The sensory equipment of a spider – A morphological survey of different types of sensillum in both sexes of Argiope bruennichi (Araneae,Araneidae)

Spiders show a wide range of sensory capabilities as evidenced by behavioural observations. Accordingly, spiders possess diverse sensory structures like mechano-, hygro-, thermo- or chemoreceptive sensilla. As to chemoreceptive structures, only trichoid tip-pore sensilla were found so far that were tested for gustation. That spiders are also able to receive airborne signals is corroborated by numerous behavioural experiments but the responsible structures have not been determined yet. Here, we provide sensilla distribution maps of pedipalps and walking legs of both sexes of the wasp spider Argiope bruennichi whose biology and mating system is well explored. By means of scanning electron microscopy, we scrutinized whether there is in fact only one type of trichoid pore sensillum and if so, if there are deviations in the outer structure of the tip-pore sensilla depending on their position on the body. We also describe the external structure and distribution of slit sense organs, trichobothria and tarsal organs. Our study shows that all four sensillum types occur on pedipalps and walking legs of both sexes. As to chemosensory organs, only tip-pore sensilla were found, suggesting that this sensillum type is used for both gustation and olfaction. The highest numbers of tip-pore sensilla were observed on metatarsi and tarsi of the first two walking legs. Mechanosensitive slit sense organs occur as single slit sensilla in rows along all podomers or as lyriform organs next to the joints. The mechanosensitive trichobothria occur on the basal part of tibiae and metatarsi. Tarsal organs occur on the dorsal side of all tarsi and the male cymbium. The distribution maps of the sensilla are the starting point for further exploration of internal, morphological differences of the sensilla from different regions on the body. Cryptic anatomical differences might be linked to functional differences that can be explored in combination with electrophysiological analyses. Consequently, the maps will help to elucidate the sensory world of spiders.     

External morphology of antennae and their sense organs in the roach Gromphadorhina brunneri (Blattoidea: Dictyoptera)

The antennae and their sense organs in nymphs and adult roaches of Gromphadorhina brunneri, were investigated and described. The number of segments and sensillae of the nymphal antennae depend on the developmental stage. Sexual dimorphism is pronounced. Males have longer antennae than females as well as an abundance of especially long sensory hairs (long wavy hairs), which are probably responsible for the perception of female sex pheromones. They also have more thin-walled sensory hairs, for instance, sensilla trichodea. On a morphological basis the sensillae of Gromphadorhina brunneri, were named and classified. Long wavy hairs and large sensory hairs appear to be present also in a related species, G. portentosa, but are lacking in others. Their distribution on the antennae varies greatly from that in G. portentosa but their structure varies only slightly. These two types of sense organs are considered to be specialized forms of sensilla chaetica. They are contact chemoreceptors, as are two other types of sensilla chaetica. Furthermore, thin-walled chemoreceptors are present, such as sensilla trichodea, sensilla basiconica, sensilla coeloconica and a typical mechanoreceptor, the sensillum campaniformium.     

Proprioceptors and sensory nerves in the legs of a spider,Cupiennius salei (Arachnida,Araneida)

Summary Retrograde CoS-impregnation was used to trace and map the course of sensory nerves and the distribution and innervation of the various proprioceptor types in all leg segments of Cupiennius salei, a Ctenid spider.1. Sensory nerve branches. In both the tibia and femur, axons of all proprioceptor types ascend in just two lateral nerves which do not merge with the main leg nerve until they reach the next proximal joint region. In the short segments — coxa, trochanter, patella, and tarsus — axons of the internal joint receptors often run separately from those of the other sensilla. Axons of the large lyriform slit sense organ at the dorsal metatarsus and of the trichobothria join with only a few hair axons and form their own nerve branches (Figs. 1, 2, 3).2. Proprioceptors. Each of the seven leg joints is supplied with at least one set of the well-known internal joint receptors, slit sensilla (single slits and lyriform organs), and long cuticular hairs. In addition, we found previously unnoticed hair plates on both sides of the coxa, near the prosoma/coxa joint; they are deflected by the articular membrane during joint movements (Fig. 4).3. Sensory cells and innervation. CoS-impregnation shows that each slit of the slit sense organs — be it a single slit or several slits in a lyriform organ — is innervated by two bipolar sensory cells (Fig. 6). We also confirm previous reports of multiple innervation in the internal joint receptors and in the long joint hairs and cuticular spines.Most of the ascending nerve branches run just beneath the cuticle for at least a short distance (Fig. 5); hence they are convenient sites for electrophysiological recordings of sensory activity even in freely walking spiders.     

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.     

The quill mite <Emphasis Type="Italic">Syringophilopsis fringilla</Emphasis> (Fritsch) (Acari: Trombidiformes: Syringophilidae): the structure of sensory organs providing feeding of the parasite in the feather quill

The structure of the sensory organs situated on palps and chelicerae of the quill mite Syringophilopsis fringilla (Fritsch, 1958) was examined with the use of scanning and transmitting electron microscopy. The tarsal segment of the palps bears 8 sensilla of three types: two contact chemo-mechanoreceptor sensilla, a single chemoreceptor (olfactory) sensillum, and five tactile mechanoreceptor sensilla. All other sensilla situated on basal palpal segments and on cheliceral stylets are represented exclusively by tactile mechanoreceptors. A proprioceptor sensillum was revealed in the movable digit of chelicerae; the modified cilia of dendrites of 5 sensory neurons of this sensillum run inside the inner non-sclerotized core of the stylet and end at different levels in its apical part, attaching to electron-dense rods connected with a sclerotized sheath of the stylet. The authors assume that the proprioceptor sensillum of the stylet detects the strength of the pressure of the stylet of the movable digit on the quill wall during its piercing, and palpal sensilla determine the optimal place for this process.     

Ultrastructural Analysis of the Sensilla of Austroperipatus aequabilis Reid, 1996 (Onychophora,Peripatopsidae)

The head of Austroperipatus aequabilis bears five types of sensilla. which were examined by electron microscopy. They differ from each other in position, shape of outer sensory elements and cuticular socket structures. Thus, we distinguish sensilla with sensory hairs, sensilla with sensory bulbs, cone-shaped sensilla. sensilla with sensory bristles, and sensilla of the lips. They are composed of up to 15 cells, which can he separated into four cell types. The most frequent cell type is the bipolar receptor cell that occurs in all sensilla. The apical surface of this primary receptor cell is characterized by one or two partly branched cilia with a basal 9 × 2 + 0 pattern of microtubules. A modified bipolar receptor cell was found in all sensilla bearing a sensory peg except for the sensilla equipped with sensory bristles. The apical dendrite extends to a long pale process which exclusively contains mitochondria and single microtubules. In all sensilla examined in this study at least one supporting cell occurs which is characterized by parallel microvilli. An additional function of this cell type as a part of the stimulus-conducting system is possible. In the sensillum with a sensory bulb two kinds of supporting cells occur. A unique cell type with an upside down position has regularly been found in all sensilla bearing a sensory peg. Apart from the sensilla they also occur within the labial epidermis. Since most sensilla contain several different receptor cells, they can be considered as complex sense organs. © 1998 The Royal Swedish Academy of Sciences. Published by Elsevier Science Ltd. All rights reserved     

Ultrastructure of chemosensory tarsal tip-pore sensilla of Argiope spp. Audouin, 1826 (Chelicerata: Araneae: Araneidae)

While chemical communication has been investigated intensively in vertebrates and insects, relatively little is known about the sensory world of spiders despite the fact that chemical cues play a key role in natural and sexual selection in this group. In insects, olfaction is performed with wall–pore and gustation with tip-pore sensilla. Since spiders possess tip-pore sensilla only, it is unclear how they accomplish olfaction. We scrutinized the ultrastructure of the trichoid tip-pore sensilla of the orb weaving spider Argiope bruennichi—a common Palearctic species the males of which are known to be attracted by female sex pheromone. We also investigated the congener Argiope blanda. We examined whether the tip-pore sensilla differ in ultrastructure depending on sex and their position on the tarsi of walking legs of which only the distal parts are in contact with the substrate. We hypothesized as yet undetected differences in ultrastructure that suggest gustatory versus olfactory functions. All tarsal tip-pore sensilla of both species exhibit characters typical of contact-chemoreceptors, such as (a) the presence of a pore at the tip of the sensillum shaft, (b) 2–22 uniciliated chemoreceptive cells with elongated and unbranched dendrites reaching up to the tip-pore, (c) two integrated mechanoreceptive cells with short dendrites and large tubular bodies attached to the sensillum shaft's base, and (d) a socket structure with suspension fibres that render the sensillum shaft flexible. The newly found third mechanoreceptive cell attached to the proximal end of the peridendritic shaft cylinder by a small tubular body was likely overlooked in previous studies. The organization of tarsal tip-pore sensilla did not differ depending on the position on the tarsus nor between the sexes. As no wall-pore sensilla were detected, we discuss the probability that a single type of sensillum performs both gustation and olfaction in spiders.     

The sensory basis for the trap-jaw mechanism in the ant Odontomachus bauri

Ants of the ponerine genus Odontomachus have evolved a mechanism that allows them to instantaneously close their long mandibles to catch prey or defend themselves. This trap-jaw action is triggered by contact of trigger hairs with a potential prey item. Two of these long mechanosensory hair sensilla reside proximally on each mandible and are supplied by giant sensory cells.Extracellular recordings demonstrate that the sensory cells respond to tactile stimulation. Their phasic responses encode amplitude and velocity of hair-deflection away from the midline, but not hair position. The discharge of action potentials follows stimulus frequencies of more than 300 Hz. During sinusoidal stimulation, the cells adapt very little, sustain discharge rates of more than 200 Hz for more than 20 s, and reach peak spike rates of about 450 Hz.The afferent axons of these sensory cells give rise to huge axon terminals within the suboesophageal ganglion. One of the afferents has a prominent contralateral branch, the other is confined to ipsilateral neuropil. Anatomical data indicate that the 4 afferents may be coupled and may serve as the substrate for a very fast reflex.Abbreviations HRP horseradish peroxidase - LGS lateral giant sensillum - MGS median giant sensillum - SEM scanning electron microscopy - SOG suboesophageal ganglion     

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.     

The pedipalp of Pseudocellus pearsei (Ricinulei, Arachnida) - ultrastructure of a multifunctional organ

Ricinulei possess movable, slender pedipalps with small chelae. When ricinuleids walk, they occasionally touch the soil surface with the tips of their pedipalps. This behavior is similar to the exploration movements they perform with their elongated second legs. We studied the distal areas of the pedipalps of the cavernicolous Mexican species Pseudocellus pearsei with scanning and transmission electron microscopy. Five different surface structures are characteristic for the pedipalps: (1) slender sigmoidal setae with smooth shafts resembling gustatory terminal pore single-walled (tp-sw) sensilla; (2) conspicuous long, mechanoreceptive slit sensilla; (3) a single, short, clubbed seta inside a deep pit representing a no pore single walled (np-sw) sensillum; (4) a single pore organ containing one olfactory wall pore single-walled (wp-sw) sensillum; and (5) gustatory terminal pore sensilla in the fingers of the pedipalp chela. Additionally, the pedipalps bear sensilla which also occur on the other appendages. With this sensory equipment, the pedipalps are highly effective multimodal short range sensory organs which complement the long range sensory function of the second legs. In order to present the complete sensory equipment of all appendages of the investigated Pseudocellus a comparative overview is provided.     

Sensilla on the antennal flagellum of the sawyer beetles Monochamus notatus (Drury) and Monochamus scutellatus (Say) (Coleoptera: Cerambycidae)

The antennae of the sawyer beetles Monochamus notatus and M. scutellatus were examined with the light and scanning electron microscopes to determine the types, number, distribution and innervation of the sense organs. Nine types of sensilla are described. Both short, thin-walled pegs (sensilla basiconica) and reversely curved thick-walled hairs (sensilla trichodea) are chemoreceptors. There are three types of long, thick-walled hairs (sensilla chaetica) which may be mechanoreceptors. One of these is modified in males to form unique snail-shaped pegs. A few dome-shaped organs, probably campaniform sensilla, were found. In addition to sense organs, many glands occur in association with the sensilla, and the antennae are well supplied with dermal glands connected by canals to small pores on the surface.     

Water detection in the desert sand scorpion,Paruroctonus mesaensis (Scorpionida,Vaejovidae)

Summary For the sand scorpion, Paruroctonus mesaensis, substrate moisture is a powerful and fast-acting stimulus of discrete behaviors related to localization and imbibitory uptake of water. These behaviors are readily observed in the field and quantified in the laboratory when free-roaming animals encounter sand substrates dampened by small amounts of water. Of 10 behaviors we monitored in laboratory tests, 5 (pedipalp-pull, rototiller-digging, prolonged stops, headstand, and backingup) occurred only after contact with a moistened substrate. These water-stimulated behaviors were selectively blocked when all 8 tarsal leg segments were coated with wax; coverings of the chemosensory pectine appendages had little to no effect. Electrophysiological recordings from chemoreceptor organs on the tarsi showed that neurons innervating the dorsal tarsal organ, were highly sensitive to humid air stimuli while the numerous, poretipped hairs on the ventral surface were responsive to aqueous solutions applied directly to their tips. Selective blocking of the 8 tarsal organs had no effect on water sensitive behavior indicating that the chemosensory hairs mediate detection of substrate moisture. Such localized, sensory triggering of a robust and directed behavior presents a useful model for further neuroethological studies.Abbreviations H headstand - PP pedipalp-pull - RD rototillerdigging - B backing-up - C wall-climbing - P pause - W walk-through - S stop - T turn - R rest - PST pacific standard time - SEM scanning electron microscopy     

Sensilla and Cuticular Appendages on the Female Abdomen of Lasioptera rubi (Schrank) (Diptera,Cecidomyiidae)

Studies by SEM and TEM revealed 6 types of integumental appendages on female uromeres VIII-X in Lasioptera rubi: microtrichia, not innervated; spines, probably without sensory function; nonporous sensory hairs, each containing one dendrite ending with a tubular body indicating a tactile function; uniporous sensory hairs, each innervated partly by 3 dendrites indicating a chemosensory function, partly by an additional dendrite with a tubular body indicating a tactile function; scoop-like sensilla, each containing partly a branched structure of dendrites in the distal half of the sensillum indicating an olfactory function, partly an unbranched dendrite ending at a pore near the base of the sensillum, most probably registrating chemical stimuli by contact or gustation; finally, nonporous bristles, all or some of them innervated, in a manner indicating a tactile function. In addition, two scolopophorous proprioceptors were found inside uromere X. The nonporous sensory hairs, the uniporous sensory hairs and the scolopophores may be used by the midge to determine the mechanical and chemical properties of potential oviposition sites. The spines and nonporous bristles may function as conidia carriers.     

Sex-specific non-pheromonal taste receptors in Drosophila

Taste receptorshave recently been reported in Drosophila [1] and [2], but little is known of the relation between receptor and response. Morphological studies of the distribution of chemosensory sensilla indicate that the fruit fly has two major sites of gustation: the proboscis and the legs [3]. The taste sensilla on both these sites are similar in structure and each sensillum generally houses four gustatory neurons [4]. Early anatomical observations have demonstrated a sexual dimorphism in the number of tarsal sensilla [5] and in their central projections [6]. We measured the electrophysiological responses of the prothoracic taste sensilla to non-pheromonal substances—salts, sugars and water—and found a clear sexual dimorphism. From the response profile of individual sensilla, we were able to distinguish three types of tarsal sensilla in females as against only two types in males. The female-specific type, which responded specifically to sugar, was absent in males except when male gustatory neurons were genetically feminised. The fact that tarsal gustatory hairs exhibit a sexual dimorphism that affects the perception of non-pheromonal compounds suggests that sexual identity is more complex than has previously been thought [7] and [8].     

Hygro- and thermoreceptors in tip-pore sensilla of the tarsal organ of the spider Cupiennius salei: innervation and central projection

The hygro- and thermoreceptive tarsal organ in the wandering spider Cupiennius salei is located on the tarsus of each walking leg and pedipalp, and consists of a tiny air-filled capsule in the cuticle. This capsule communicates with the outside world through a small aperture and contains seven nipple-shaped sensilla, each with a pore at its tip. In both their external morphology and internal structure, the sensilla are indistinguishable, although one sensillum is innervated by only two sensory cells, whereas the other six sensilla contain three sensory cells. Their dendrites are unbranched and terminate at the tip-pore, where they are enveloped by amorphous material that appears to limit their exposure to the atmosphere. Cobalt fillings reveal that each tarsal organ projects to three different areas within the suboesophageal ganglionic mass: (1) the sensory longitudinal tract 3 and 4; (2) the corresponding pedipalpal or leg ganglion; (3) a structured neuropil (here termed the Blumenthal neuropil) beneath the oesophagus. The multiple representation of sensory afferents from each tarsal organ in different regions of the suboesophageal ganglionic mass suggests parallel processing of hygro-/thermoreceptive information.     

Detecting substrate engagement: responses of tarsal campaniform sensilla in cockroaches

Sensory signals of contact and engagement with the substrate are important in the control and adaptation of posture and locomotion. We characterized responses of campaniform sensilla, receptors that encode forces as cuticular strains, in the tarsi (feet) of cockroaches using neurophysiological techniques and digital imaging. A campaniform sensillum on the fourth tarsal segment was readily identified by its large action potential in nerve recordings. The receptor discharged to contractions of the retractor unguis muscle, which engages the pretarsus (claws and arolium) with the substrate. We mimicked the effects of muscle contractions by applying displacements to the retractor apodeme (tendon). Sensillum firing did not occur to unopposed movements, but followed engagement of the claws with an object. Vector analysis of forces suggested that resisted muscle contractions produce counterforces that axially compress the tarsal segments. Close joint packing of tarsal segments was clearly observed following claw engagement. Physiological experiments showed that the sensillum responded vigorously to axial forces applied directly to the distal tarsus. Discharges of tarsal campaniform sensilla could effectively signal active substrate engagement when the pretarsal claws and arolium are used to grip the substrate in climbing, traversing irregular terrains or walking on inverted surfaces.     

Central nervous projections of mechanoreceptors in the spider Cupiennius salei Keys.

Summary The basic organization of sensory projections in the suboesophageal central nervous system of a spider (Cupiennius salei Keys.) was analyzed with anterograde cobalt fills and a modified Golgi rapid method. The projections of three lyriform slit sense organs and of tactile hairs located proximally on the legs are described and related to central nerve tracts. There are five main longitudinal sensory tracts in the central region of the suboesophageal nervous mass arranged one above the other. Whereas the three dorsal ones contain fibers from the lyriform organs, the two ventral ones contain axons from the hair receptors. Axons from all three lyriform organs have typical shapes and widely arborizing ipsilateral intersegmental branches and a few contralateral ones. The terminal branches of the afferent projections from identical lyriform organs on each leg form characteristic longitudinal pathways, typical of each organ: U-shaped, O-shaped, or two parallel bundles. The terminations of the hair sensilla are ipsilateral and intersegmental. Two large bilaterally arranged longitudinal sensory association tracts receive inputs from all legs including the dense arborizations from tactile hairs, lyriform organs, and other sense organs. These tracts may serve as important integrating neuropils of the suboesophageal central nervous system.     

Single Sensillum Recordings in the Insects Drosophila melanogaster and Anopheles gambiae

The sense of smell is essential for insects to find foods, mates, predators, and oviposition sites³. Insect olfactory sensory neurons (OSNs) are enclosed in sensory hairs called sensilla, which cover the surface of olfactory organs. The surface of each sensillum is covered with tiny pores, through which odorants pass and dissolve in a fluid called sensillum lymph, which bathes the sensory dendrites of the OSNs housed in a given sensillum. The OSN dendrites express odorant receptor (OR) proteins, which in insects function as odor-gated ion channels^{4, 5}. The interaction of odorants with ORs either increases or decreases the basal firing rate of the OSN. This neuronal activity in the form of action potentials embodies the first representation of the quality, intensity, and temporal characteristics of the odorant^{6, 7}.Given the easy access to these sensory hairs, it is possible to perform extracellular recordings from single OSNs by introducing a recording electrode into the sensillum lymph, while the reference electrode is placed in the lymph of the eye or body of the insect. In Drosophila, sensilla house between one and four OSNs, but each OSN typically displays a characteristic spike amplitude. Spike sorting techniques make it possible to assign spiking responses to individual OSNs. This single sensillum recording (SSR) technique monitors the difference in potential between the sensillum lymph and the reference electrode as electrical spikes that are generated by the receptor activity on OSNs^{1, 2, 8}. Changes in the number of spikes in response to the odorant represent the cellular basis of odor coding in insects. Here, we describe the preparation method currently used in our lab to perform SSR on Drosophila melanogaster and Anopheles gambiae, and show representative traces induced by the odorants in a sensillum-specific manner.Open in a separate windowClick here to view.^{(78M, flv)}     

Spider mechanoreceptors

Spiders have highly developed mechanosensory systems, some of which provide access to forms of stimulation alien to our own sensations. Studies of hair-shaped air movement detectors (trichobothria) and tactile sensors have uncovered an outstanding refinement of the processes of stimulus uptake and stimulus transformation, which reflect details of both stimulus physics and behavioral significance. They also emphasize the potential contained in the seemingly simple Bauplan of arthropod cuticular hairs. Embedded into the spider exoskeleton are several thousands of strain detectors (slit sensilla) measuring compressive exoskeletal strains induced by various forms of loads and forces. A compound slit sensillum (lyriform organ) on the leg has become an important model system for studies of mechanoreceptor primary processes at the cellular and membrane level.     

Sensory organs of the thoracic legs of the moth Manduca sexta

Summary The thoracic legs of the moth Manduca sexta acquire a new form and develop a new complement of sensory organs and muscles during metamorphosis from larva to adult. Because of our interest in the reorganization of neural circuitry and the acquisition of new behaviors during metamorphosis, we are characterizing sensory elements of larval and adult legs so that we may determine the contribution of new sensory inputs to the changes in behaviors. Here we describe the sensory structures of adult legs using scanning electron microscopy to view the external sensilla and cobalt staining to examine innervation by underlying sensory neurons. We find that, in contrast to larval legs, the adult legs are covered with a diverse array of sensilla. All three pairs of thoracic legs contain scattered, singly innervated scalelike sensilla. Campaniform sensilla occur singly or in clusters near joints. Hair plates, consisting of numerous singly innervated hairs, are also present near joints. Other more specialized sensilla occur on distal leg segments. These include singly innervated spines, two additional classes of singly innervated hairs, and three classes of multiply innervated sensilla. Internal sensory organs include chordotonal organs, subgenual organs, and multipolar joint receptors.