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The Life-Cycle, Abundance and Seasonal Movement of the Wolf Spider, Lycosa (Pardosa)lugubris, in Central ScotlandAuthor(s): Walter D. EdgarSource: Journal of Animal Ecology, Vol. 40, No. 2 (Jun., 1971), pp. 303-322Published by: British Ecological SocietyStable URL: http://www.jstor.org/stable/3248 .

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303

THE LIFE-CYCLE, ABUNDANCE AND SEASONAL MOVEMENT OF THE WOLF SPIDER, LYCOSA (PARDOSA)

LUGUBRIS, IN CENTRAL SCOTLAND

BY WALTER D. EDGAR

Department of Zoology, University of Glasgow

INTRODUCTION

Wolf spiders belong to the family Lycosidae, a group of ground-dwelling spiders which are among the largest and most conspicuous members of the terrestrial invertebrate fauna of temperate regions. Adult females of the European representatives of the genus Lycosa are about 6 mm in length. Lycosids have relatively good eyesight, by spider standards (Homann 1931) they are good sprinters and they do not make use of silk to catch their prey. They are said to chase and catch their prey (Savory 1928; Nielsen 1932; Bristowe 1958) although this has been questioned (Edgar 1969). The egg mass is wrapped in a layer of silk and this cocoon or egg sac is carried by the female, attached to her spinnerets. Their large size, conspicuousness and high density in favourable habitats, makes them very suitable for population studies.

The wolf spider, Lycosa lugubris (Walckenaer), occurs over the whole of Western Europe, from Finland in the north to Spain and Italy in the south. It is found in Asiatic as well as European Russia (Bristowe 1939), and has also been recorded in Japan (Yagi- numa 1962). Most authors describe its habitat as being in or near woods (Dahl 1908; Norgaard 1945; Locket & Millidge 1951; Wiebes 1959; Hallander 1967a). Locket & Millidge say 'it never seems to occur far from woods' while Wiebes says it is 'very com- mon in clearings in woods'. All the specimens of L. lugubris which I have taken in Scot- land have been collected in or near deciduous woodland.

This paper, which is part of a 4-year study on the ecology of L. lugubris, attempts to elucidate the close association of this spider with woodlands. This has been done by studying the life-cycle, and the habitats occupied by the spider at different stages in its life-cycle.

Most of the field work was carried out in an oak woodland (Quercus petraea (Matt.)) near Glasgow University Field Station on the Ross Peninsula on the east bank of Loch Lomond. The average height of the trees was between 40 and 50 ft and they were 70-100 years old. Clearings had been made in a number of areas to allow for the planting of conifers. Vaccinium myrtillus L. is the dominant plant over large areas of the Peninsula while Pteridium aquilinum (L.) is dominant in places.

In this paper frequent reference is made to whether the sampling areas are shaded or whether they are situated in clearings. Reference is made to eight sampling areas. Areas I and 2 were partly shaded, areas 3-6 were shaded and areas 7 and 8 were in clearings. The following is an account of the vegetation at a site which included three of the largest sampling areas. Two of these (areas 7 and 8) were located in a clearing and one (area 6) in a shaded area. The vegetation at this site, which consisted of a stand of oak trees and adjacent clearing, was fairly typical of that found in all the sampling areas.

The nature of the vegetation, in the clearing in which areas 7 and 8 were situated, is



304 Life-cycle and seasonal movement of Lycosa lugubris

indicated in Plate 6(a); and that in the shaded area, in which area 6 was located, in Plate 6(b). The clearing was an area of acid grassland dominated by Deschampsiaflexuosa (L.). It was species-poor and had little vertical layering. There was a fairly dense mat of Galium saxatile L. and several clumps of bracken (Pteridium aquilinium). Also present were Thelypteris limbosperma (All.), Oxalis acetosella L. and Luzula pilosa L. The most abundant mosses were Hylocomium splendens B and S., Pseudoscleropodium purum (Hedw.) and Pleurozium schreberi (Brid.). In the shaded area the bracken was much more dense and there was a considerable amount of leaf litter. Other ferns present included Blechnum spicant (L.) and Thelypteris limbosperma. The grass was much less dense than in the clearing and consisted of Deschampsiaflexuosa, Agrostis canina L. and a few clumps of Molinia caerulea (L.). Bluebells (Campanula rotundifolia L.) were fairly abundant and the more important mosses present were Polytrichum species and Hypnum cupressiforme Hedw.

Mid-winter

Dead bracken Area 6

stms /Leaef Are 7 Log9\L

stems\ ~~~~~~~~litter

Mid-summer

Short i3 - Area 6 Bracker dense

Area 7 grass Log

FIG. 1. Diagrammatic representation of the locality in which sampling areas 6 and 7 werQ situated, in winter and suummer.

As will be seen later, the seasonal changes in the vegetation were considered to be important with regard to the seasonal distribution of Lycosa lugubris. These changes were particularly striking with respect to the state of the oak trees and the bracken. A diagrammatic representation of areas 6 and 7, in mid-winter and in mid-summer, is given in Fig. 1. As the trees and most of the bracken were present in area 6, the seasonal vegetation changes were more striking in this area than in area 7. In mid-winter there were large accumulations of leaf litter and dead bracken stems in area 6. The dead bracken stems served to bind the leaf litter and thus prevented the dead leaves from being blown away. Towards the end of April the young bracken stems appeared. In mid-May the oak trees broke their leaves and these exerted their full shading effect by the end of May. By early



Journal of Animal Ecology Volume 40, Plate 6

e-~~~~~~~~~~J N

(a) Vegetation in clearing in summer. (b) Vegetation in shaded area in summer. (Photo- graphs by W. D. Edgar.)

(Facing p. 304)



WALTER D. EDGAR 305

August the bracken was about 1 m in height. It started to die off in autumn and in late autumn and early winter the oak trees shed their leaves.

THE LIFE-CYCLE OF LYCOSA LUGUBRIS

The life-cycle of Lycosa lugubris is illustrated in Figs. 2 and 3. The information on which this account is based was collected during the course of an investigation on density,

}Oinuary

July

FIG. 2. The life-cycle of Lycosa lugubris. Closed areas, second instar-sub-adult; stippled areas, sub-adult; hatched areas, adult male; open areas, adult female.

Sub-adult males

Sub-adult females Adult males

Adult females

Egg sacs carried Spiderlings emerge

Most females Most females carrying egg sacs carrying egg sacs

Significant events First matings Most males Females start observed have died to die

I I A

I i I i

I A

II M A M J J A S 0

FIG. 3. Phenology of the adult stage of the life-cycle of Lycosa lugubris.

natality, mortality and growth rates of L. lugubris. Density estimates were obtained using the Lincoln Index (Lincoln 1930) and by quadrat sampling (see later). Natality was esti- mated by counting the numbers of eggs in the egg sacs, and growth rates were obtained by taking representative samples of the spiders, at regular intervals, and weighing the ani-




mals in these samples. The results of the investigation on mortality, natality and growth rates of L. lugubris are reported elsewhere (Edgar 1971).

Females of L. lugubris normally produce two egg sacs during their life. The spiderlings from the first egg sac emerge about the middle of July; those from the second about mid- September. By the time they are ready to overwinter the majority of these spiderlings have reached the fourth instar stage, some are third or fifth instars while a few are in their second instar. The fifth and most of the fourth instars are products of the first egg sac. They overwinter in the leaf litter and during the following spring and summer continue their development so that by the beginning of September most are sub-adult, i.e. at the next moult they become adult. These sub-adults overwinter for a second time in the leaf litter.

On mild sunny days during the winter a few sub-adults can be found but they do not become generally active until early March. The first adult males appear in late March, and early in April adult females can be found. Under field conditions males are considered to be adult in instar eight and females in instar nine (Edgar unpublished). Most of the sub-adults become adult by the end of April at which time the first mating pairs can be found. By the middle of May the first females carrying egg sacs are present and by the end of May most females have egg sacs. These are carried for 5-6 weeks before the female tears open the egg sac to allow the spiderlings to emerge. In Fig. 2 the period of carrying the first egg sac is indicated as covering a period of 11 weeks. This is the total period of time when females carrying first egg sacs can be found and not the time that individual spiders carry their egg sacs. The spiderlings climb on to their mother's back and are carried in this fashion for up to a week before dispersing. Most of the males have died by late June; the females retain the spermatozoa from the spring matings to fertilize the second egg sac (Vlijm, Kessler & Richter 1963). By mid-August most of the females are carrying a second egg sac and the spiderlings from this egg sac emerge about the middle of Septem- ber. By the end of September the females beging to die and by the end of October few adult females can be found. Thus the females live, on average, just over 2 years, while the males live for just under 2 years.

SEASONAL MOVEMENT OF LYCOSA LUGUBRIS AND ITS EFFECT ON DENSITY ESTIMATES OF ADULT AND SUB-ADULT ANIMALS

The distribution of L. lugubris in the oak woodland of the Ross Peninsula was markedly discontinuous. In March 1965, sampling areas were selected in localities where L. lugubris was abundant. Due to the discontinuous distribution mentioned above, most of the sampling areas were relatively small, e.g. 5 5 x 5 5 m. In early March the sub-adult animals were still in overwintering areas. The spiders overwintered in leaf litter hence the sampling areas had large accumulations of leaf litter, being situated beneath oak trees.

A simple Lincoln Index method (Lincoln 1930) was used to obtained density values for sub-adult and adult animals. Density estimates of the smaller instars were obtained by quadrat sampling and these will be discussed separately.

Spiders were collected by hand by proceeding round the perimeter of the area and gradually working inwards. This was called the initial capture and the type of sampling was termed 'systematic sampling'. The spiders were brought back to the laboratory, anaesthetized with carbon dioxide, placed on their backs under a binocular microscope and marked on the sternum and underside of the coxae with Humbrol 1-h enamel paint. A marking code was used such that females carrying egg sacs, females with spiderlings



WALTER D. EDGAR 307

and females without egg sacs or spiderlings, were distinguished. The spiders were retained overnight to allow the paint to dry and were then released in the sampling area from which they had been taken. After a suitable release period, experience showed 1 day to be the most appropriate (Fig. 4), a second collection was made in the same area, and the number of marked and unmarked spiders recorded. The total population in the area was calculated using the formula:

an r

where P = total population, n = total number of individuals in second sample, a total number marked, r = total recaptures.

40

35 - * n=28\ ~10

-, 30 -

0.- n=7 O25 n=3

* 20

a) 15 n=3 1 5

E * o n= \

0= \

5

l , ,I, ,1 ,_ 1 , I , ,1 1 ,I 1 2 3 4 5 6 7 8 9 10

Duration of release period (days)

FIG. 4. Relation between percentage of marked specimens of Lycosa lugubris recaptured, in mark and recapture experiments, and the duration of the release period. (The regression line was calculated using all the fifty-six observations, but only the mean values are plotted:

n = number of observations from which mean value was calculated.)

The release period was in all cases 24 h or more and it was considered that so long as the marked animals were left overnight in the field they would be mixed at random with the rest of the population. The high percentage of marked animals recaptured, when the release period was short (1 or 2 days) (Fig. 4), suggests that the movement of marked animals out of the area during the release period was not too important a source of error. Marked spiders, kept in the laboratory for several months, retained their marks well and showed no ill-effects from having been marked. Marked animals were recovered in the field, several months after marking, with their marks still clearly visible and several spiders marked in the autumn were recovered the following spring. Thus the marks would also seem to be retained in the field. Anaesthetizing has been shown to have a detrimental effect on certain arthropods (Southwood 1966) and Hallander (1 967b) found a high mortality among specimens of L. lugubris which had been narcotized. Edgar (unpublished) reared specimens of L. lugubris from fourth instar to adult, anaesthetizing the spiders with carbon dioxide after each moult. All the spiders developed normally which




suggests that carbon dioxide has no inimical effects. Marking the spiders ventrally avoided the possibility of bias in subsequent captures and also possible selective predation on marked animals.

120 -

, 80 _

4-

0 0

Z 40 -

A M J

FIG. 5. Fall in numbers of sub-adult and adult specimens of Lycosa lugubris in the overwintering areas in spring. 0, Area 4; 0, area 5.

200 -

175 -

150 -

0~~~~~~~ E 125 -

.'0I0Q 0 0 o / C-

P75

50 i

0

25 _

I r 1 t I t 1 1 10 20 30 40 50 60 70 80 90

No.spiders in initial capture

FIG. 6. Relationship between the total number of specimens of Lycosa lugubris in an area, as determined by the Lincoln Index, and the number of spiders in the initial capture of a

mark and recapture experiment. 0, Sub-adult animals; 0, adult females.

(a) Seasonal movement of L. lugubris The attempt to measure the density of L. lugubris was affected by adult animals

moving away from the areas in which they overwintered. This movement was detected due to its effect on the numbers of spiders which remained in the overwintering areas after moulting to adult had occurred.

In this section frequent reference is made as to whether areas are shaded or not. Oak trees break their leaves relatively late in the year, about mid-May, and the term



WALTER D. EDGAR 309

shaded is used not only for areas which are shaded at the time of sampling but also for areas which are not shaded in the spring but will become shaded when the oak trees break their leaves.

In the spring of 1965, the sampling areas were sampled approximately every 3 weeks. Towards the end of April the density of adult and sub-adult animals started to drop and by mid-May few spiders were present in the overwintering areas (Fig. 5). In parenthesis it should be noted here that, since no marking of spiders was undertaken when densities were low, the 'numbers of spiders' plotted in Figs. 5, 7 and 8 are the numbers taken in the initial (systematic) capture of a mark and recapture experiment and not the density estimates. This is justified since there is a direct relationship, both for adult females and sub-adults, between the population estimates of the Lincoln Index and numbers in the initial captures (Fig. 6). By the end of May a few areas had relatively large numbers of

60 Area 2

00

20 -

q 60 _ 0 Ara0 6 Area E

z

40 _

20

A M I J

FIG. 7. Differences in numbers of sub-adult and adult specimens of Lycosa lugubris in shaded areas and clearings in spring and early summer. 0, Shaded area; o, clearing.

spiders in them again (Fig. 7). Most of these spiders were adult females with egg sacs. A number of sampling areas had been sub-divided and on examining these areas which had been recolonized it was clear that they differed from those that had not, in that they were wholly or partly shaded by oak trees, which break their leaves in mid-May.

The fact that few spiders were present in most of the sampling areas in late May resulted in several new sampling areas being selected. These were in clearings and almost all the spiders present were adult females with egg sacs. The largest of these areas was designated area 7. Area 7 was first sampled on 6 July 1965. When it was next sampled on 1 August 1965, the numbers of spiders had dropped considerably. By late August the numbers had increased. A similar drop in numbers at the end of July followed by an increase in early August occurred in 1966 at area 7 and also at area 8, an adjoining area




in the clearing (Fig. 8), so that it would appear to be a permanent feature of the population. Thus the numbers of sub-adult and adult spiders in all the overwintering areas fell to

low levels in mid-May. A few areas were recolonized in late May, mainly by females with egg sacs, and these differed from the others in that they were unshaded. The number of

80 _

*\A 60 _

c4 400

0

20

I J I A S 0

Fio. 8. Fluctuations in the numbers of adult females of Lycosa lugubris in a clearing in the summer and autumn of 1965 and 1966. *, Area 7 1965; 0, area 7 1966; A, area 8 1966.

120 - Males

80_

40 -

0~

c 25- Females

z -

15__

L L I I

A M J J A

FIG. 9. Numbers of sub-adult and adult specimens of Lycosa lugubris taken in pit-fall traps in spring and summer in a shaded area in 1965. Black parts of columns, sub-adult animals.

adult females in the clearings dropped suddenly in late July and then rose again in August (Fig. 8).

These fluctuations in the numbers of spiders require some explanation. The fall in the numbers of spiders in the overwintering areas followed by the recoloniza-

tion of the few unshaded areas by adult females with egg sacs, raises two questions.



WALTER D. EDGAR 311

Firstly, why is there a drop in the numbers of spiders in the overwintering areas in late April and early May, at a time when these areas have not become shaded? Secondly, why are unshaded areas colonized by adult females with egg sacs?

The initial drop in the number of spiders in the overwintering areas in spring coincides with the period when moulting to adult is occurring (Fig. 3). A number of pit-fall traps were present in the sampling areas and the results from these traps indicated that there was an increase in activity in late April (Fig. 9). The peak in acitivity at the end of April coincides with the time when mating pairs are observed. Mark and recapture experiments indicated that proportionally more sub-adult than adult animals were recaptured (Table 1, x2 = 21 01, df = 1, P< 0 001), suggesting that the adult animals are more active than the sub-adults. The drop in the numbers of spiders could thus be explained by the more active adult animals expanding their range out of the somewhat restricted overwintering areas.

Table 1. Proportions of marked sub-adult and adult specimens of Lycosa lugubris recaptured in mark and recapture experiments in spring 1965

Sub-adults Adults No. marked 446 241 No. recaptured 126 31 % recaptured 28-3 12-9

Table 2.. Density of Auchenorhyncha in a shaded area and clearinig in 1966 and 1967 (sampled with a quadrat of area 009 m2)

Clearing Shaded area _ _ - & \ -

No. No. No. No. Date Auchenorhyncha/m2 quadrats Auchenorhyncha/m2 quadrats

26 April 1966 19-2 50 3 May 1966 24-1 72

15 May 1966 29-4 38 8.8 180 13 April 1967 23-4 30 4*0 30 27 April 1967 31-0 30 6-1 30 8 June 1967 3-4 30 1-2 30

In the 2 or 3 weeks prior to egg sac production the females feed at a much increased rate. Quadrat samples have indicated that the density of homopterans (Auchenorhyncha), which form an important part of the diet of L. lugubris (Edgar 1969), is much higher in clearings than in the overwintering areas (Table 2). Hence the expansion in the range of the adult animals may result in many of the adult females arriving in areas with a more abundant food supply.

With regard to the question of why unshaded areas are colonized by adult females with egg sacs: wolf spiders sun their egg sacs (N0rgaard 1945, 1951; Vlijm et al. 1963), and N0rgaard (1951) showed that females of Pirata piraticus prefer higher temperatures when they are carrying egg sacs. Thus it seems likely that adult females of Lycosa lugubris do not recolonize the shaded areas because of the need to sun their egg sacs.

The increase in the numbers of spiders in the clearings in late May and early June (Fig. 7) coincides with the decrease in numbers, to a low level, in the shaded areas (Figs. 5 and 7). As adult males are dying at this time, adult females predominate in both shaded

J.A.E. B




and unshaded areas and it is suggested that the females still in the shaded areas move into the clearings on producing their egg sacs. This hypothesis is supported by the results of hand collections of sub-adult and adult females in shaded areas and clearings in 1967 (Table 3).

This evidence is very suggestive, but to prove that the females with egg sacs which were present in the clearings in June, were the same individuals which were present without egg sacs in the adjacent shaded areas in May, two marking experiments were carried out.

Table 3. Hand samples of sub-adult and adult females of Lycosa lugubris collected in shaded areas and clearings in 1967

No. spiders % of % with % in collected adults egg sacs shaded area

Early April 74 0 0 93-2 Late April 39 974 0 66-7 Early May 130 992 0 577 Mid-May 69 100 0 0 56 5 Late May 112 1000 18.8 429 Early June 105 100 0 65-7 28-6 Mid-June 98 100 0 92.9 6-1

Table 4. Timed collections of adult females of Lycosa lugubris in a shaded area and clearing in 1969

Duration Shaded area Clearing of , I 1 _

collection No. No. with No. No. with Date (min) collected egg sacs collected egg sacs

12 May 45 158 0 40 0 3 June 30 67 55 60 54

11 June 30 4 0 32 30 12 June 30 7 3 60 60 24 June 15 0 0 34 34

The first of these was carried out in an area which consisted of a large clearing with an adjacent shaded area. A series of timed collections of adult females was carried out between 12 May and 24 June 1969 in both the shaded area and clearing (Table 4). These confirmed the results cited above (Table 3) in that there were more adult females in the shaded area in early May but the situation was reversed as the females produced egg sacs.

In conjunction with these timed samples systematic collections were made in the shaded area and clearing. Between 12 and 16 May 1969, 819 adult females, none of which had produced an egg sac, were collected in the shaded area and during the same time forty- eight females were collected from the clearing. These animals were marked on the cephalo- thorax and released in the areas in which they had been captured. Between 11 June and 14 July 1969, a period when most females were carrying egg sacs, a number of collections were made in the two areas. Any marked spiders found in the clearing were removed and killed. The results of this experiment are given in Table 5. Of the 819 females which were marked in the overwintering area in May, at a time when none was carrying an egg sac, 232 (29-0 %). were recaptured in the adjacent clearing during a period when most of the females had egg sacs. Bearing in mind that the last marked spiders were released on 16 May 1969, and recaptures were not started until 11 June 1969, this is a strikingly high



WALTER D. EDGAR 313

recovery rate; the percentage recapture of marked spiders which had originally been present in the clearing was only slightly greater (33-3 %). This experiment, in conjunction with the timed samples, lends strong support to the hypothesis that females in the shaded areas move into the clearings on producing their egg sacs, while those females already in the clearings, as a result of the range expansion of the adult animals, remain there.

Table 5. Results of a marking experiment on adult females of Lycosa lugubris in which females were captured in a shaded area in May and recaptured

in an adjacent clearing in June and July

No. recaptured in clearing

r In shaded area 819 232 (29%) No. caught, marked and released -

L In clearing 49 16 (33%)

Table 6. Timed collections of adult females of Lycosa lugubris in a shaded area and adjacent clearing in 1969

Duration Shaded area Clearing of ,&---, -

I collection No. No. with No. No. with

Date (min) collected egg sac collected egg sac 9 June 30 6 3 42 40

25 June 15 0 0 41 41 1 July 15 0 0 42 42

As a final check on this hypothesis a second marking experiment was carried out. The sampling area likewise consisted of a large clearing with an adjacent shaded area. A timed sample on 9 June 1969, indicated that many more adult females were present in the clearing than in the shaded area (Table 6). Almost all these females had egg sacs. On 10 and 11 June, 329 females were collected in the clearing. They were marked and released in the adjacent shaded area. Two subsequent timed samples (Table 6) indicated that few, if any, of these spiders were still present in the shaded area 2-3 weeks later, and between 24 June and 1 July 1969, ninety-four marked females (28 6 %) were recaptured in the clearing. This again indicates that females with egg sacs had a marked propensity to move from shaded areas to clearings.

Summarizing: the density of spiders dropped in the overwintering areas in April and early May (Fig. 5). This drop coincided with moulting to adult (Fig. 3) and was thought to have been due to a general increase in the range of the more active adult spiders (Table 1, Fig. 9). Towards the end of May, when adult females predominated there was an increase in the numbers of spiders in the clearings (Fig. 7, Table 3). This was attributed to those adult females, which were not already in the clearings, moving into the clearings to sun their egg sacs (Tables 3 and 5).

The period from late July to early August, when the number of adult females in the clearings falls (Fig. 8), was the period between the first and second egg sac batches (Fig. 3). As mentioned above, N0rgaard (1951) has shown that females of Pirata piraticus have a lower temperature preference when they are not carrying an egg sac. If, as seems likely, the same thing holds for Lycosa lugubris, then the drop in numbers in late July may well




have been due to spiders without egg sacs moving back to the shaded areas. Support for this comes from the results obtained from pit-fall traps placed in a clearing and shaded area (Fig. 10). No adult females were taken in the shaded area from the end of May until mid-July and none of those taken in mid-July had an egg sac. Likewise, in the clearing there was an increase in the numbers of spiders falling into the traps in mid-July and few of them were carrying egg sacs. Further, five marked females were taken in the shaded area in late July. None of these animals was carrying an egg sac but earlier in July they had been carrying egg sacs in the adjacent clearing. This postulated movement of females out of the clearings between egg sac batches would have the further advantage of removing them from the area where they have just released their spiderlings, during a time when they are feeding actively.

25 - Clearing

o 20 r Shaded area

5 I

M U J

3 A I S

FIGi 10. Numbers of adult females o3fLycosa lugubris taken in pit-fall traps in a clearing and shaded area in spring and summenr in 1966. Black parts of columns, females with egg sacs.

Table 7. Numbers of large D)iptera caught on sticky trap?s in a shaded area and clearing (traps exposed from 1-8 August 1966)

Clearing Shaded area Trap 1 57 87 Trap 2 37 69 Trap 3 51 54 Mean and standard deviation 48 ?1043 70? 16-5

Sticky traps, placed in the shaded area and in the clearing in early August, indicated that more large Diptera, which are known to constitute an important part of the diet of L. luubris, at this time of the year (Edgar 1969), were caught on the traps in the shaded area (Table 7). This difference was not statistically significant (t-1*97, df =4,

P> 0*l) but it does indicate, in contrast to the situation with Auchenorhyncha in the spring, that there was at least as much food present in the shaded area as in the clearing at this time of year.

(b) Density of sub-adfult and adalt specimens The efect of the seasonal movement of L. lugubris described above was that the only



WALTER D. EDGAR 315

Table 8. Mark and recapture experiments on sub-adult specimens of Lycosa lugubris in late March 1965

Area 2 3 4 5 No. marked 50 25 29 84

No. marked 23 8 8 33 Second sample

No. unmarked 41 9 21 26 No. spiders in area (?SE) 139?23-2 53?13 7 105+31-6 105?17-7 No. spiders/M2 5.5 2-2 1-7 40

Table 9. Mark and recapture experiments on sixth and seventh instar and sub-adult specimens of Lycosa lugubris in area 6 from early August to early

October 1965

Date of initial capture 5 Aug. 25 Aug. 14 Sept. 5 Oct. No. marked 93 162 143 135

{ No. marked 21 45 45 42 Second sample

No. unmarked 117 78 63 27 No. spiders in area (? SE) 611 + 122-2 443 ? 52 3 343 ? 39-1 222 ? 20-9 No. spiders/rn2 10-3 7.4 5-8 3-7

Table 10. Mark and recapture experiments on adult females of Lycosa lugubris in area 7 in 1965

Date of initial capture 6 July 1 Aug. 22 Aug. 12 Sept. 29 Sept. No. marked 74 38 49 57 19

No. marked 32 14 20 30 7 Second sample

L No. unmarked 43 29 34 21 8 No. spiders in area (?SE) 173+23-2 103+19.8 132+23 5 97?11-4 41?11 3 No. spiders/m2 5 8 3.5 4.4 3-2 1 3

Table 11. Mark and recapture experiments on adult females of Lycosa lugubris in area 7 in 1966

Date of initial capture 11 July 27 July 15 Aug. 11 Sept. No. marked 22 8 20 27

F No. marked 6 (captured, 5 8 Second sample . not marked)

L No. unmarked 12 20 20 No. spiders in area (? SE) 66?22.0 100?40 0 91 ?27-3 No. spiders/M2 2-2 3-4 3 0

Table 12. Mark and recapture experiments on adultfemales of Lycosa lugu bris in area 8 in 1966

Date of initial capture 11 July 27 July 15 Aug. 11 Sept. No. marked 68 23 32 25

{No. marked 27 9 11 11 Second sample

No. unmarked 37 10 25 14 No. spiders in area (?SE) 161?23 8 49+11 9 105?25-8 57?12 7 No. spiders/m2 54 1 7 3*5 1.9




reliable density values obtained were for sub-adult animals in spring and autumn, and for adult females in summer. The densities of sub-adult animals in four areas in late March are given in Table 8. The mean value for the four areas was 3 4 spiders/m2. Densities of sixth and seventh instar and sub-adult animals for one area, from early August to early October, are given in Table 9. The density dropped from 10-3 spiders/m2 in early August to 3-7 spiders/m2 in early October.

Table 13. Mark and recapture experiments on sub-adult specimens of Lycosa lugubris in 1965, indicating that proportionally more females than males are

recaptured

Area 2 3 4 5 6 6 Date of initial capture 31 March 31 March 30 March 8 April 14 Sept. 5 Oct. No. spiders/M2 5 5 2-2 40 1-7 49 4 2

Sex ratio males 1.1 2-5 0.8 03 1 8 21 females

% males recaptured 39 3 23 5 13 3 31-8 25-2 27.7 % females recaptured 54 5 50 0 42-9 41-9 58-3 37-3

Table 14. Percentage of adult males and females of Lycosa lugubris recaptured in mark and recapture experiments

Adult males Adult females No. marked 162 79 No. recaptured 16 15 % recaptured 9*9 19.0

Table 15. Percentage offemales of Lycosa lugubris, of various categories, recaptured in mark and recapture experiments

Females Females Females without with with egg sacs or

spiderlings egg sacs spiderlings No. marked 30 369 52 No. recaptured 12 157 14 % recaptured 400 42 6 26 9

During most of the summer adult females occurred in clearings on the edge of the wood. Their density, in two sampling areas, 7 and 8, situated in one such clearing, in 1965 and 1966, are given in Tables 10, 11 and 12. The fluctuations in the numbers of adult females in the clearing in the two years are indicated in Fig. 8. The changes in spider abundance throughout the summer, in the two years, were similar, although the density was lower in area 7 in 1966.

In a series of marking experiments on sub-adult animals in 1965, a consistently higher proportion of females was recaptured, although the areas sampled varied considerably with regard to spider density and the sex-ratio in the samples (Table 13). The most likely explanation for this difference is that males were more active than females, and that during the release period more males moved out of the sampling areas. Marking experiments involving adult males and females, soon after moulting to adult occurred, similarly indicated that males were more active than females (Table 14, x2 = 3.93, df = 1, P<



WALTER D. EDGAR 317

0 05); while further experiments showed that females with egg sacs were less active than those without egg sacs (Table 15, x2 = 4-61, df = 1, P<0 05).

DENSITY OF SMALL INSTARS AND DISPERSAL OF SPIDERLINGS

Obtaining density estimates of the small instars posed a number of problems which were not fully overcome. Marking these small spiders was difficult and time-consuming and a pilot mark and recapture experiment indicated that the recovery of marked animals was low. It was decided, therefore, to obtain density estimates using a metal quadrat with an area of 0 09 m2.

Quadrat sampling was not possible during most of the summer in the shaded area due to the height of the bracken. An attempt was made to place the quadrat on the ground without looking to see where it was being placed. It was pressed firmly into the soil and the vegetation was then disturbed by hand and all the spiders within the quadrat counted and scored as to instar. The results are summarized in Tables 16 and 17.

Table 16. Density of small instars of Lycosa lugubris in shaded areas (sampled with a quadrat of area 0 09 m2)

Date No. quadrats No. spiders/m2 Late May 1965 495 6-2 Early Oct. 1965 20 5 4 Early May 1966 296 6-0 Mid-Sept. 1966 20 8-6

Table 17. Density of small instars of Lycosa lugubris in clearings (sampled with a quadrat of area 0 09 m2)

No. quadrats No. spiders/m2 26 April 1966 60 1-7 17 August 1966 24 22-4

In the shaded area samples were possible only in spring and autumn, due to the presence of bracken, and the spider densities at these times, over the two years of the study, were relatively constant. In spring the density in the clearing was significantly lower than in the shaded area. This supports the account given in the section on seasonal movement; according to which the small instars should be in the shaded area among the leaf litter. The high density of small instars in the clearing in mid-August is also to be expected. These spiders were mainly second and third instars, being the products of the first egg sac.

Before mid-April the small instars were not conspicuous in the shaded areas. They became active in the spring later than the sub-adults and only appeared in large numbers when many of the adult animals had moved out of these areas. This presumably reduced predation between the two groups. Despite this the density estimates were not much higher than those obtained for sub-adults in spring and autumn. The reason for this is probably that the small instars were more widely distributed than the sub-adult animals.

During the autumn there was a gradual movement of spiderlings into the overwintering areas. It is considered that this movement was a random rather than a directed one, the spiderlings moving until they came into conditions of relatively constant temperature




and high humidity such as is found among leaf litter. A high degree of cannibalism might be expected between sub-adult animals and the small spiderlings in the overwintering areas, but Edgar (1969) has shown that the three-dimensional medium of leaf litter tended to reduce predation between the two age groups: the sub-adults running on the surface of the litter and the small instars in 'tunnels' within the litter.

Bristowe (1939) suggested that the spiderlings of certain lycosid spiders disperse by ballooning. To determine whether this was so for Lycosa lugubris sixteen sticky traps were put out in a clearing which had numerous females with egg sacs. The traps were exposed from 5 July to 3 August 1966, and this covered the period when spiderlings were released from the first egg sac batch. The traps consisted of inverted jam jars on bamboo canes and were approximately 1 m above the ground. The jars were smeared with banding grease, as was a zone of the supporting cane, to preclude the possibility of climbing arthropods reaching the traps. A wide variety of insects were caught on the traps but no lycosid spiderlings were taken. Thus it seems that ballooning is not important as a means of dispersal in L. lugubris. Richter (1967) likewise found no ballooning in L. lugubris spiderlings although it occurs in other members of the genus.

DISCUSSION

The main sampling method used in this study, hand sampling, was preferred to pit-fall trapping for a variety of reasons. Pit-fall traps would have to have been the 'non-killing' type (Cherrett 1964; Vlijm & Kessler-Geschiere 1967). To avoid cannibalism such traps have to be emptied frequently, and this was not considered advisable, because of the effect of trampling on the vegetation. Relatively few juvenile spiders are taken in pit-fall traps (Hackman 1957; Breymeyer 1966; Vlijm & Kessler-Geschiere 1967). The results of hand sampling, like those of pit-fall traps, are affected by spider activity which is in turn affected by weather. With experience wolf spider activity can be forecast from the nature of the weather (Vlijm & Kessler-Geschiere 1967) and most sampling of Lycosa lugubris was done during 'active weather' (Vlijm & Kessler-Geschiere 1967).

With regard to the mark and recapture experiments the absolute number of spiders taken is not too important. What is important, for an accurate estimate of density, is that a reasonably large number of marked spiders is recaptured. The percentage of marked animals recaptured decreased as the release period increased (Fig. 4) but it was relatively high with a release period of I or 2 days.

Timed hand samples were used as a quick method for obtaining comparative estimates of spider abundance. Timed hand samples have been used by N0rgaard (1945). Provided the spiders can be collected with equal ease in two areas the method is valid for comparative purposes. Low numbers of spiders collected are even more significant than the numbers suggest, as where many spiders are present a considerable amount of the collecting period is spent transferring spiders to collecting tubes.

The life-cycle of spiders of the genus Lycosa in the Netherlands and in England would appear to differ from L. lugubris in Scotland in that the spiders overwinter only once (Gabbutt 1956; Wiebes 1960; Duffey 1962; Vlijm et al. 1963; Vlijm & Kessler-Geschiere 1967). The life-cycle of L. lugubris in Sweden is, however, the same as in Scotland (Hal- lander, personal communication). A possible explanation for this is that the life-cycle is shorter in more southerly latitudes; as Bonnet (1930) found with Dolomedes.

Spiders of the genus Lycosa on the continent appear later in spring and disappear earlier in autumn than in Scotland (Dahl 1908; Vlijm & Kessler-Geschiere 1967). The



WALTER D. EDGAR 319

rate of development over the 'growing season' is, however, higher; these spiders catching up with their Scottish counterparts by the time the first egg sac is produced. This is to be expected in view of the colder winters and warmer summers on the continent. Vlijm et al, (1963) noted that different populations of L. amentata could be at different stages of development, depending on the degree of exposure of the habitat to the sun. The same was true for L. lugubris; spiders on a south-west facing slope could be 2 weeks ahead in development of spiders on a north-east slope.

The greater activity of adult males compared with females and of females without egg sacs compared with those with egg sacs, as indicated in this study by mark and recapture experiments (Table 14 and 15) and pit-fall trapping (Figs. 9 and 10), has been found by other workers for both laboratory and field experiments (Vlijm & Richter 1966; Vlijm & Kessler-Geschiere 1967; Hallander 1967b). Vlijm & Richter (1966) found that females with spiderlings were more active than females with egg sacs and suggest that this is an adaptation to facilitate dispersal. Hallander (1967b) found no difference in activity between the two categories. The limited evidence available for L. lugubris (Table 15) would seem to support Hallander's findings.

The fact that different stages in the life-cycle of L. lugubris occupy different habitats has been illustrated in the section on seasonal movement. Vlijm & Kessler-Geschiere (1967) stressed the importance of obtaining precise information as to the habitat preference of different stages in the life-cycle of spiders of the genus Lycosa. Their data, on spiders taken from vegetation and ground traps, indicate that females with egg sacs and juveniles of L. pullata and L. nigriceps occur more frequently in the vegetation than males and females without egg sacs. This may be analogous to the seasonal movement found in L. lugubris; the movement in this case occurring in a vertical instead of a horizontal plane.

Vlijm et al. (1963) noted that L. amentata females with egg eacs could be found sitting in crevices in the soil. Their egg sacs were exposed to the sun while they themselves were in the shadow of the crevice. N0rgaard (1951) found that Pirata piraticus females 'migra- ted' vertically to the surface of the sphagnum carpet in which they live to sun their egg sacs. Lycosa lugubris has been found to exhibit similar behaviour. Females with egg sacs can readily be found sunning their egg sacs on logs, or similar elevated objects, which are present in the clearings. Periodically, they move to the underside of the log, presumably when they become too hot. It seems likely that such mechanisms will exist in all species of the genus Lycosa. For example, L. amentata and L. monticola females with egg sacs have been observed to make use of dry cow dung. They sun their egg sacs on the surface of the cow-pat and periodically move to the shaded underside of the cow-pat.

Duffey (1966) stressed the importance of examining habitat structure as well as micro- climatic conditions when studying the distribution of spiders. The very striking differences in the vegetation structure, in shaded areas and clearings, with respect to the presence or absence of trees, bracken and leaf litter, have been mentioned in the Introduction. It has been suggested that the structural nature of the leaf litter reduces cannibalism in L. lugubris and also, as a result of its structure, that leaf litter provides suitable micro- climatic conditions for L. lugubris during the winter. The structural nature of the oak trees and bracken, through their shading effect, makes the overwintering areas unsuitable for adult females with egg sacs. In the clearings the short dense grass (Deschampsia flexuosa) and the logs, provided structural conditions suitable for sunning of egg sacs.

From the preceding account of the life-cycle and seasonal movement of Lycosa lugubris it can be seen that this animal is nicely adapted to exploit the resources of the habitat in which it lives. The expansion in the range of spiders, after moulting to adult, allows




some of the females to exploit an abundant food resource in the clearings. The logs in the clearings provide suitable sites for the females to sun their egg sacs. The fact that the small instars do not become active in spring until about a month after the sub-adult animals presumably reduces cannibalism. By the time these small instars are conspicuous, the sub- adults have become adult, and these adults have moved out of the overwintering areas. Almost all the adult males die before the spiderlings emerge from the first egg sac. As these males have served their biological function it would, teleologically speaking, be wasteful for them to remain as potential predators of their own spiderlings. The adult females leaving the clearings during the period of active feeding between the egg sacs would likewise seem to be a mechanism likely to reduce cannibalism, as the females would eat their own spiderlings if they stayed in the clearings. The adult females, on moving into the shaded areas to feed, are not potential predators of the spiders of the second overwintering year group as these are almost at the sub-adult stage and conse- quently are too large to be readily eaten by the adults. The spiderlings, by being released in the clearings, are spatially separated from these spiders which have remained in the overwintering areas. The spiderlings, on moving into the overwintering areas, tend to be spatially separated from the sub-adult animals by the three-dimensional medium of the leaf litter.

L. lugubris normally occurs only on the perimeter of woodlands and in clearings within woods. This somewhat restricted distribution can, on the basis of the account given above, be explained by the fact that the animal, at certain times in its life-cycle, utilizes the conditions found within the wood while at other times it exploits certain characteristics of the clearings.

ACKNOWLEDGMENTS

I should like to thank Sir Maurice Yonge, F.R.S., formerly Regius Professor of Zoology, and, latterly, Professor D. R. Newth, for the facilities provided for this research in the Zoology Department of the University of Glasgow and Glasgow University Field Station.

I thank also Dr A. F. G. Dixon who supervised the work, and Mr C. J. J. Richter, Professor L. Vlijm and Miss R. M. White for their advice. I am particularly indebted to Mr P. S. Meadows for his helpful discussion and criticism of the manuscript. The work was carried out while in receipt of an N.E.R.C. Research Studentship.

SUMMARY

(1) The life-cycle of Lycosa lugubris in central Scotland differs from published accounts of the life-cycle of spiders of the genus Lycosa in England and The Netherlands in that the spiders have to overwinter twice before reaching maturity. Females of L. lugubris produce two egg sacs during their life; one in May and one in August. The spiderlings from these egg sacs overwinter as third, fourth or fifth instars. By the following autumn they reach the sub-adult stage (the instar before adult). They overwinter as sub-adults and moult to adult the following spring. Most of the adult males have died by late June while the females live until October.

(2) L. lugubris normally occurs on the perimeter of woodlands and in clearings within woods. This somewhat restricted distribution is explained on the basis that at certain times in its life-cycle the spider utilizes the conditions found within the wood while at other times it exploits the characteristics of the clearings. The sub-adult animals over-



WALTER D. EDGAR 321

winter among leaf litter along with the smaller instars which emerge from the egg sacs during the summer and autumn. Thus there are two year classes in the overwintering areas. In spring the sub-adults moult to adult and these adult animals extend their range out of the overwintering areas. This brings many of them into areas with a more abundant food supply. It further serves to reduce cannibalism by spatially separating them from the smaller individuals of the other year group with which they overwintered. These smaller individuals remain in the overwintering areas during the spring and summer and by the autumn have reached the sub-adult stage.

In late May, when the oak trees break their leaves, the adult females accumulate in the clearings to sun their egg sacs. In the period between the two egg sacs, when they are feeding actively, the females leave the clearings. This spatially separates them from the spiderlings which are released in the clearings. The spiderlings gradually move into the overwintering areas during autumn. Cannibalism between them and the sub-adult animals tends to be reduced by the fact that they run within the litter and the larger sub- adults run on the surface of the litter.

(3) Density estimates were obtained for adult females, sub-adults and small instars of L. lugubris. The highest values recorded were 5'8, 1013 and 22'4 spiders/m2, respectively.

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(Received 21 January 1970)

