Structure and mechanics of the tarsal chain in the hornet, Vespa crabro (Hymenoptera: Vespidae): implications on the attachment mechanism

Two combined mechanisms on the hornet tarsus are adapted to attachment to the substrate: a friction-based (claws and spines) and an adhesion-based one (arolium). There are two ranges of substrate roughness optimal for attachment, either very smooth or very rough. There is an intermediate range of substrate grains of small but non-zero size, where both of these mechanisms fail. The optimal size of substrate grains for hornet grasping was 50-100 microm. Maximal hold to the substrate was achieved when surface irregularities were clamped between the claws of opposite legs. In such a position, the insect could withstand an external force which was almost 25 times larger than its own weight. The tarsal chain is an important part of the entire attachment mechanism. The articulations in the kinematic chain of tibia-tarsus-pretarsus are monocondylar. Three tarsal muscles and one head of the claw retractor muscle originate in the tibia. On pull to the retractor tendon, the tarsus bends in a plane. All elements of the tarsal kinematic chain have one active degree of freedom. The distance between the intertarsomeric articulation point and the tendon of the claw retractor (75-194 microm) corresponds to an efficiency of 1 degrees per 1-3 mircom of pulling distance travelled by the tendon. The claw turns about 1 degrees per 4.3-5.0 microm of pulling distance travelled by the unguitractor. The arolium turns forward and downward simultaneously with flexion of the claws. The kinematic chain of the arolium lacks real condylar joints except the joint at the base of the manubrium. Other components are tied by flexible transmissions of the membranous cuticle. The walking hornet rests on distal tarsomeres of extended tarsi. If the retractor tendon inside the tarsus is fixed, passive extension of the tarsomeres might be replaced by claw flexion. Tarsal chain rigidity, measured with the force tester, increased when the retractor tendon was tightened. Probably, pull to the tendon compresses the tarsomeres, increasing friction within contacting areas of rippled surfaces surrounding condyles within articulations.     

The problem of the number of tarsomeres in the regenerated cockroach leg.

There are 5 tarsomeres in the normal cockroach leg, but this number is often reduced in regenerated legs. In order to examine this complicated situation, fore-, mid-, and hindlegs of German cockroaches were amputated at 11 different tarsal levels and at 18 different times during the last instar. When tarsi were amputated at or proximal to the 3rd tarsomere, 4-segmented tarsi regenerated. When legs were amputated distal to the 3rd tarsomere, the regenerated tarsi had 5 segments. Three-segmented tarsi rarely regenerated when legs were amputated proximal to 3rd tarsomere and in the latter half of the instar period. The lengths of all tarsomeres of regenerated tarsi were measured together with those of unoperated contralateral tarsomeres, and the ratios of the former to the latter were calculated. The ratios ranged from 28 to 138% for the various tarsomeres and levels of amputation. From a comparison of the ratios and morphological observations, it was suggested that the 3rd tarsomere of the normal 5-segmented tarsus has disappeared in the regenerated 4-segmented tarsus. Pads and disto-lateral spines of tarsomeres were observed on unoperated and regenerated tarsi. It was of interest that double spines were often found on the 4-segmented tarsi, mostly on the 2nd tarsomere, just proximal to the position of the missing 3rd tarsomere. This observation supported the idea that the 3rd tarsomere has not simply disappeared, but has probably fused with the 2nd tarsomere.     

Structure of the tarsi in some Stenus species (Coleoptera,Staphylinidae): external morphology,ultrastructure, and tarsal secretion

SEM studies show that the differentiation among Stenus species with respect to the formation of the tarsi (wide bilobed vs. slender tarsomeres) takes place with a considerable augmentation of tarsal ventral setae in wide bilobed tarsomeres. The structural diversity of ventral tarsal setae among and within species is discussed with respect to 1) their different roles as mechanosensilla and tenent setae, respectively, and 2) the different selection pressures in terms of adhesive requirements along the longitudinal tarsus axis. The tarsi are provided with four groups of tarsal mechanosensilla, comprising hair and bristle sensilla, campaniform sensilla, and scolopidia. The tarsus wall is supported by an epidermis, which forms three different types of glands pouring their secretion via different exit paths onto the outer cuticle. The organization and ultrastructure of each of these glands is described. Only one (unicellular) gland is directly associated with the ventral tenent setae and is thus considered to form the main part of the adhesive secretion. The beetles appear to release the tarsal secretion through mediation of the tenent setae, which contains a lipid and a proteinaceous fraction. I propose that the secretion is discharged to the outside via a system of very fine pore canals in the wall of the setal shaft. Gas chromatography and infrared spectroscopy revealed that the lipid fraction of the secretion is a mixture of unsaturated fatty acid glycerides and aliphatic hydrocarbons whose spectra are similar to those of extractions of the superficial lipid coating of the body surface.     

Gripping ease in southern green stink bugs Nezara viridula L. (Heteroptera: Pentatomidae): Coping with geometry,orientation and surface wettability of substrate

The southern green stink bug Nezara viridula L. (Heteroptera, Pentatomidae) is highly polyphagous, preferring apically situated seeds and fruits on more than 150 plant species belonging to over 30 plant families all over the world. This forces them to move over highly variable terrains, including plant stems, leaves, pods and buds, which requires efficient attachment. Stink bugs have long slender legs and feet (tarsi) equipped with paired curved claws, paired soft adhesive pads (pulvilli), and flattened lanceolate hairs (setae), which arise ventrally on the first and second foot segments (tarsomeres). To characterize their attachment abilities on well‐defined test substrates, here we comparatively measured and analyzed the traction forces of bugs walking horizontally and vertically on hydrophilic (water attractive) and hydrophobic (water repellent) glass plates and rods. The latter correspond to the geometry of preferred feeding sites of stink bugs in the field. The results show a clear contribution of tarsal flattened lanceolate hairs to the stink bug's attachment. Higher traction forces are generated on a glass rod than on a glass plate, corresponding to up to individual maximum of 43 times the stink bug's body weight. Substrate hydrophobicity promotes the attachment, while the measured forces are up to eight times lower when tarsal hairs are disabled. The combination of smooth and hairy tarsal pads results in a remarkable attachment ability, which enables N. viridula to climb unstable apical plant parts, and supports their invasive behavior and global dispersion.     

Giant stick insects reveal unique ontogenetic changes in biological attachment devices

A strong modification of tarsal and pretarsal attachment pads during the postembryonic development is described for the first time. In the exceptionally large thorny devil stick insect Eurycantha calcarata a functional arolium is only present in the immature instars, enabling them to climb on smooth surfaces, especially leaves. Nymphs are also characterized by greyish and hairy euplantulae on tarsomeres 1–4. The gradual modifications of the arolium and the euplantula of tarsomere 5 in the nymphal development are probably mainly related to increased weight. The distinct switch in the life style between the leaf-dwelling nymphal stages and the ground-dwelling adults results in the final abrupt change of the adhesive devices, resulting in a far-reaching reduction of the arolium, the presence of a fully-developed, elongated euplantula on tarsomere 5, and white and smooth euplantulae on tarsomeres 1–4. The developmental remodelling of attachment pads also reflects a phylogenetic pattern. The attachment devices of the earlier instars are similar to those found in the basalmost lineage of extant stick insects, Timema, which is characterized by a very large pan-shaped arolium and a hairy surface of the tarsal and pretarsal attachment pads.     

Bowl is required downstream of Notch for elaboration of distal limb patterning

In the Drosophila leg, activation of Notch leads to the establishment of the joints that subdivide the appendage into segments. We find that mutations in bowl result in similar phenotypes to Notch, causing fusion and truncations of tarsal segments (tarsomeres) and, like its close relative Odd-skipped, Bowl is produced in response to Notch signalling at a subset of segment boundaries. However, despite the fact that bowl mutant clones result in fusion of tarsomeres, Bowl protein is only found at the t1/tibial and t5/pretarsal boundaries, not at tarsomere joints. One hypothesis to reconcile these data is that bowl has a role at an earlier stage in tarsal development. We therefore investigated the effects of bowl mutations on the expression of leg 'gap' genes that confer regional identity on the developing leg. Several of these genes have altered expression in bowl mutant cells. For example, bric-a-brac2 is normally expressed in the central part of the tarsus domain but expands into distal and proximal regions in bowl clones. Conversely, ectopic bowl leads to a reduction in bric-a-brac2, with a concomitant expansion of proximal (t1) and distal (t5) tarsomere fates. The bowl gene is therefore required for the elaboration of pattern in the tarsus and its effects suggest a progressive model for the determination of P/D identities. This mechanism might be important in the diversification of arthropod limbs, because it explains how segmented tarsomeres could have arisen from an ancestral limb with an unsegmented tarsus.     

On Heels and Toes: How Ants Climb with Adhesive Pads and Tarsal Friction Hair Arrays

Ants are able to climb effortlessly on vertical and inverted smooth surfaces. When climbing, their feet touch the substrate not only with their pretarsal adhesive pads but also with dense arrays of fine hairs on the ventral side of the 3^rd and 4^th tarsal segments. To understand what role these different attachment structures play during locomotion, we analysed leg kinematics and recorded single-leg ground reaction forces in Weaver ants (Oecophylla smaragdina) climbing vertically on a smooth glass substrate. We found that the ants engaged different attachment structures depending on whether their feet were above or below their Centre of Mass (CoM). Legs above the CoM pulled and engaged the arolia (‘toes’), whereas legs below the CoM pushed with the 3^rd and 4^th tarsomeres (‘heels’) in surface contact. Legs above the CoM carried a significantly larger proportion of the body weight than legs below the CoM. Force measurements on individual ant tarsi showed that friction increased with normal load as a result of the bending and increasing side contact of the tarsal hairs. On a rough sandpaper substrate, the tarsal hairs generated higher friction forces in the pushing than in the pulling direction, whereas the reverse effect was found on the smooth substrate. When the tarsal hairs were pushed, buckling was observed for forces exceeding the shear forces found in climbing ants. Adhesion forces were small but not negligible, and higher on the smooth substrate. Our results indicate that the dense tarsal hair arrays produce friction forces when pressed against the substrate, and help the ants to push outwards during horizontal and vertical walking.     

Friction force reduction triggers feet grooming behaviour in beetles

In insects, cleaning (grooming) of tarsal attachment devices is essential for maintaining their adhesive ability, necessary for walking on a complex terrain of plant surfaces. How insects obtain information on the degree of contamination of their feet has remained, until recently, unclear. We carried out friction force measurements on walking beetles Gastrophysa viridula (Coleoptera, Chrysomelidae) and counted grooming occurrence on stiff polymer substrata with different degrees of nanoroughness (root mean square: 28-288 nm). Since nanoscopically, rough surfaces strongly reduced friction and adhesion without contaminating feet, we were able to demonstrate, for the first time to our knowledge, that friction force between tarsal attachment pads and the substrate provides an insect with information on the degree of contamination of its attachment structures. We have shown that foot grooming occurrence correlates not only with the degree of contamination but also with the decrease of friction force. This result indicates that insects obtain information about the degree of contamination, not statically but rather dynamically and, presumably, use mechanoreceptors monitoring either tensile/compressive forces in the cuticle or tensile forces between leg segments.     

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.     

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.     

Elasticity and movements of the cockroach tarsus in walking

Anatomical, kinematic and ablation studies were performed to evaluate the contribution of elasticity in use of the cockroach tarsus (foot) in walking. The distal tarsus (claws and arolium) engages the substrate during the stance phase of walking by the action of a single muscle, the retractor unguis. Kinematic and ablation studies demonstrated that tarsal disengagement occurs at the end of stance, in part via the action of elastic elements at the penultimate tarsal joint. In isolated legs, this joint exhibits very rapid (less than 20 ms duration) recoil to extension when released from the engaged position, and recoil is even more rapid (less than 10 ms) after removal of the retractor tendon (apodeme). The joint also possesses an enlarged cuticular condyle which is the attachment for ligaments and articular membranes, some of which fulfill morphological criteria consistent with the presence of the elastic protein resilin. Measurements of restoring forces generated by joint displacement indicate that they are graded but could readily lift the mass of the distal tarsus. This biomechanical design can facilitate efficient use of the tarsus in walking while under active control by only a single muscle and may also be highly advantageous when cockroaches very rapidly traverse irregular terrain. Accepted: 16 September 1998     

Leg coordination during turning on an extremely narrow substrate in a bug, Mesocerus marginatus (Heteroptera, Coreidae)

The turning movement of a bug, Mesocerus marginatus, is observed when it walks upside-down below a horizontal beam and, at the end of the beam, performs a sharp turn by 180 degrees . The turn at the end of the beam is accomplished in three to five steps, without strong temporal coordination among legs. During the stance, leg endpoints (tarsi) run through rounded trajectories, rotating to the same side in all legs. During certain phases of the turn, a leg is strongly depressed and the tarsus crosses the midline. Swing movements rotate to the same side as do leg endpoints in stance, in strong contrast to the typical swing movements found in turns or straight walk on a flat surface. Terminal location is found after the search through a trajectory that first moves away from the body and then loops back to find substrate. When a leg during stance has crossed the midline, in the following swing movement the leg may move even stronger on the contralateral side, i.e. is stronger depressed, in contrast to swing movements in normal walking, where the leg is elevated. These results suggest that the animals apply a different control strategy compared to walking and turning on a flat surface.     

Evolution of locomotory attachment pads in the Dermaptera (Insecta)

This contribution is the first comparative SEM study of tarsal and pretarsal structures of 18 dermapteran species, including epizoic Hemimeridae, rare Apachyidae, as well as basal Pygidicranidae. Our data reject the apparent uniformity of this taxon and show that representatives of Dermaptera have independently evolved both types of attachment mechanisms: hairy and smooth. Dermaptera possess a wide spectrum of attachment devices: arolia, euplantulae, tarsal surfaces covered with specialised tenent setae and other types of cuticular outgrowths. The groundpattern of the pretarsal and tarsal attachment structures was reconstructed by mapping their characters onto a cladogram, generated without tarsal characters. In the groundpattern of recent Dermaptera, the tarsus consists of three tarsomeres. Presumably, the last common ancestor of the Dermaptera possessed an arolium, since this structure occurs in the most basal taxa: Diplatyidae, Karschiellidae (partim, adults), Pygidicranidae partim, and Apachyidae. The absence of arolium in two of the pygidicranid taxa is probably due to a secondary loss. The arolium seems to be reduced in the 'higher Dermaptera' and amongst them, only the Geracinae, which belong to the Spongiphoridae and, hence, to the well supported Eudermaptera [European Journal of Entomology, 98 (2001), 445], evolved this structure convergently. The character state distribution for euplantulae suggests their evolution being similar to that of the arolium. All species of Tagalina possess a specialised tarsus with a strongly dilated second tarsomere. The same applies to the Forficulidae. However, their relatively remote phylogenetic position to Tagalina burri is a convincing reason to assume convergent evolution of this character. The Chelisochidae, with a slender, elongated second tarsomere, possess a unique structure, which supports their monophyly. The special, heart shaped structure of the second tarsal segments in the Forficulidae suggests their monophyly. The attachment structures of Hemimerus vosseleri are highly derived and probably autapomorphic for this taxon.     

Tarsal chemoreception in the polyphagous grasshopper Schistocerca americana: behavioural assays, sensilla distributions and electrophysiology

ABSTRACT Behavioural and electrophysiological responses of Schistocerca americana (Drury) (Orthoptera: Acrididae) to chemical stimulation of the tarsi were investigated. Using restrained insects, differences in leg-waving behaviour were observed following stimulation by sucrose and nicotine hydrogen tartrate (NHT), compared to control stimulations by water. Furthermore, free-walking insects were able to detect NHT on leaf surfaces, resulting in leg-raising to avoid tarsal contact.
SEM studies showed the presence of numerous peg chemoreceptor sensilla on the ventral surface of the tarsus. Tip recordings from such pegs showed activity from up to three chemosensitive neurones, plus a mechanoreceptor neurone. Stimulation by NaCl and KC1 elicited similar responses from two or three neurones in all sensilla tested, with increased firing rates at higher concentrations. Sucrose caused an increase in firing rate in few sensilla. In such cases several neurones were stimulated, and there was no evidence of a specific neurone sensitive to sucrose. In contrast, NHT elicited rapid firing in a single neurone, which was not sensitive to NaCl. Stimulation by NHT also inhibited the activity of the NaCl-sensitive neurones.
Possible mechanisms for chemical discrimination in S. americana tarsi are compared with those previously proposed for grasshopper mouthpart sensilla, and the significance of a NHT-sensitive neurone in tarsal sensilla is discussed.     

The tarsus of rhynchocephalian reptiles

A new reconstruction of the tarsus of Stenaulorhynchus stockleyi differs from that previously offered by von Huene (1938) and Schaeffer (1941), and leads to a revaluation of the tarsi of other rhynchosaurs and rhynchocephalians. The tarsi of the rhynchosaurs Stenaulorhynchus, Scaphonyx, Cephalonia, Rhynchosaurus, Hyperodapedon, Howesia and Mesosuchus appear to comprise a proximal row of three bones (tibiale, intermedium, and fibulare) and a distal row of three bones associated with metatarsals–4. The metatarsals increase in length but decrease in thickness from the first to fourth, and the fifth is "hooked". The first metatarsal of Stenaulorhynchus, Scaphonyx , and Rhynchosaurus is unusually short. Among the remaining rhynchocephalians, the tarsi of claraziids and pleurosaurids are simplified in accord with aquatic habits involving the use of the limbs as paddles, and those of sphenodontids and Sapheosaurus are alike in the following respects. An astragalus (intermedium+tibiale+ centrale) and calcaneum (fibulare), or a single conjoined bone, occupy the proximal row of the tarsus and the distal row comprises four (first to fourth), three (second to fourth), or but two (third and fourth) bones; the fourth is the largest. The metatarsals increase in length from first to fourth but do not decrease in thickness. In both rhynchosaurian and spheno-dontid types tarsal movement is largely mesotarsal, a condition derived from their eosuchian ancestors and not independently developed as Schaeffer (1941) has thought. The specialization of the sphenodontid tarsus parallels that seen in lizards which have the same eosuchian ancestry.     

Sperm transfer and mating in Ricinoides hanseni (Ricinulei: Arachnida)

Ricinuleid reproduction involves indirect sperm transfer using the highly modified distal podomeres of the third legs of the male. This is homologous with the apparatus and technique used by male spiders, which possess elaborate pedipalps. The interpretation of the method of sperm transfer is based upon morphological studies of the male's third legs and the female's genital atrium and the behaviour of males during mating. The male charges the emboli of his modified leg tarsi with sperm from his penis. After climbing on to the back of a receptive female he delicately and precisely places a modified tarsus in the genital atrium of the female. A series of lobes on the tip of part of the modified tarsus fit into a number of vesicular evaginations of the female's genital atrium. The lobes form part of the mechanism which provides a firm attachment of the male's tarsal elements with the female's genital atrium during sperm transfer. A tubular element of the modified tarsus fits into a spermatheca of the female. Sperm and seminal fluid are then injected from the male's embolus into the female's spermatheca.     

Two functional types of attachment pads on a single foot in the Namibia bush cricket Acanthoproctus diadematus (Orthoptera: Tettigoniidae)

Insects have developed different structures to adhere to surfaces. Most common are smooth and hairy attachment pads, while nubby pads have also been described for representatives of Mantophasmatodea, Phasmida and Plecoptera. Here we report on the unusual combination of nubby and smooth tarsal attachment structures in the !nara cricket Acanthoproctus diadematus. Their three proximal tarsal pads (euplantulae) have a nubby surface, whereas the most distal euplantula is rather smooth with a hexagonal ground pattern resembling that described for the great green bush-cricket Tettigonia viridissima. This is, to our knowledge, the first report on nubby euplantulae in Orthoptera and the co-occurrence of nubby and smooth euplantulae on a single tarsus in a polyneopteran species. When adhering upside down to a horizontal glass plate, A. diadematus attaches its nubby euplantulae less often, compared to situations in which the animal is hanging upright or head down on a vertical plate. We discuss possible reasons for this kind of clinging behaviour, such as morphological constrains, the different role of normal and shear forces in attachment enhancement of the nubby and smooth pads, ease of the detachment process, and adaptations to walking on cylindrical substrates.     

Positive force feedback in development of substrate grip in the stick insect tarsus

The mechanics of substrate adhesion has recently been intensively studied in insects but less is known about the sensorimotor control of substrate engagement. We characterized the responses and motor effects of tarsal campaniform sensilla in stick insects to understand how sensory signals of force could contribute to substrate grip. The tarsi consist of a chain of segments linked by highly flexible articulations. Morphological studies showed that one to four campaniform sensilla are located on the distal end of each segment. Activities of the receptors were recorded neurographically and sensilla were identified by stimulation and ablation of their cuticular caps. Responses were characterized to bending forces and axial loads, muscle contractions and to forces applied to the retractor apodeme (tendon). The tarsal sensilla effectively encoded both the rate and amplitude of loads and muscle forces, but only when movement was resisted. Mechanical stimulation of the receptors produced activation of motor neurons in the retractor unguis and tibial flexor muscles. These findings indicate that campaniform sensilla can provide information about the effectiveness of the leg muscles in generating substrate adherence. They can also produce positive force feedback that could contribute to the development of substrate grip and stabilization of the tarsal chain.     

The Immediate Source of the Oviposition-Deterring Pheromone Produced by Larvae of Adalia bipunctata (L.) (Coleoptera, Coccinellidae)

As is the case for other insects ovipositing on or in resources that are limited in time and/or space, the two-spot ladybird beetle, Adalia bipunctata (L.) produces an oviposition-deterring pheromone (ODP), which is produced by the larval stages. Foraging larvae touch the substrate with their tarsi and the anal disk on the tenth abdominal segment. The aim of this paper was to determine whether the ODP produced by larvae was deposited by the tarsi or the anal disk. Fourth instar larvae either had their anal disk and tarsi, or anal disk, or tarsi coated with a water-soluble mounting medium. Larvae so treated were allowed to walk on filter paper that was subsequently presented to gravid females. The tracks of larvae that had both their tarsi and anal disk masked did not inhibit oviposition. However, the tracks of larvae that had only their tarsi masked significantly inhibited oviposition but those of larvae that had only their anal disk masked did not. It is concluded that the ODP is deposited on the substrate by the anal disk on the tenth abdominal segment of larvae.     

Detection of vibrations in sand by tarsal sense organs of the nocturnal scorpion,Paruroctonus mesaensis

Summary The scorpionParuroctonus mesaensis locates prey by orienting to substrate vibrations produced by movements of the prey in sand. At the end of each walking leg of this scorpion there are two sense organs, the basitarsal compound slit sensillum and tarsal sensory hairs (Figs. 1, 3) that are excited by substrate vibrations conducted through sand. The slit sensilla appear to be most sensitive to surface (Rayleigh) waves while the tarsal sensory hairs respond best to compressional waves (Fig. 7). Both mechanoreceptors were activated by nearby disturbances of the substrate (Fig. 6) but only the slit sensilla responded to insects moving more than 15 cm away. Both receptors are highly sensitive to small amplitude (less than 10 Å) mechanical stimuli applied to the tarsus (Fig. 5).Behavioral studies of scorpions with ablated sense organs (Fig. 2) indicate that the basitarsal compound slit sensilla are necessary for determining vibration source direction.Abbreviation BCSS basitarsal compound slit sensillum (a) Supported by PHS Environmental Science and Regents Intern Fellowships (PB), and by intramural research funds from the University of California (RDF)