology, 12: 281–289.

Li, N., Andorfer, C.A., and Duman, J.G. 1998. En-hancement of insect antifreeze protein activity bysolutes of low molecular mass. Journal of Experi-mental Biology, 201: 2243–2251.

Lloyd, M., and Dybas, H.S. 1966. The periodical ci-cada problem. I. Population ecology; II. Evolu-tion. Evolution, 20: 133–139; 466–505.

Luker, L.A., Hatle, J.D., and Juliano, S.A. 2002. Re-productive responses to photoperiod by a southFlorida population of the grasshopper Romaleamicroptera (Orthoptera: Romaleidae). Environ-mental Entomology, 31: 702–707.

Madura, J.D., Baran, K., and Wierzbicki, A. 2000.Molecular recognition and binding of thermalhysteresis proteins to ice. Journal of MolecularRecognition, 13: 101–113.

Masaki, S. 2002. Ecophysiological consequences ofvariability in diapause intensity. European Journalof Entomology, 99: 143–154.

Matsumoto, M., Saito, S., and Ohmine, I. 2002. Mo-lecular dynamics simulation of the ice nucleationand growth process leading to water freezing. Na-ture (London), 416: 409–413.

Meidell, E.-M. 1983. Diapause, aerobic and anaero-bic metabolism in alpine, adult Melasoma collaris(Coleoptera). Oikos, 41: 239–244.



Meier, P., and Zettel, J. 1997. Cold hardiness inEntomobrya nivalis (Collembola, Entomo-bryidae): annual cycle of polyols and antifreezeproteins, and antifreeze triggering by temperatureand photoperiod. Journal of Comparative Physiol-ogy B: Biochemical, Systemic, and EnvironmentalPhysiology, 167: 297–304.

Miller, K. 1982. Cold-hardiness strategies of someadult and immature insects overwintering in inte-rior Alaska. Comparative Biochemistry and Physi-ology A, 73: 595–604.

Miller, L.K. 1978. Physiological studies of arctic an-imals. Comparative Biochemistry and PhysiologyA, 59: 327–334.

Miller, L.K., and Smith, J.S. 1975. Production of threitoland sorbitol by an adult insect: association withfreezing tolerance. Nature (London), 258: 519–520.

Miller, L.K., and Werner, R. 1987. Extreme super-cooling as an overwintering strategy in three spe-cies of willow gall insects from interior Alaska.Oikos, 49: 253–260.

Moreira, G.R.P., and Peckarsky, B.L. 1994. Multipledevelopmental pathways of Agnetina capitata(Plecoptera: Perlidae) in a temperate foreststream. Journal of the North AmericanBenthological Society, 13: 19–29.

Morimoto, R.I., Tissieres, A., and Georgopoulos, C.(Editors). 1994. Biology of heat shock proteinsand molecular chaperones. Cold Spring HarborLaboratory Press, Cold Spring Harbor, New York.

Morissey, R.E., and Baust, J.G. 1976. The ontogenyof cold tolerance in the gall fly Eurosta solidaginis.Journal of Insect Physiology, 22: 431–437.

Mousseau, T.A., and Dingle, H. 1991. Maternal ef-fects in insect life histories. Annual Review ofEntomology, 36: 511–534.

Nakamura, K., and Numata, H. 2000. Photoperiodiccontrol of the intensity of diapause and diapausedevelopment in the bean bug, Riptortus clavatus(Heteroptera: Alydidae). European Journal of En-tomology, 97: 19–23.

Neal, J.W., Jr., Chittams, J.L., and Bentz, J.-A. 1997.Spring emergence by larvae of the eastern tentcaterpillar (Lepidoptera: Lasiocampidae): a hedgeagainst high-risk conditions. Annals of the Ento-mological Society of America, 90: 596–603.

Nedv�d, O. 2000. Snow white and the seven dwarfs:a multivariate approach to classification of coldtolerance. CryoLetters, 21: 339–348.

Nelson, D.E., and Lee, R.E., Jr. 2004. Cuticularlipids and desiccation resistance in overwinteringlarvae of the goldenrod gall fly, Eurostasolidaginis (Diptera: Tephritidae). ComparativeBiochemistry and Physiology B, 138: 313–320.

Nishizuka, M., Azuma, A., and Masaki, S. 1998.Diapause response to photoperiod and temperaturein Lepisma saccharina Linnaeus (Thysanura:Lepismatidae). Entomological Science, 1: 7–14.

Niva, C.C., and Becker, M. 1998. Embryonic exter-nal morphogenesis of Rhammatocerus conspersus

(Bruner) (Orthoptera: Acrididae: Gomphocerinae)and determination of the diapausing embryonicstage. Anais da Sociedade Entomologica doBrasil, 27: 577–583.

Norling, U. 1976. Seasonal regulation in Leucor-rhinia dubia (van der Linden) (Anisoptera:Libellulidae). Odonatologica, 5: 245–263.

Parry, W.H. 1986. The overwintering strategy ofAphidecta obliterata in Scottish coniferous for-ests. In Ecology of Aphidophaga. Edited by I.Hodek. Academia, Prague; Dr. W. Junk,Dordrecht. pp. 179–184.

Pullin, A.S. 1994. Evolution of cold hardiness strate-gies in insects. CryoLetters, 15: 6–7.

Pullin, A.S., and Bale, J.S. 1989. Influence ofdiapause and temperature on cryoprotectant syn-thesis and cold hardiness in pupae of Pierisbrassicae. Comparative Biochemistry and Physi-ology A, 94: 499–503.

Ramløv, H., Wharton, D.A., and Wilson, P.W. 1996.Recrystallization in a freezing tolerant Antarcticnematode, Panagrolaimus davidi, and an alpineweta, Hemideina maori (Orthoptera; Steno-pelmatidae). Cryobiology, 33: 607–613.

Ricci, C. 2001. Dormancy patterns in rotifers.Hydrobiologia, 446/447: 1–11.

Ring, R.A. 1977. Cold-hardiness of the bark beetle,Scolytus ratzeburgi Jans. (Col., Scolytidae). Nor-wegian Journal of Entomology, 24: 125–136.

Ring, R.A. 1981. The physiology and biochemistryof cold tolerance in arctic insects. Journal ofThermal Biology, 6: 219–229.

Ring, R.A. 1982. Freezing-tolerant insects with lowsupercooling points. Comparative Biochemistryand Physiology A, 73: 605–612.

Ring, R.A. 1983. Cold tolerance in Canadian arcticinsects. In Plant, animal, and microbial adapta-tions to terrestrial environments. Edited by N.S.Margaris, M. Arianoutsou-Faraggitaki, and R.J.Reiter. Plenum, New York. pp. 17–29.

Ring, R.A., and Danks, H.V. 1994. Desiccationand cryoprotection: overlapping adaptations.CryoLetters, 15: 181–190.

Ring, R.A., and Danks, H.V. 1998. The role oftrehalose in cold-hardiness and desiccation.CryoLetters, 19: 275–282.

Ring, R.A., and Tesar, D. 1981. Adaptations to coldin Canadian Arctic insects. Cryobiology, 18: 199–211.

Rivers, D.B., Antonelli, A.L., and Yoder, J.A. 2002.Bags of the bagworm Thyridopteryx ephemer-aeformis (Lepidoptera: Psychidae) protectdiapausing eggs from water loss and chilling in-jury. Annals of the Entomological Society ofAmerica, 95: 481–486.

Ryan, R.B. 1975. Photoperiod effects on develop-ment of the larch casebearer, Coleophora laricella(Lepidoptera: Coleophoridae). The Canadian En-tomologist, 107: 1305–1310.


Danks 21

Sabelis, M.W., and Janssen, A. 1994. Evolution oflife-history patterns in the Phytoseiidae. In Mites:ecological and evolutionary analyses of life-history patterns. Edited by M.A. Houck. Chapmanand Hall, New York. pp. 70–98.

Sakagami, S.F., Tanno, K., Tsutsui, H., and Honma,K. 1985. The role of cocoons in overwintering ofthe soybean pod borer Leguminivora glycinivorella(Lepidoptera: Tortricidae). Journal of the KansasEntomological Society, 58: 240–247.

Shimada, K., and Riihimaa, A. 1988. Cold acclima-tion, inoculative freezing and slow cooling: essen-tial factors contributing to the freezing-tolerancein diapausing larvae of Chymomyza costata(Diptera: Drosophilidae). CryoLetters, 9: 5–10.

Sinclair, B.J. 1999. Insect cold tolerance: How manykinds of frozen? European Journal of Entomology,96: 157–164.

Sinclair, B.J. 2001. Field ecology of freeze toler-ance: interannual variation in cooling rates,freeze–thaw and thermal stress in the microhabitatof the alpine cockroach Celatoblatta quin-quemaculata. Oikos, 93: 286–293.

Sinclair, B.J., and Chown, S.L. 2003. Rapid responsesto high temperature and desiccation but not to lowtemperature in the freeze tolerant sub-Antarcticcaterpillar Pringleophaga marioni (Lepidoptera,Tineidae). Journal of Insect Physiology, 49: 45–52.

Šlachta, M., Vambera, J., Zahradn��ková, H., andKošt’ál, V. 2002. Entering diapause is a prerequi-site for successful cold-acclimation in adultGraphosoma lineatum (Heteroptera: Pentatom-idae). Journal of Insect Physiology, 48: 1031–1039.

Sømme, L. 1974a. The overwintering of Pelophilaborealis Payk. III. Freezing tolerance. NorskEntomologisk Tidsskrift, 21: 131–134.

Sømme, L. 1974b. Anaerobiosis in some alpineColeoptera. Norsk Entomologisk Tidsskrift, 21:155–158.

Sømme, L. 1996. Anhydrobiosis and cold tolerancein tardigrades. European Journal of Entomology,93: 349–357.

Sømme, L. 1999. The physiology of cold hardinessin terrestrial arthropods. European Journal of En-tomology, 96: 1–10.

Sømme, L., and Birkemoe, T. 1997. Cold toleranceand dehydration in Enchytraeidae from Svalbard.Journal of Comparative Physiology B: Biochemi-cal, Systemic, and Environmental Physiology,167: 264–269.

Soszynska, A., and Durska, E. 2002. Cold-adaptedscuttle-flies species of Triphleba Rondani(Diptera: Phoridae). Annales Zoologici, 52: 279–283.

Spieth, H.R. 2002. Estivation and hibernation ofPieris brassicae (L.) in southern Spain: synchro-nization of two complex behavioral patterns. Pop-ulation Ecology, 44: 273–280.

Stearns, S.C., and Kawecki, T.J. 1994. Fitness sensi-tivity and the canalization of life-history traits.Evolution, 48: 1438–1450.

Stevens, D.J., Hansell, M.H., Freel, J.A., andMonaghan, P. 1999. Developmental trade-offs incaddis flies: increased investment in larval de-fence alters adult resource allocation. Proceedingsof the Royal Society Biological Sciences Series B,266: 1049–1054.

Stevens, D.J., Hansell, M.H., and Monaghan, P.2000. Developmental trade-offs and life histories:Strategic allocation of resources in caddis flies.Proceedings of the Royal Society Biological Sci-ences Series B, 267: 1511–1515.

Storey, K.B., and Storey, J.M. 1988. Freeze tolerancein animals. Physiological Reviews, 68: 27–84.

Storey, K.B., and Storey, J.M. 1992. Biochemical ad-aptations for winter survival in insects. Advancesin Low-Temperature Biology, 1: 101–140.

Storey, K.B., Baust, J.G., and Beuscher, P. 1981. Deter-mination of water “bound” by soluble subcellularcomponents during low-temperature acclimation inthe gall fly larva, Eurosta solidagensis. Cryobiology,18: 315–321.

Storey, K.B., Churchill, T.A., and Joanisse, D.R.1993. Freeze tolerance in hermit flower beetle(Osmoderma eremicola) larvae. Journal of InsectPhysiology, 39: 737–742.

Strong-Gunderson, J.M., Lee, R.E., Jr., and Lee,M.R. 1989. Ice-nucleating bacteria promotetranscuticular nucleation in insects. Cryobiology,26: 551.

Tanaka, S. 1983. Seasonal control of nymphaldiapause in the spring ground cricket, Ptero-nemobius nitidus (Orthoptera: Gryllidae). InDiapause and life cycle strategies in insects.Edited by V.K. Brown and I. Hodek. SeriesEntomologica. Vol. 23. Dr. W. Junk, The Hague.pp. 35–53.

Tanno, K. 1977. Ecological observation and frost-resistance in overwintering pre-pupae, Sciara sp.(Sciaridae). Low Temperature Science, Series BBiological Sciences, 35: 63–74.

Tillman, P.G., and Powell, J.E. 1991. Developmentaltime in relation to temperature for Microplitiscroceipes, M. demolitor, Cotesia kazak (Hyme-noptera: Braconidae), and Hyposoter didymator(Hymenoptera: Ichneumonidae), endoparasitoids ofthe tobacco budworm (Lepidoptera: Noctuidae).Environmental Entomology, 20: 61–64.

Townsend, G.D., and Pritchard, G. 1998. Larvalgrowth and development of the stoneflyPteronarcys californica (Insecta: Plecoptera) inthe Crowsnest River, Alberta. Canadian Journal ofZoology, 76: 2274–2280.

Turnock, W.J., and Boivin, G. 1997. Inter- and intra-population differences in the effects of tempera-ture on postdiapause development of Deliaradicum. Entomologia Experimentalis etApplicata, 84: 255–265.



Turnock, W.J., Reader, P.M., and Bracken, G.K.1990. A comparison of the cold hardiness of pop-ulations of Delia radicum (L.) (Diptera:Anthomyiidae) from southern England and theCanadian prairies. Canadian Journal of Zoology,68: 830–835.

Turnock, W.J., Boivin, G., and Ring, R.A. 1998.Interpopulation differences in the coldhardiness ofDelia radicum (Diptera: Anthomyiidae). The Ca-nadian Entomologist, 130: 119–129.

Vali, G. 1995. Principles of ice nucleation. In Bio-logical ice nucleation and its applications. Editedby R.E. Lee, Jr., G.J. Warren, and L.V. Gusta.American Phytopathological Society, St. Paul,Minnesota. pp. 1–28.

van der Woude, H.A., and Verhoef, H.A. 1988. Re-productive diapause and cold hardiness in temper-ate Collembola Orchesella cincta and Tomocerusminor. Journal of Insect Physiology, 34: 387–392.

Vernon, P., and Vannier, G. 2002. Evolution of freez-ing susceptibility and freezing tolerance in terres-trial arthropods. Comptes Rendus Biologies, 325:1185–1190.

Voituron, Y., Mouquet, N., de Mazancourt, C., andClobert, J. 2002. To freeze or not to freeze? Anevolutionary perspective on the cold-hardinessstrategies of overwintering ectotherms. AmericanNaturalist, 160: 255–270.

Wassmer, G.T., and Page, T.L. 1993. Photoperiodictime measurement and a graded photoperiodic re-sponse in a cockroach. Journal of BiologicalRhythms, 8: 47–56.

Watanabe, M., and Tanaka, K. 1998a. Adultdiapause and cold hardiness in Aulacophoranigripennis (Coleoptera: Chrysomelidae). Journalof Insect Physiology, 44: 1103–1110.

Watanabe, M., and Tanaka, K. 1998b. Effect of juve-nile hormone analogs on diapause termination andmyo-inositol content in Aulacophora nigripennisadults (Coleoptera: Chrysomelidae). Applied En-tomology and Zoology, 33: 259–262.

Watanabe, M., and Tanaka, K. 1999. Cold tolerancestrategy of the freeze-intolerant chrysomelid,Aulacophora nigripennis (Coleoptera: Chryso-melidae), in warm-temperate regions. EuropeanJournal of Entomology, 96: 175–181.

Wathen, B., Kuiper, M., Walker, V., and Jia, Z. 2003.A new model for simulating 3D crystal growthand its application to the study of antifreeze pro-teins. Journal of the American Chemical Society,125: 729–737.

Williams, K.S., and Simon, C. 1995. The ecology,behavior, and evolution of periodical cicadas. An-nual Review of Entomology, 40: 269–295.

Wilson, P.W., and Leader, J.P. 1995. Stabilization ofsupercooled fluids by thermal hysteresis proteins.Biophysical Journal, 68: 2098–2107.

Wilson, P.W., Heneghan, A.F., and Haymet, A.D.J.2003. Ice nucleation in nature: supercooling point(SCP) measurements and the role of heteroge-neous nucleation. Cryobiology, 46: 88–98.

Wipking, W., and Mengelkoch, C. 1994. Control ofalternate-year flight activities in high-alpine Ring-let butterflies (Erebia, Satyridae) and Burnetmoths (Zygaena, Zygaenidae) from temperate en-vironments. In Insect life-cycle polymorphism:theory, evolution and ecological consequences forseasonality and diapause control. Edited by H.V.Danks. Series Entomologica 52. Kluwer Aca-demic Publishers, Dordrecht. pp. 313–347.

Wolfe, J., Bryant, G., and Koster, K.L. 2002. What is‘unfreezable water’, how unfreezable is it andhow much is there? CryoLetters, 23: 157–166.

Worland, M.R., and Block, W. 2003. Desiccationstress at sub-zero temperatures in polar terrestrialarthropods. Journal of Insect Physiology, 49: 193–203.

Worland, M.R., Grubor-Lajsic, G., and Montiel, P.O.1998. Partial desiccation induced by sub-zerotemperatures as a component of the survival strat-egy of the Arctic collembolan Onychiurusarcticus (Tullberg). Journal of Insect Physiology,44: 211–219.

Worland, M.R., Block, W., and Grubor-Lajsic, G.2000. Survival of Heleomyza borealis (Diptera,Heleomyzidae) larvae down to –60 °C. Physiolog-ical Entomology, 25: 1–5.

Yoder, J.A., Denlinger, D.L., Dennis, M.W., andKolattukudy, P.E. 1992. Enhancement ofdiapausing flesh fly puparia with additional hy-drocarbons and evidence for alkane biosynthesisby a decarbonylation mechanism. Insect Biochem-istry and Molecular Biology, 22: 237–243.

Zachariassen, K.E., and Hammel, H.T. 1976. Nucle-ating agents in the haemolymph of insects tolerantto freezing. Nature (London), 262: 285–287.

Zachariassen, K.E., Kristiansen, E., Pedersen, S.A.,and Hammel, H.T. 2004. Ice nucleation in solu-tions and freeze-avoiding insects — homogeneousor heterogeneous? Cryobiology, 48: 309–321.

Zwick, P. 1996. Variable egg development ofDinocras spp. (Plecoptera, Perlidae) and thestonefly seed bank theory. Freshwater Biology,35: 81–100.


Danks 23

Documents

Insect adaptations to cold and changing environments, Sømme (1999), and Bale (2002). These studies have led to the discovery of more and more elements that contribute to insect cold