1873The Journal of Experimental Biology 200, 18731879 (1997)Printed in Great Britain The Company of Biologists Limited 1997JEB0905CENTRAL PROJECTIONS OF THE MAXILLARY AND ANTENNAL NERVES IN THEMOSQUITO AEDES AEGYPTI

PAUL G. DISTLER* AND JRGEN BOECKHInstitut fr Zoologie, Universitt Regensburg, D-93040 Regensburg, Germany

Accepted 23 April 1997In the mosquito Aedes aegypti, CO2-sensitive receptorneurones are located together with two other types ofchemoreceptor neurones in club-shaped sensilla basiconicaon the most distal segment of the maxillary palps. In orderto identify the central target neuropiles of these neuronesand to determine whether antennal receptor neuronesproject into the same area, the palpal and antennal nerveswere labelled by anterograde staining with horseradishperoxidase and by experimentally induced degeneration.The different methods revealed a consistent projectionpattern. (1) Maxillary afferents project into thesuboesophageal ganglion and ascend further into the

ipsilateral antennal lobe. There, they terminate withinan identified glomerulus of the ventroposterior lobe.(2) Afferents of the antennal flagellum project into allglomeruli of the ipsilateral antennal lobe, with theexception of the glomerulus innervated by the maxillarynerve. The present anatomical findings suggest thatprimary processing of information about CO2 levels takesplace in a defined glomerulus which also receives inputfrom other palpal chemoreceptor neurones.

Key words: insect, olfactory system, carbon dioxide, palpalprojections, mosquito, Aedes aegypti.

SummaryMosquitoes, like a number of other blood-sucking insects, areequipped with sensory cells responding to host emanations suchas moisture, heat and, of particular importance, body odours.Among the latter, lactic acid and carbon dioxide have beenshown to alarm and/or attract female mosquitoes (e.g. Daykinet al. 1965; Mayer and James, 1969; Smith et al. 1969; Friendand Smith, 1977; Galun, 1977; Gillies, 1980; for reviews, seeBowen, 1991; Takken, 1991; for more recent results, see Geier,1995; Steib, 1995). Behavioural and electrophysiologicalstudies have shown that the yellow fever mosquito Aedesaegypti (Diptera: Culicidae) has receptor organs that detect verylow concentrations and also extremely small changes in carbondioxide (CO2) concentration in air (Kellogg, 1970; Grant et al.1995; Geier, 1995; Steib, 1995). The CO2-detecting primaryreceptor neurones are located on the most distal, fourth segmentof the maxillary palps (Fig. 1). Fine-structure andelectrophysiological studies have shown that these neurones liein triads together with other types of chemosensory receptorcells within club-shaped sensilla basiconica (also referred to aspalpal pegs). Approximately 30 such sensilla are found on asingle maxillary palp of a female A. aegypti (McIver andCharlton, 1970; Chaika, 1977; McIver, 1982). More recently,tracing of the axonal projections of these neurones withfluorescent dye has revealed their projections in a very distinctglomerulus-like terminal region within the ipsilateral antennal

Introduction*e-mail: Paul.Distler@biologie.uni-regensburg.de.lobe (Anton, 1996). Basically similar projection patterns existin other insects. Both physiological and anatomical evidence fora single and specifically reserved glomerulus in the antennallobe for CO2 input from the labial palps has been reported inan arctiid moth, Rhodogastria luteibarba (Bogner et al. 1986),and in two other lepidopteran species, Manduca sexta (Kent etal. 1986) and Pieris rapae (Lee and Altner, 1986).

Our interest in the neuronal processing of host odorants inmosquitoes has led us to the question of the location of CO2-elicited activity in the brain of A. aegypti. As a first step inanswering this question, we have studied the afferentprojections of the maxillary and antennal nerves using twodifferent labelling techniques: (1) anterograde horseradishperoxidase (HRP) labelling and (2) experimentally induceddegeneration. In the second step, nerve cell activity elicited byCO2 stimulation in the central nervous system is located using2-deoxyglucose labelling (B. Bausenwein and J. Boeckh,unpublished) in order to identify further the pathway for theprocessing of this important olfactory input.

Materials and methodsAnimals

Adult 1- to 2-week-old females of the yellow fever mosquitoAedes aegypti L. (Diptera: Culicidae) were used as

1874 P. G. DISTLER AND J. BOECKHexperimental animals. They were taken from a colony rearedin the Institutes vivarium under standard conditions (Geier,1995).

Anterograde tracing of the maxillary and antennal nerves

HRP labelling

The animals were immobilized by exposure to cold air (8 C)and subsequently fixed in a position favourable formanipulations on the maxillary palps. In 20 mosquitoes, thefourth segment of one maxillary palp was cut off just distal toits base using a fine blade. A small drop of a 5 % aqueoussolution of HRP (Boehringer, Mannheim, Germany) wasimmediately placed onto the lesion and allowed to infiltrate fora few seconds. Finally, the preparation was sealed with Vaseline.A similar labelling procedure had been used previously to tracethe central projections of the cockroach antennal nerve (Kirn,1996). To label the antennal nerve with HRP, the flagellum wascut off just distal to the pedicel in 14 specimen, and HRP wasapplied onto the lesioned site as described above. The animalswere kept in a moist chamber for between 2.5 and 8 h at roomtemperature (20 C) before the preparations were fixed.

Induced degeneration

Degeneration studies (see Boeckh et al. 1970) of both themaxillary and the antennal nerve projections were performedin four specimens each. The fourth segment of the maxillarypalp or the flagellum, respectively, was removed according tothe procedure described above, in order to induce degenerationof receptor axons. These preparations were fixed 24 or 48 hpost lesion. During the degeneration period, the animals couldmove freely.

Fixation of the brains

After decapitation of the mosquitoes, the head capsules werefixed using rapid-setting glue on their posterior surface andsuperfused with 2.5 % glutaraldehyde in 0.1 mol l1 phosphatebuffer (PB), pH 7.4. In order to achieve immediate fixation ofthe brain, the head capsules were carefully opened by ablatingsmall areas of cuticle in the dorsal region using sharp glasscapillary tips. The free-floating heads were fixed forapproximately 11.5 h at room temperature (RT). Preparationsthat had been induced to degenerate were subsequently post-Fig. 1. Scanning electron micrographs of thehead and mouthparts of a female Aedesaegypti (photographs kindly provided by A.Khn). (A) Head with antenna (an) andmaxillary palps (mp). (B) Distal (fourth)segment of the maxillary palp showing thesensilla basiconica (arrow) and non-innervated hairs (arrowhead). (C) Singlebasiconic sensillum (arrow) at highermagnification. Scale bars, A, 200 m; B,50 m; C, 10 m.fixed in OsO4 and embedded in resin (see below). HRP-labelled brains were embedded in a mixture ofovalbumin/gelatin, fixed for a further 10 min, and transferredinto 0.01 mol l1 PB, pH 7.4.

Histochemical reaction of HRP

HRP-labelled brains were cut into 30 m thick serialsections in a horizontal plane using a vibroslicer. The sectionswere collected in 0.05 mol l1 TrisHCl buffer (pH 7.5). HRPwas visualized by reaction with nickel-intensifieddiaminobenzidine (Ni-DAB) (0.02 % DAB in TrisHCl buffer,pH 7.5, with 0.1 % nickel ammonium sulphate and 0.003 %hydrogen peroxide). The reaction was monitored under themicroscope and stopped after 5 min by removing the Ni-DABsolution. The sections were postfixed in 1 % OsO4 dissolved indistilled water for 20 min at RT and, after dehydration inethanol, embedded in Epon 812. The polymerizationtemperature was 57 C.

Semi- and ultrathin sectioning

For reconstruction of the nerve projections, 3m thicksemithin section series were cut from whole brains (induceddegeneration) or from selected 30m thick sections (HRPlabelling) and counterstained with Richardson Blue. Formaxillary nerve labelling, three brains with induced degeneration(24 and 48h) and five brains with HRP labelling (between 5 and8h) were studied. For antennal nerve labelling, three brains withinduced degeneration (24 and 48h) and six brains with HRPlabelling (between 5 and 8h) were studied. In addition, ultrathinsections were cut from (a) the ventroposterior glomerulus withthe terminal arborizations of the maxillary afferents (threedifferent preparations) and (b) the nerve bundle in thesuboesophageal ganglion (one preparation). These sections werecounterstained with uranyl acetate (20min) and lead citrate(3min), and examined in a Zeiss electron microscope (EM 109).

ResultsAnterograde tract tracing of the maxillary palp nerve

HRP labelling of maxillary afferents revealed a distinct fibrebundle that projected into the suboesophageal ganglion andascended further into the ipsilateral antennal lobe. There, all

1875Maxillary and antennal nerve projections

Fig. 2. Maxillary nerve afferents. (A) 30 m thick horizontal sectionthrough a brain with horseradish-peroxidase (HRP)-labelled maxillaryafferents (right maxillary nerve labelled, 5.5 h). A densely innervatedglomerulus (g) is evident in the ipsilateral ventroposterior region ofthe antennal lobe (AL). No labelled fibre processes are present in thecorresponding contralateral brain region. The innervated glomeruluslies lateral to the basal region of the antenno-cerebral tract (act)and medial to the oesophagus. For further details, see Fig. 3.(B) Richardson-Blue-stained 3 m thick horizontal section through abrain with experimentally induced degeneration of maxillary afferents(24 h). Degeneration granules are present in only one ventroposteriorglomerulus (g). Note that the corresponding contralateral glomerulus(asterisk) is not innervated. The spatial positions of both glomeruliidentified by HRP labelling and by induced degeneration are identical.act, antenno-cerebral tract; cb, central body; PC, protocerebrum. Scalebars, 50 m.labelled axons terminated within a single, oval-shaped andparticularly large glomerulus approximately 3050 m indiameter (Figs 25). This projection pattern was identified in10 brains. No morphological differences with regard to theprojection site or the number of innervated glomeruli wereevident in brains stained for between 2.5 and 8 h. Theinnervated glomerulus was located in the ventroposteriorantennal lobe. Medially, it bordered the oesophagus andlaterally it bordered the root of the antenno-cerebral tract(Fig. 2A). No axonal projections were detected within thecontralateral antennal lobe. In the suboesophageal ganglion,however, a few smaller processes crossed the midline andprojected over a short distance into the contralateral neuropile.Experimentally induced degeneration of the maxillary palpnerve for 24 and 48 h revealed a similar projection pattern (Fig.2B). The characteristic degeneration granules were found inthe same glomerulus that had been identified by HRP labelling.

Fine-structure analysis showed that the glomerulus wasdemarcated within the surrounding neuropile by a thin glialsheath (Fig. 3). Only laterally, next to the basal antenno-cerebral tract, was the glial border not clearly identifiable. TheHRP-labelled axon processes were distributed throughout theglomerulus neuropile. A subdivision of this into regions witheither dense or less-dense innervation was not evident. Byanalogy with the light microscopic findings, the correspondingcontralateral glomerulus revealed no (two brains) or only veryfew degenerated processes (one brain). Cross sections throughthe maxillary nerve in the suboesophageal ganglion revealedthat the majority of fibres were HRP-labelled (Figs 4, 5). Thecorresponding unlabelled maxillary nerve in the contralateralhemibrain contained approximately 110, mostly small, axons.

Anterograde tract tracing of the antennal nerve

HRP labelling of the antennal (flagellar) axons revealed thatsuch afferents innervated all the antennal lobe glomeruli of theipsilateral hemibrain, with the exception of the ventroposteriorglomerulus that had specifically received the maxillaryafferents (Fig. 6A,B). Contralateral projections were notevident, even after HRP labelling for up to 8 h (14 brains).Slight differences were found, however, between differentpreparations with regard to both the number and the intensityof labelled receptor cell axons. Induced degeneration of theantennal nerve for periods of 24 and 48 h (three brains)revealed a similar projection pattern (Fig. 6C).

DiscussionMorphology of maxillary and antennal afferents

Both HRP labelling and induced degeneration of themaxillary afferents consistently revealed projections into asingle glomerulus located in the ipsilateral ventroposteriorantennal lobe (Figs 24). Its contralateral counterpart did notcontain labelled processes, except for a very few in onespecimen. These presumably belonged to receptor cells of thecontralateral maxillary palp, which could have been severedduring the operation at the closely neighbouring palp andthereby been caused to degenerate. Preparations labelled forvarying periods, from 2.5 to 48 h, did not show significantdifferences in the projection pattern and the terminal regionsof the axonal processes, indicating that the nerve had beencompletely labelled. This was further supported by the findingthat all axons within the maxillary nerve appeared to belabelled. Earlier morphological and fine-structureinvestigations on the maxillary receptor cell types of the fourthpalpal segment had described a similar number of receptorcells (McIver, 1982). The ventroposterior glomerulusidentified here is located in the same antennal lobe region asthe axon terminals found in the fluorescent dye labellings of

1876 P. G. DISTLER AND J. BOECKH

Fig. 3. Ultrathin section throughthe ventroposterior glomerulusinnervated by HRP-labelledmaxillary afferents (same sectionas in Fig. 2A). Stained processes(arrows) are distributed throughoutthe glomerulus. act, basal region ofthe antenno-cerebral tract; AL,antennal lobe neuropile; G,ventroposterior glomerulus. Scalebar, 10 m.

Fig. 4. HRP-labelled right maxillary nerve in the suboesophagealganglion (sog) (same brain as in Fig. 2A). The fibres (arrow) projecttowards the centre of the neuropile, where small ramifications areevident (arrowhead). Few processes cross the midline (none shownhere). 30 m thick horizontal section. og, optic ganglia. Scale bar,100 m.Anton (1996). A slight difference between the studies concernsthe number of innervated glomeruli. In the present study,terminal ramifications of afferent fibres were found only in onelarge glomerulus (Figs 24), and the fluorescence appeared tobe distributed over two smaller, very closely neighbouringglomeruli.

In contrast to other cyclorrhaphic Diptera (Boeckh et al.1970; Stocker et al. 1990; Stocker, 1994), the maxillaryafferents projected exclusively into the ipsilateral antennallobes. Only in the suboesophageal ganglion were a few fibresfound to cross the midline. The majority of the maxillaryafferents identified in Drosophila melanogaster bifurcate andproject into both antennal lobes. There, they terminate withina small, mirror-symmetrical group of three (five)ventroposterior glomeruli (Stocker et al. 1990; Singh andNayak, 1985). Morphologically, the maxillary afferents in D.melanogaster and A. aegypti differ with regard to (a) theprojection site and (b) the number of antennal lobe glomeruliinnervated. Such differences might be due, for example, todifferent numbers of chemoreceptor cells on the palps in thetwo species (for a review, see Stocker, 1994). Inhemimetabolous insects (Locusta migratoria, Ernst et al. 1977;Altman and Kien, 1986; Periplaneta americana, Boeckh andErnst, 1987), the maxillary nerve projects into both the ipsi-and the contralateral brain hemisphere. There, the terminalramifications are located in the so-called lobus glomeratus.The maxillary projection pattern in A. aegypti shows certainparallels with the labial afferents in some lepidopteran species.Bilateral axonal projections from each labial palp have beenshown to terminate mirror-symmetrically in a singleglomerulus in each antennal lobe in R. luteibarba (Bogner et

1877Maxillary and antennal nerve projections

Fig. 5. Ultrathin sectionthrough the maxillary nervein the suboesophagealganglion. (A) HRP-labellednerve. (B) Maxillary nerve(unlabelled) in thecontralateral hemiganglion.Arrows, axonal processes oflabelled (A) and unlabelled(B) receptor neurones. Scalebar, 1 m.al. 1986), P. rapae (Lee and Altner, 1986) and M. sexta (Kentet al. 1986). In Rhodogastria, furthermore, it has beendemonstrated that the respective receptor cells in the distallabial pits are very sensitive to CO2 and only slightly sensitiveto other odorants (Bogner, 1990). The CO2-sensitive receptorneurones constitute the majority of labial afferents inRhodogastria.

Anterograde tracing of the antennal nerve in A. aegyptirevealed projections only into the ipsilateral antennal lobe. Allthe glomeruli, with the exception of the maxillaryglomerulus, were innervated (Fig. 6). These findings are inaccordance with data reported in a previous fluorescencelabelling study (Anton, 1996). Ipsilateral projections ofantennal chemoreceptor cells are typical of many insect species(for reviews, see Boeckh and Tolbert, 1993; Hildebrand, 1996).In cyclorrhaphic Diptera such as Musca and Drosophila,however, the majority of antennal chemoreceptors have axonprojections into both the ipsi- and the contralateral antennallobe (Boeckh et al. 1970; Stocker 1994). In D. melanogaster,it has been reported that glomeruli innervated by the maxillaryafferents receive no other antennal afferents (Singh and Nayak,1985; Stocker et al. 1990). These findings are consistent withthe situation in A. aegypti, where no antennal axons terminatedin the ventroposterior glomerulus. Studies on the centralFig. 6. Antennal nerve afferents. (A) 30 m thick horizontal section(overview) through a brain with HRP-labelled antennal afferents(right antennal nerve stained, 5 h). (B) All glomeruli of the ipsilateralantennal lobe, with the exception of the ventroposterior glomerulus,are innervated. (C) Induced degeneration (24 h) of the right antennalnerve reveals a similar innervation pattern. (B,C) 3 m thickhorizontal sections counterstained with Richardson Blue. Arrows,innervated glomeruli; asterisks, ventroposterior glomeruli. act,antenno-cerebral tract; AL, antennal lobe; cb, central body; PC,protocerebrum. Scale bars, A, 100 m; B,C, 50 m.

1878 P. G. DISTLER AND J. BOECKHprojections of the antennal nerve in Lepidoptera (M. sexta,Kent et al. 1986; P. rapae, Lee and Altner, 1986;Rhodogastria, Bogner et al. 1986) also revealed innervation ofall antennal lobe glomeruli with the exception of the onereceiving the labial afferents.

Functional considerations

In the present study, all the labelled maxillary afferentsterminated within a single glomerulus, suggesting that thelatter might be a distinct region of the antennal lobe specializedfor the primary processing of CO2. The hypothesis of a CO2-specific glomerulus is supported by the observation that in A.aegypti no other appendages (for example, the antenna) areknown to have CO2-sensitive receptor cells that could form anadditional afferent pathway. However, this glomerulus, inaddition to the CO2-sensitive axonal projections, also receivesinput from other palpal receptor neurones, which are sensitiveto a variety of chemical vapours, such as n-heptane, acetoneand amyl acetate (Kellogg, 1970). Since the latter are co-localized with the CO2-sensitive receptor neurones in the sametype of basiconic sensillum (McIver, 1982), the labelled axonscould not be identified as being sensitive to either CO2 orodorants. Therefore, the identified glomerulus is presumed tobe a sensory neuropile in which signals from both CO2-sensitive and other maxillary receptor cells (see above) areprocessed.

Further insect antennal lobe glomeruli have been identifiedas specific target neuropiles of certain chemo- andthermosensitive receptor neurones (for reviews, see Boeckh etal. 1990; Stocker, 1994; Hildebrand, 1996). In cockroachesand moths, for example, pheromone-specific chemoreceptorcells project exclusively into a single large glomerulus, the so-called macroglomerulus. A small set of glomeruli in theantennal lobe of Periplaneta americana receives axonalprojections from cold receptor neurones, and other glomeruliare the target neuropile of afferents from hygroreceptorneurones on the antenna (Nishikawa et al. 1995). In D.melanogaster, 23 identified glomeruli are thought to form thesensory neuropile in which the axons of certainthermosensitive neurones from aristal sensilla terminate(Stocker, 1994). However, it still remains unclear in all thesecases whether a particular glomerulus receives inputexclusively from a single receptor type or also from other, sofar unidentified, receptor cell types.

The present findings show that the glomerulus describedhere in A. aegypti represents probably the only and specifictarget area for CO2-sensitive afferents, which is analogous tothe situation in R. luteibarba. However, there is combinedanatomical and physiological evidence for otherchemoreceptor afferents in this glomerulus, suggesting that, incontrast to the situation in R. luteibarba, further odorants areprobably processed in this neuropile in A. aegypti.

We thank Mrs E. v. Ciriacy for expert technical assistanceand Mrs H. Hallmer for photographic assistance.ReferencesALTMAN, J. S. AND KIEN, J. (1986). Functional organization of the

subesophageal ganglion in arthropods. In Arthropod Brain: ItsEvolution, Development, Structure and Functions (ed. A. P. Gupta),pp. 265302. New York: Wiley.

ANTON, S. (1996). Central olfactory pathways in mosquitoes and otherinsects. In Proceedings of the Ciba Foundation, Symposium no.200, Olfaction in MosquitoHost Interactions, pp. 184196.Chichester: Wiley.

BOECKH, J., DISTLER, P., ERNST, K. D., HSL M. AND MALUN, D.(1990). Olfactory bulb and antennal lobe. In ChemosensoryInformation Processing (ed. D. Schild), pp. 201228. Berlin:Springer.

BOECKH, J. AND ERNST, K.-D. (1987). Contribution of single unitanalysis in insects to an understanding of olfactory function. J.comp. Physiol. A 161, 549565.

BOECKH, J., SANDRI, C. AND AKERT, K. (1970). Sensorische Eingngeund synaptische Verbindungen im Zentralnervensystem vonInsekten: Experimentelle Degeneration in der antennalenSinnesbahn im Oberschlundganglion von Fliegen und Schaben. Z.Zellforsch. mikrosk. Anat. 103, 429446.

BOECKH, J. AND TOLBERT, L. P. (1993). Synaptic organization anddevelopment of the antennal lobe in insects. Microsc. Res. Techn.24, 260280.

BOGNER, F. (1990). Sensory physiological investigation of carbondioxide receptors in Lepidoptera. J. Insect Physiol. 36, 951957.

BOGNER, F., BOPPR, M., ERNST, K.-D. AND BOECKH, J. (1986). CO2-sensitive receptors on the labial palps of Rhodogastria moths(Lepidoptera: Arctiidae): physiology, fine structure and centralprojections. J. comp. Physiol. A 158, 741749.

BOWEN, M. F. (1991). The sensory physiology of host-seekingbehavior of mosquitoes. A. Rev. Ent. 36, 139158.

CHAIKA, S. Y. (1977). Relationship of the number of club shapedpalpal sensilla to the type of attack on the host by blood suckingmosquitoes. Vestn. Mosk. Univ. Ser. XVI Biol. 2, 8587.

DAYKIN, P. N., KELLOGG, F. E. AND WRIGHT, R. H. (1965). Host-finding and repulsion of Aedes aegypti. Can. Ent. 97, 236263.

ERNST, K.-D., BOECKH, J. AND BOECKH, V. (1977). A neuroanatomicalstudy on the organization of the central antennal pathways ininsects. II. Deutocerebral connections in Locusta migratoria andPeriplaneta americana. Cell Tissue Res. 176, 285308.

FRIEND, W. G. AND SMITH, J. J. B. (1977). Factors affecting feedingby blood-sucking insects. A. Rev. Ent. 22, 309331.

GALUN, R. (1977). Responses of blood-sucking arthropods tovertebrate hosts. In Chemical Control of Insect Behavior (ed. H. H.Shorey and J. J. McKelvey Jr), pp. 103115. New York: JohnWiley & Sons.

GEIER, M. (1995). Verhaltensversuche mit Gelbfiebermcken Aedesaegypti zur Aufklrung des attraktiven Reizmusters bei derolfaktorischen Wirtsfindung. PhD thesis, Universitt Regensburg.

GILLIES, M. T. (1980). The role of carbon dioxide in host-finding bymosquitoes (Diptera: Culicidae): a review. Bull. ent. Res. 70,525532.

GRANT, A. J., WIGTON, B. E., AGHAJANIAN, J. G. AND OCONNEL, R.J. (1995). Electrophysiological responses of receptor neurons inmosquito maxillary palp sensilla to carbon dioxide. J. comp.Physiol. A 177, 389396.

HILDEBRAND, J. G. (1996). Olfactory control of behavior in moths:central processing of odor information and the functionalsignificance of olfactory glomeruli. J. comp. Physiol. A 178, 519.

1879Maxillary and antennal nerve projectionsKELLOGG, F. E. (1970). Water vapour and carbon dioxide receptors inAedes aegypti. J. Insect Physiol. 16, 99108.

KENT, K. S., HARROW, I. D., QUARTARARO, P. AND HILDEBRAND, J. G.(1986). An accessory olfactory pathway in Lepidoptera: the labialpit organ and its central projections in Manduca sexta and certainother sphinx moths and silk moths. Cell Tissue Res. 245, 237245.

KIRN, C. (1996). Synaptische Verbindungen antennalerRezeptorfasern und GABA-immunreaktiver zentraler Neurone inden Glomeruli des Antennallobus der Schabe Periplanetaamericana. PhD thesis, Universitt Regensburg.

LEE, J.-K. AND ALTNER, H. (1986). Primary sensory projections of thelabial palp-pit organ of Pieris rapae L. (Lepidoptera: Pieridae). Int.J. Insect Morph. Embryol. 15, 439448.

MAYER, M. S. AND JAMES, J. D. (1969). Attraction of Aedes aegypti(L.): responses to human arms, carbon dioxide and air currents ina new type of olfactometer. Bull. ent. Res. 58, 629641.

MCIVER, S. B. (1982). Sensilla of mosquitoes (Diptera: Culicidae). J.med. Ent. 19, 489535.

MCIVER, S. B. AND CHARLTON, C. C. (1970). Studies on the senseorgans of selected culicine mosquitoes. Can. J. Zool. 48, 293295.

NISHIKAWA, M., YOKOHARI, F. AND ISHIBASHI, T. (1995). Centralprojections of the antennal cold receptor neurons and hygroreceptorneurons of the cockroach Periplaneta americana. J. comp. Neurol.361, 165176.

SINGH, R. N. AND NAYAK, S. (1985). Fine structure and primarysensory projections of sensilla on the maxillary palp of Drosophilamelanogaster Meigen (Diptera: Drosophilidae). Int. J. InsectMorph. Embryol. 14, 291306.

SMITH, C. N., SMITH, N., GOUGH, H. K., WEIDHAAS, D. E., GIBERT, I.H., MAYER, M. S., SMILLE, B. J. AND HOFBAUER, A. (1969). L-Lacticacid as a factor in the attraction of Aedes aegypti (Diptera:Culicidae) to human hosts. Ann. ent. Soc. Am. 63, 760768.

STEIB, B. (1995). Kohlendioxid bei der Wirtsfindung vonGelbfiebermuecken Aedes aegypti (L.) (Diptera: Culicidae).Diplomarbeit, Universitt Regensburg.

STOCKER, R. F. (1994). The organization of the chemosensory systemin Drosophila melanogaster: a review. Cell Tissue Res. 275, 326.

STOCKER, R. F., LIENHARD, M. C., BORST, A. AND FISCHBACH, K.-F.(1990). Neuronal architecture of the antennal lobe in Drosophilamelanogaster. Cell Tissue Res. 262, 934.

TAKKEN, W. (1991). The role of olfaction in host-seeking ofmosquitoes: A review. Insect Sci. Appl. 12, 287295.