The fine structure of thermo-/hygrosensitive sensilla in the silkmoth Bombyx mori: Receptor membrane substructure and sensory cell contacts

Summary The thermo-/hygroreceptive sensilla styloconica of the silkmoth Bombyx mori are located on the tips of the antennal branches. A small poreless cuticular peg is innervated by three sensory cells. The outer dendritic segments of two type-1 receptor cells, the presumed hygroreceptors, almost completely fill the peg lumen and are in close contact with each other. The outer dendritic segment of the third (type-2) receptor cell, the presumed thermoreceptor, forms lamellae below the peg base. The membranes of these lamellae are studded with knobs in orthogonal array, protruding into the extracellular space with the same orientation on facing lamellae. This Bossy Orthogonal Surface Substructure (BOSS) is assumed to play a role in thermoreceptor function. Contacts are observed between the outer dendritic segments, between the inner dendritic segments immediately below the ciliary segments, and between the sensory cell somata. These contacts, which are not found in the olfactory sensilla (s. trichodea and basiconica) of this species, indicate electrical interactions between the three sensory cells of the styloconic sensillum and possibly are involved in the antagonistic and/or bimodal response characteristics of thermo-/hygroreceptor cells.     

Antennal sensilla ofNeomysis integer (leach)

Summary The most frequent type of the hair sensilla on the antennae ofNeomysis integer is investigated by electron microscopic methods. The cellular properties of the sensilla are compared with those of other arthropods in order to detect possible homologies.The hairs are innervated by 2, 3, 6, 8, 9, or 10 sensory cells. The dendrites show an inner and outer dendritic segment. Five or six enveloping cells belong to a sensillum. In intermoult stage, processes of all the enveloping cells except the innermost one extend into the hair shaft. The sensory hairs possess only a single liquor cavity, which morphologically is homologous to the inner lymph cavity of insect sensilla. Around the liquor cavity, a supporting structure is located which seems to be identical to the scolopale of chordotonal organs. The six-to tenfold-innervated hairs possess two groups of differently structured dendrites which are regularly arranged on opposite sides of the liquor cavity. The outer dendritic segments are enclosed in a dendritic sheath. It is secreted by the innermost enveloping cell (= dendritic sheath cell of insect sensilla). All the outer dendritic segments terminate in the distal region of the hair shaft which shows a pore at its tip. The possible function of the sensilla is discussed. The double and triple-innervated hairs are considered to be mechano-receptors, whereas the sensilla associated with six to ten sensory cells might be mechano-chemoreceptors.     

Reduction of sensory cells in antennal sensilla correlated with changes in feeding behavior in the beetle Loricera pilicornis (Insecta,Coleoptera, Carabidae)

Summary The structure of the setae on the proximal antennal segments of the beetle Loricera pilicornis is described using electron microscopical methods. These setae are part of a prey-capturing apparatus and are inserted within flexible sockets. They have no central lumen.Four or five sensory cells are connected to each seta. One cell is characterized as a mechanoreceptor due to the presence of a tubular body and the location of its dendritic outer segment. The other sensory cells are of two types. One type shows the usual features of sensillar receptors except that the dendritic outer segments end beneath the seta within the cuticular sheath. In the other type all parts of the cell, including the perikaryon, appear undersized, and no axon was found. In a single case a sixth cell was found which lacks any process, although, due to its location, it belongs to the sensory cell group.The enveloping cells also deviate from the usual pattern. Trichogen and tormogen cells have no membrane folds nor microvilli. From the membrane of the thecogen cell, where it borders on the inner receptor lymph cavity, invaginations have developed which form voluminous membrane whorls. Portasomes are found on these membranes.On the basis of the structural features we hypothesize that the setae represent sensilla undergoing stepwise reduction, losing primordial gustatory units whilst the prey-capturing mechanism is optimized.Dedicated to Professor Dr. Dietrich Schneider on occasion of his 65th birthday     

The structure of the postantennal organ in Onychiurus sp. (Insecta: Collembola) and its connection to the central nervous system

Summary The postantennal organ in Onychiurus (group armatus) is a sensory organ comprising one sensory cell, several enveloping cells and cuticular structures.The perikaryon of the sensory cell is located in the central nervous system and distally gives off a dendrite in which one inner and two outer segments are distinguishable. Two ciliary structures connect the outer dendritic segments with the inner segment. The outer segments divide repeatedly, basal to the cuticular structures, into small branches which end distally beneath the cuticular wall. The wall of the cuticular structures is very thin and is pierced by numerous funnel-shaped pores. The pores are filled with electron-dense material which forms a continuous sheath underneath the cuticle. This material encases the small dendritic branches and the processes of the enveloping cells which occupy the lumen of the cuticular structures. There are three types of enveloping cells: one inner, several outer and one basal. Their processes differ in the manner in which they envelop the various regions of the dendrite.At the beginning of moulting outer dendritic branches are not found within the cuticular structures of the organ. They may be assumed to retract inwardly. However, in the later stages, when the cuticle is fully formed, the outer dendritic segments appear to divide. It is assumed that the small dendritic branches reach their targets before ecdysis. The electrondense material which clogs the intermoult cuticular pores is absent until the final stages of the moulting cycle.Supported by a grant from the Deutscher Akademischer Austauschdienst.     

Multicellular antennal sensilla containing a sensory cell with a short dendrite and dense-core granules in the insect,Hypogastrura socialis (Collembola): intermolt and molting stages

Summary At the antennal tip of the collembolan insect Hypogastrura socialis two terminal-pore sensilla are located, which, in addition to normally structured and most probably chemosensitive sensory cells, also contain aberrant sensory cells. Portions of these cells resemble chemoreceptors but also shown are features that, as a rule, occur in mechanoreceptors. One cell in each sensillum is remarkable in two characteristics: (1) Its dendritic outer segment does not reach the cuticular outer structures of the sensillum; (2) it contains dense-core granules (diameter 60–110 nm) within its perikaryon, its dendritic inner segment and its axon. Additionally, these two cells do not show lengthening of their dendritic outer segment during molt as do all other sensory cells. Among the fibers of one major branch of the antennal nerve within the head capsule a single axon was observed to contain dense-core granules. This axon was traced to its termination where normal synaptical contacts were found. Based on the assumption that the axon belongs to one of the granule-containing sensory cells two alternative hypotheses are proposed: (1) an individual sensory cell of a sensillum may synthesize a transmitter that is different from that of the other sensory cells of this sensillum; (2) the aberrant cells have lost exteroceptive functions but act as neuromodulatorsSupported by the Deutsche Forschungsgemeinschaft (SFB 4/G1)     

Morphology of the prothoracic discs and associated sensilla of Acanthocnemus nigricans (Coleoptera, Acanthocnemidae)

The Australian ‘little ash beetle’ Acanthocnemus nigricans (Coleoptera, Cleroidea, Acanthocnemidae) is attracted by forest fires. A. nigricans has one pair of unique prothoracic sensory organs and it has been speculated that these organs may play a role in fire detection. Each organ consists of a cuticular disc, which is fixed over an air-filled cavity. On the outer surface of the disc, about 90 tiny cuticular sensilla are situated. The poreless outer peg of a sensillum is 3–5 μm long and is surrounded by a cuticular wall. One ciliary sensory cell innervates the peg. As a special feature, the outer dendritic segment is very short already terminating below the cuticle. A massive electron-dense cylindrical rod, which most probably represents the hypertrophied dendritic sheath, extends through the cuticular canal connecting the tip of the outer dendritic segment to the peg. The dendritic inner segment and the soma are fused indistinguishably. Thin, leaflike extensions of glial cells deeply extend into that conjoint and considerably enlarged compartment which also contains large numbers of mitochondria. In summary, the sensilla of the sensory disc of A. nigricans represent a new type of insect sensillum of hitherto unknown function. The possible role of the prothoracic sensory organ in fire detection is discussed.     

Fine structure of the galeal styloconic sensilla of larval Lymantria dispar (Lepidoptera: Lymantriidae)

Lepidopteran larvae possess two pairs of styloconic sensilla located on the maxillary galea. These sensilla, namely the lateral and medial styloconic sensilla, are each comprised of a smaller cone, which is inserted into a style. They are thought to play an important role in host-plant selection and are the main organs involved in feeding. Ultrastructural examination of these sensilla of fifth instar Lymantria dispar (L.) larvae reveal that they are each approximately 70 um in length and 30 um in width. Each sensillum consists of a single sensory peg inserted into the socket of a large style. Each peg bears a slightly subapical terminal pore averaging 317 nm in lateral and 179 nm in medial sensilla. Each sensillum houses five bipolar neurons. The proximal dendritic segment of each neuron gives rise to an unbranched distal dendritic segment. Four of these dendrites terminate near the tip of the sensillum below the pore and bear ultrastructural features consistent with contact chemosensilla. The fifth distal dendrite terminates near the base of the peg and bears ultrastructural features consistent with mechanosensilla. Thus, these sensilla each bear a bimodal chemo-mechanosensory function. The distal dendrites lie within the dendritic channel and are enclosed by a dendritic sheath. The intermediate and outer sheath cells enclose a large sensillar sinus, whereas the smaller ciliary sinus is enclosed by the inner cell. The neurons are ensheathed successively by the inner, intermediate, and outer sheath cells.     

Fine structure of the tibiotarsal and pretarsal sensory organs in Monobella grassei banyulensis Deharveng (Collembola : Neanuridae)

The fine structure of the tibiotarsal and pretarsal sensory organs of Monobella grassei banyulensis Deharveng (Collembola : Neanuridae) has been examined by electron microscopy.Three types of sensory organs have been observed. (1) the most numerous setae of the tibiotarsus are classic mechanosensitive setae with one bipolar sensory cell, whose distal outer segment ends in a tubular body. (2) Two small setae are arranged on each side of the basal part of the claw; they show 3 sensory cells, 2 of which are mechanosensitive cells of the scolopidial type; the outer segments of the 2 mechanosensitive cells end at the base of the sensory hair. The dendrite of the 3rd sensory cell extends into the hair shaft. (3) Two similar chordotonal sensilla link the tibiotarsus and the pretarsus; each sensillum is composed of 2 bipolar sensory cells enveloped in sheath cells. The first type of sensory organ shows the characteristics of insect exteroceptive mechanosensitive hairs. The mechanosensitive cells of the 2nd and 3rd tibiotarsus sensory organs are probably proprioceptive and control the movements of the pretarsus in relation to the tibiotarsus. Two features are noteworthy: (1) the association of the scolopidial cells with a chemosensitive one has never been observed in other insect sensory organs, except in the Collembola; and (2) there is an important morphological diversity in the ciliary roots of the various scolopidial cells, which are in other respects very similar.     

The structure of bimodal chemo-, thermo-, and hygroreceptive sensilla on the antenna of Locusta migratoria

Summary Coeloconic sensilla on the antenna of Locusta migratoria were investigated electrophysiologically, labeled and then examined under the scanning or transmission electron microscope. They can be categorized into two main morphological types: 1) sensilla with wall pores and two concentric cuticular walls (wp-dw sensilla), and 2) sensilla lacking wall pores (np-sensilla).Hygroreceptors were observed only in np-sensilla. Olfactory receptors were observed in wp-dw sensilla, but not in np-sensilla. Cold-sensitive units were found in both types. Wp-dw sensilla with a cold-sensitive unit also manifest a fatty acid olfactory receptor. This cold-sensitive unit appears to be less sensitive to drops in temperature than the cold receptor combined with hygroreceptors in np-sensilla. Recordings from both types revealed up to three or sometimes four units that could be distinguished on the basis of impulse amplitude.The number of np-sensilla on each antennal segment ranges from 0 to 6. These sensilla are pegs 4–5 m long with a terminal swelling. The dendritic outer segments of two sensory cells fit tightly into these pegs. The dendrite of the third sensory cell ends at its base. All three are up to 1 m thick, unbranched, and densely filled with microtubules. The cuticular wall and socket of the peg are characterized by (1) an electron-dense inner layer surrounding the dendrites and continuous with electron dense clusters at the base of the peg, and (2) clefts in the cuticle at the insertion of the peg.The present observations favor the hypothesis that insect sensilla combining the triad of moist air, dry air and cold receptors generally lack pores. Specifical, though still alternative roles in stimulus transduction can be suggested for substructures demonstrated in the present and previous electron microscopic investigations.Supported by the Deutsche Forschungsgemeinschaft (SFB 4:G1/D)     

Functional morphology of a double-walled multiporous olfactory sensillum: the sensillum coeloconicum of Bombyx mori (Insecta, Lepidoptera)

The fine structure of coeloconic sensilla of Bombyx mori was studied in cryofixed specimens. These sensilla belong to the category of double-walled wall-pore sensilla. The pegs are approximately 10 mum long, located in pits on the dorsal side of the antennal branches, and longitudinally grooved in their distal half (grooved surface approximately 30 mum(2)). The central lumen contains the outer dendritic segments of usually five receptor cells, and is surrounded by up to 15 partially fused cuticular fingers. The peripheral lumina of these cuticular fingers are filled with material resembling wax-canal filaments. Radial spoke channels ( approximately 600 per peg), each 10-20 nm wide, connect the central lumen with the longitudinal groove channels. Groove and spoke channels are assumed to mediate the transport of odorant molecules from the outer epicuticular surface layers to the sensory dendrites. Thus the double-walled wall-pore sensilla represent a bauplan essentially different from single-walled wall-pore sensilla; the reason, however, why the two types are found together throughout the insect orders remains enigmatic. Other peculiar features of the coeloconic sensilla of the silkmoth are invaginations of the outer dendritic segments and direct contacts between the receptor cell somata. The latter may be the structural correlate to electrophysiological observations indicative of peripheral interaction between the receptor neurons. All three auxiliary cells have elaborately folded apical plasma membranes studded with portasomes and associated with an abundance of mitochondria; basally they often contact tracheal branches. As compared to the auxiliary cells of the single-walled olfactory sensilla of the same species, all the mentioned features are much more prominent and hint to a higher ion pumping activity at the border to the sensillum-lymph cavities.     

Projection of sensory neurons from a homeotic mutant appendage,Antennapedia, in Drosophila melanogaster

Central projections of sensory neurons from homeotic mutant appendages (Antennapedia) of Drosophila melanogaster were compared with those of wild-type antennae and wild-type legs by means of degeneration and cobalt backfilling methods. Sensory axons originating from wild-type thoracic legs terminate within the ventral ipsilateral half of the corresponding neuropile segment and do not project to the brain. Sensory fibers from the third antennal segment (AIII) of wild-type animals project into the ipsilateral antennal glomerulus (AG) and to a lesser extent into the contralateral AG, whereas those from the second antennal segment terminate principally within the ipsilateral posterior antennal center. The sensory terminals of femur, tibia, and tarsi of the homeotic leg show a distribution very similar to that of the homologous wild-type antennal segment AIII, differing to a minor degree only in the size and precise localization of terminals within the antennal glomeruli. No degenerating axons were evident in ultrastructural examination of neck connectives after removal of homeotic legs. It is thus very improbable that any sensory fibers of the homeotic leg project to normal leg projection areas in the thoracico-abdominal ganglion. Several alternative explanations are offered for the apparent retention of antennal specificity by axons from the transformed appendage.     

Antennal sensilla of whiteflies: Trialeurodes vaporariorum (Westwood), the glasshouse whitefly,and Aleyrodes proletella (Linnaeus), the cabbage whitefly, (Homoptera : Aleyrodidae). Part 2: Ultrastructure

A transmission electron microscope study of the antennal sensilla of the whitefly Trialeurodes vaporariorum and Aleyrodes proletella (Homoptera : Aleyrodidae) revealed that of the sensilla unique to the antennal flagellum (basiconic, coeloconic and small digitate-tipped sensory pegs), basiconic and coeloconic sensilla occur as subtypes. Four subtypes of basiconic cone sensilla occur on the flagella of T. vaporariorum and 3 on A. proletella. All the subtypes of basiconic sensilla have an ultrastructure typical of olfactory sensilla and probably have a primary olfactory function. Two subtypes of coeloconic sensilla occur on the flagella of both species. Their ultrastructure suggests primarily a chemosensory function. The digitate-tipped sensory peg of both species possesses a triad of neurones which have ultrastructural characteristics similar to the known thermo-/hygroreceptors of other insect species. The other sensilla, which occur on the antennae of the whiteflies, include cheatae, campaniform and subcuticular sensilla, all of which have an ultrastructure typical of mechanoreceptors.     

The role of homeotic genes in determining the segmental pattern of chordotonal organs in Drosophila

The homeotic genes are instrumental in establishing segment-specific characteristics. In Drosophila embryos there is ample evidence that the homeotic genes are involved in establishing the differences in the pattern of sense organs between segments. The chordotonal organs are compound sense organs made up of several stretch receptive sensilla. A set of serially homologous chordotonal organs, lch3 in the 1st thoracic segment, dch3 in the 2nd and 3rd thoracic segments and lch5 in abdominal segments 1 to 7, is composed of different numbers of sensilla with different positions and orientations. Here we examine this set of sense organs and a companion set, vchA/B and veh1, in the wild type and mutants for Sexcombs reduced, Antennapedia, Ultrabithorax, and abdominal-A, using immunostaining. Mutant phenotypes indicate that Ultrabithorax and abdominal-A in particular influence the formation of these sense organs. Differential expression of abdominal-A and Ultrabithorax within compartments of individual parasegments can precisely modulate the types of sense organs that will arise from a segment.     

Synaptic input from serial chordotonal organs onto segmentally homologous interneurons in the grasshopper Schistocerca gregaria

We investigated the synaptic inputs from the serially homologous pleural, tympanal and wing-hinge chordotonal organs onto a set of identified homologous interneurons (714, 539, 529) in the ventral nerve cord of the grasshopper Schistocerca gregaria. Cobalt backfills show that afferents from all chordotonal organs project into stereotypic tracts in the central nervous system in which intracellular staining reveals the interneurons to have dendritic arborizations. Neuron 714 was found to receive excitatory bilateral synaptic input from all the serial chordotonal organs tested, from the second thoracic segment down to the seventh abdominal segment. Neuron 531, by contrast, only receives input from the chordotonal afferents on the first abdominal segment; those on the axon side are excitatory, while those on the soma side are inhibitory. The pattern of chordotonal input onto neuron 529 is similar to that seen for neuron 714, with the exception that neuron 529 receives no input from the forewing chordotonal organs. The pattern of afferent connectivities onto neurons 714, 531 and 529 differs with respect to those afferents which synapse directly or indirectly with the respective neuron. The synaptic inputs demonstrate a segmental specialization in the chordotonal system and thereby offer an insight into information processing in a modular sensory system.     

TEM studies on the lattice organs of cirripede cypris larvae (Crustacea, Thecostraca, Cirripedia)

 Lattice organs consist of five pairs of sensory organs situated on the dorsal carapace in cypris larvae of the Crustacea Cirripedia. The lattice organs in cypris larvae of Trypetesa lampas (Acrothoracica) and Peltogaster paguri (Rhizocephala) represent the two main types found in cirripedes, but only minor differences exist at the TEM level. Each lattice organ is innervated by two bipolar, primary receptor cells. The inner dendritic segment of each receptor cell carries two outer dendritic segments. The outer dendritic segments contain modified cilia with a short ciliary segment (9×2+0 structure). Two sheath cells envelop the dendrite except for the distal ends of the outer dendritic segments. This distal end enters a cavity in the carapace cuticle and reaches a terminal pore situated at the far end of the cavity. The cuticle above the cavity is modified. In both species the epicuticle is partly perforated by numerous small pores and the underlying exocuticle is much thinner and less electron dense than the regular exocuticle. Lattice organs very probably have a chemosensory function and are homologous with the sensory dorsal organ of other crustacean taxa. Accepted: 18 August 1998     

Ultrastructure of a possible new type of crustacean cuticular strain receptor in Carcinus maenas (crustacea,decapoda)

A hitherto unknown sensillum type, the “intracuticular sensillum” was identified on the dactyls of the walking legs of the shore crab, Carcinus maenas. Each sensillum is innervated by two sensory cells with dendrites of “scolopidial” (type I) organization. The ciliary segment of the dendrite is 5–6 μm long and contains A-tubules with an electron-dense core and dynein arm-like protuberances; the terminal segment is characterized by densely packed microtubules. The outer dendritic segments pass through the endo- and exocuticle enclosed in a dendritic sheath and a cuticulax tube (canal), which is suspended inside a slit-shaped cavity by cuticular lamellae. The dendrites and the cavity terminate in a cupola-shaped invagination of the epicuticle. External cuticular structures are lacking. Three inner and four to six outer enveloping cells are associated with each intracuticular sensillum. The innermost enveloping cell contains a large scolopale that is connected to the ciliary rootlets inside the inner dendritic segments by desmosomes. Scolopale rods are present in enveloping cell 2. Since type I dendrites and a scolopale are regarded as modality-specific structures of mechanoreceptors, and since no supracuticular endorgan is present, the intracuticular sensilla likely are sensitive to cuticular strains. The intracuticular sensilla should be regarded as analogous to insect campaniform sensilla and arachnid slit sense organs.     

Compensatory regeneration of antennae after removal of the distal segment in Riptortus clavatus (Thunberg) (Heteroptera : Alydidae)

In Riptortus clavatus (Thunberg) (Heteroptera : Alydidae), growth and development of cuticular structures were compared between normal antennae and the antennae whose distal (4th) segment had been amputated during the 1st instar. The total length of the remaining 3 segments was 51% of the normal antenna. From the 2nd ecdysis onwards, the 2nd and 3rd segments grew excessively, and after adult emergence, the length of the operated antennae was 84% that of a normal antenna, although a typical 4th segment, separated from the 3rd segment by an intersegmental membrane, never developed. On the new distal (3rd) segment of operated antennae, long fine sensory hairs and grooved pegs, which characterize the normal distal (4th) segment, began to appear at the 2nd ecdysis, and successively increased in number between molts. Thus, when the distal segment was removed, the remaining segments tended to gradually compensate for the loss, both in terms of length and cuticular structures.     

Embryonic development and evolutionary origin of the Orthopteran auditory organs

Two different types of ears characterize the order of Orthopteran insects. The auditory organs of grasshoppers and locusts (Caelifera) are located in the first abdominal segment, those of bushcrickets and crickets (Ensifera) are found in the tibiae of the prothoracic legs. Using neuron-specific antibody labelling, we describe the ontogenetic origin of these two types of auditory organs, use comparative developmental studies to identify their segmental homologs, and on the basis of homology postulate their evolutionary origin. In grasshoppers the auditory receptors develop by epithelial invagination of the body wall ectoderm in the first abdominal segment. Subsequently, at least a part of the receptor cells undergo active migration and project their out-growing axons onto the next anterior intersegmental nerve. During this time the receptor cells and their axons express the cell-cell adhesion molecule, Fasciclin I. Similar cellular and molecular differentiation processes in neighboring segments give rise to serially homologous sensory organs, the pleural chordotonal organs in the pregenital abdominal segments, and the wing-hinge chordotonal organs in the thoracic segments. In more primitive earless grasshoppers pleural chordotonal organs are found in place of auditory organs in the first abdominal segment. In bushcrickets the auditory receptors develop in association with the prothoracic subgenual organ from a common developmental precursor. The auditory receptor neurons in these insects are homologous to identified mechanoreceptors in the meso- and metathoracic legs. The established intra- and interspecies homologies provide insight into the evolution of the auditory organs of Orthopterans.     

Ultrastructure and physiology of the CO2 sensitive sensillum ampullaceum in the leaf-cutting ant Atta sexdens

The sensilla ampullacea on the apical antennomere of the leaf-cutting ant Atta sexdens were investigated regarding both their responses to CO2 and their ultrastructure. By staining the sensillum during recording, we confirmed that the sensilla ampullacea are responsible for CO2 perception. We showed that the sensory neurons of the sensilla ampullacea are continuously active without adaptation during stimulation with CO2 (test duration: 1 h). This feature should enable ants to assess the absolute CO2 concentration inside their nests. Sensilla ampullacea have been found grouped mainly on the dorso-lateral side of the distal antennal segment. Scanning and transmission electron microscopic investigations revealed that the external pore opens into a chamber which connects to the ampulla via a cuticular duct. We propose protection against evaporation as a possible function of the duct. The ampulla houses a peg which is almost as long as the ampulla and shows cuticular ridges on the external wall. The ridges are separated by furrows with cuticular pores. The peg is innervated by only one sensory neuron with a large soma. Its outer dendritic segment is enveloped by a dendritic sheath up to the middle of the peg. From the middle to the tip numerous dendritic branches (up to 100) completely fill the distal half of the peg. This is the first report of a receptor cell with highly branched dendrites and which probably is tuned to CO2 exclusively.     

Ultrastructure of the galeal sensory complex in adults of the Colorado potato beetle, Leptinotarsa decemlineata

ABSTRACT. The structure of galeal sensilla of the Colorado potato beetle, Leptinotarsa decemlineata Say (Coleoptera: Chrysomelidae), is described using electron microscopical methods. Previous electro-physiological studies indicate that these sensilla respond to amino acids, sucrose and plant saps. One physiological type is particularly sensitive to L-alanine and gamma amino butyric acid (GABA).
Three morphologically different types of sensilla occur on the galeal tip. The more numerous apical pegs are not distinguishable from one another on the basis of external structure, although they differ physiologically. Five sensory cells are associated with most apical pegs. One apical peg, the α-sensillum, contains only four cells. All apical pegs have one cell with a tubular body. The remaining cells have unbranched dendrites and are associated with a single apical pore.
Apical hairs differ from the apical pegs by having double innervation. Within the hair shaft, a dendritic sheath is lacking and the sensillar sinus extends to the base of the hair. The function of this hair type is not known.
Numerous mechanosensory hairs which surround the other sensilla are singly innervated and contain a tubular body at the level of the outer dendritic segments.