J. exp. BioL (1980), 87, 163-175 163JVith 6 figures

in Great Britain

THE FUNCTION OF HAIR SENSILLA ON THE LOCUST'SLEG: THE ROLE OF TIBIAL HAIRS

BY HANS-JOACHIM PFLt)GER*tDepartment of Biology, University, P.O. Box 8640D-4800 Bielefeld 1, Federal Republic of Germany

{Received 31 October 1979)

SUMMARY

In Schistocerca americana gregaria hairs of the simple trichoid sensillumtype on the hind tibia near the tibia-tarsus joint can mediate an avoidanceresponse during which the hind leg is lifted by flexion of several joints. Theresponsiveness of this reflex can be improved by partially isolating the meta-thoracic ganglion. In these operated animals, the response can even be elicitedafter stimulation of one hair.

Myograms from various muscles in the tibia, femur, coxa and thoraxrevealed which muscles were active during the avoidance response. Thedelay times between the stimulus (bending several hairs with an eyelash)and the first muscle activity are variable (median values 150-200 msec). It isconcluded that the pathway is polysynaptic and situated within the meta-thoracic ganglion.

INTRODUCTION

Hairs (= trichoid sensilla) are a conspicuous feature of the cuticle of insects butonly occasionally have attempts been made to reveal their role in behaviour. Tactilestimuli applied to the upper surface of the tarsus or to a tibial spine of a leg of a cock-roach lead to a momentary lifting of this leg (Pringle, 1940). Light touches elicitedspikes only in the slow motoneurones, whereas stronger stimuli also excite the fastmotoneurones of the extensor tibiae muscle (levator). The excitation of the extensortibiae muscle (levator) was accompanied by a corresponding inhibition of the extensortrochanteris muscle (depressor) (Pringle, 1940). As these tactile stimuli excite cam-paniform and hair sensilla, it is not clear whether stimulation of the hairs alone wouldbe sufficient to elicit the movement of the leg. Stimulation of a single prominenttrichoid sensillum on the hind tarsus of different species of locusts can activate the fastextensor tibiae muscle (Runion & Usherwood, 1968). In order to activate the slowextensor tibiae motoneurone or the common inhibitor neurone, it was necessary tostimulate at least four sensilla either simultaneously or in close succession. A groomingreflex of the locust's front leg is mediated by hair sensilla of the sternal region (Rowell,1969). Stimulation of interommatidial hairs in Gryllus campestris can induce the com-

• Present address: Dept. of Biology, University, P.O. Box 5560, D-7750 Konstanz, Fed. Rep. ofnany.With support from the DFG.

Germany

164 H.-J. PFLUGER

plex grooming movements of a foreleg (Honegger, Reif & Miiller, 1979). Filiforp*hairs can also detect airborne vibrations and mediate complex escape responses iffcockroaches (Roeder, 1963; Nicklaus, 1965) and caterpillars (Tautz & Markl, 1978).

The numerous trichoid sensilla on the legs are organized either into distinct platesat the proximal joints (coxal-thoracic and coxal-trochanteral) or distributed over thewhole leg, often in rows along the ridges of the appendages. Only hairs distributed overthe whole leg will be considered in this article; hair plates in locusts will be discussedin a following paper (H.-J. Pfluger, in preparation).

Most hairs are of the simple 'sensilla trichodea' type (see Dethier, 1963), whichvary considerably in length. Those hairs which are organized into distinct plates on thecoxa and trochanter do not exceed 50 /im in length. Sensilla of this type are also foundon all segments of the leg, but their number is considerably smaller than of the longer'sensilla trichodea'.

I shall show in this paper that after stimulation of hairs at the tibial-tarsal joint ofthe hind leg of the locust, this leg is lifted and protracted. The leg is therefore movedaway from the offending stimulus (avoidance-response). When the metathoracicganglion, which controls the hind leg is isolated from the rest of the CNS, the move-ment of the leg can be elicited more reproducibly.

METHODSBehavioural experiments

Locusts, Schtstocerca americana gregaria (Dirsh, 1974) of either sex, were takenfrom our own crowded cultures. The intact or operated animals were restrained in anormal posture on a plasticine platform by fixing their front and middle legs. Motionof the hind leg was allowed (i) about all joints, or (ii) about one joint with the restfixed by dental wax (Scutan, Espe-Chemie). In a series of experiments movementabout each of the three joints considered was permitted in sequence. In other experi-ments motion of the hind leg was prevented by rigidly fixing all segments of the legto the platform.

Single hairs on the tibia were stimulated by an eyelash glued to a glass rod whichwas moved by hand. The movement of the rod was viewed through a binocularmicroscope to be sure that only hairs were stimulated. When the eyelash was aboutto touch a hair, an electrical contact was closed by the fingers holding the glass rod.The delay between this electrical signal and the first sensory spike was calculatedfrom the recordings of the tibial nerve (see below). In all tests this delay time rangedfrom 20 to 40 ms (see Results). The hind leg was filmed by a 16 mm cine-camera (BolexHS 16). The films were then analysed frame by frame (Lafayette Analyzer).

Electrophysiological experiments

In some experiments the intact or operated animals were firmly fixed by theirventral surface to a platform. The hind legs were free to move. Differential recordingswith 50 /tm copper wires insulated but for the tip were made from the followingmuscles during the avoidance response (numeration after Snodgrass, 1929): M 118,M 125, M 126 (promotors of the coxa), M 129 (elevator of the coxa), M 132

Journal of Experimental Biology, Vol. 87 Fig. 1

Fig. 1. (a) Scanning micrograph of the ventral surface of the hind tibia near the tibial-tarsaljoint. (6) Enlargement of the white rectangle (a) in order to show typical hairs. Calibration500 /im (a), 167 fun (6). The scanning micrographs were given by courtesy of DrR. Foelix,University of Bochum.

H.-J. PFLUGER {Factng t

Function of hair sensilla on locust's leg 165tf)f the trochanter), M 133 (depressor of the trochanter), M 135 (extensor of the tibia),W/l 136 (flexor of the tibia) and M 137 (levator of the tarsus). The best recordingswere obtained from electrodes placed where the muscles insert on the cuticle. In otherexperiments, recordings were made additionally from one of the two tibial nerves,usually the anterior one using silver hook electrodes. The hind leg was then positionedwith the femur and tibia at 900 horizontal to the long axis of the body. In most experi-ments the tibia was opened just proximal to the first cuticular spine, which allowedstimulation of many hairs located more distally. The recordings were stored on dc-tape (Philips Analog 7) and later filmed (Grass Oscilloscope Camera). When intra-cellular recordings from motoneurones were made, the animal was fixed on its backand opened up ventrally. A platform covered with 'red-utility-wax' (ADAK-Electronics) was used to support the ganglion. A dilute solution of Protease (PronaseB, Type II, Sigma) was applied for up to 45 s. Glass capillary microelectrodes werefilled with 2 M-K acetate and had resistances between 20 and 30 M.Q.. For furtherdetails see Hoyle & Burrows (1973).

Operations

After anaesthetizing the animals with CO2 a small flap was cut in the ventralcuticle and deflected to reveal the metathoracic ganglion. Both connectives anteriorand posterior to the ganglion were cut, but the tracheae were left intact. In mostoperated animals the small nerves projecting to the abdomen, including nerve 6, thetympanal nerve, were also cut. In some animals nerve 1 was cut as well, thus leavingonly nerves 2-5 intact. Animals operated in such a way were considered to possessa 'partially-isolated' metathoracic ganglion. Animals treated in this way were usedafter a 24 h rest.

RESULTS

Anatomical description

At first glance it is clear that the hairs on the hind tibia are unequally distributed.There are more hairs on the anterior and dorsal side than on the posterior and ventralside of the tibia (definition as if the hind leg is at 900 horizontal from the long axis ofthe body, according to Snodgrass, 1929, page 72, see Fig. ia). Near the tibial-tarsaljoint the hairs are distributed all over the cuticle, whereas more proximally they areorganized in rows along the ridges (mainly on the ventral side of the tibia) or betweenthe cuticular spines. They are angled toward the tibial-tarsal joint (Fig. 1). Theirlength is very variable: for the rows of the ventral side the range is 200-450 jim,for the tibial-tarsal joint the range is 280-500 fim. The longest hairs of the legare between the tibial spines (max. length 600 fim). The other segments, the tarsus(Runion & Usherwood, i960; Kendall, 1970), the femur, the coxa, and the differentcuticular plates of the thorax are covered with hairs of the same kind (max. length800 /*m).

Fig. 2 shows a schematic drawing of the nerves within the tibia. If the tibia isdivided along the dorsal-ventral axis all the hairs anterior to this midline enter theanterior tibial branch of the main leg nerve (a). The hairs of the posterior side enter

posterior tibial branch (b). Both tibial nerves (a) and (b) originate in the tarsus.

166

Posterior

H.-J. PFLUGER

Anterior Posterior Anterior

Femur

Tibia

Tibia

Tarsus

Fig. a. Diagram of the proximal (a) and distal (6) part of the tibia. The tibia is disected fromthe ventral side, (i) Apodeme of the retractor unguis, ia, ib: retractor unguis muscle, (2)depressor muscle of the tarsus (right bundles not drawn) with apodeme, (3) levator muscle ofthe tarsus with apodeme, (a) anterior branch of the tibial nerve, (a:) motor branch inner-vating the retractor unguis, (a,) sensory branch originating from the spurs at the tibial—tarsusjoint, (a,) motor branch innervating the depressor and levator muscle of the tarsus, (b)posterior branch of the tibial nerve, (c) main leg nerve (nsb2 after Heitler & Burrows, 1977),(d) nsbi (after Heitler & Burrows, 1977). cs: campaniform sensilla; sbgo: subgenual organ.Calibration: 1 mm.

The anterior tibial nerve (a) corresponds to the 'ventral' nerve, the posterior tibialnerve (b) to the 'dorsal' nerve labelled by Kendall (1970) (Fig. 26). As revealed indissections stained by methylene blue, two side branches from nerve (a) innervate themuscles in the tibia: (ax) innervates the retractor unguis muscle and (a3) the levatorand depressor muscle of the tarsus. Branch (a2) is sensory and originates at the jointand in the two moveable spurs (Uvarov, 1966) at the tibial-tarsal joint. Within thetibia the thicker anterior branch runs along a small trachea, the smaller posteriorbranch opposite to it along a big trachea. Both branches merge at the level of thefemoral-tibial joint (Fig. 2 a, c). From there they form the nerve that is commonlycalled the 'main leg nerve' (labelled as n5b2 by Heitler & Burrows, 1977).

Extracellular recordings from nerves 3, 4 and 5 just before they enter the meta-thoracic ganglion show that the sensory axons from the tibial hairs stay within themain leg nerve (n$) exclusively. Only in recordings from the main leg nerve (n5) can aseries of sensory spikes be seen after stimulation of tibial hairs. This is consistent withthe anatomical projections of trichoid sensilla from coxa and trochanter that arerevealed by silver-intensified cobalt stains (H.-J. Pfluger, in preparation).

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Fig. 3. Histograms of (a) the duration of individual bursts of sensory spikes (mean valuewith standard deviation (8.D.): * = 88±51 ms; the arrow shows the median value of thedistribution), (6) the time to half frequency (thf) of a burst (mean value with S.D.: x = 67 ±24ms, arrow: median value). The inserted figure shows which value equals thf. (c) The numberof spikes in a burst (mean value with 8.D.: 14 ±7, arrow: median value).

Hair cell response

If one hair is touched by an eyelash and bent in different directions, spikes of onlyone height are recorded in the nerve. Bending it proximally towards the femoral-tibial joint seems to be the best stimulus, but no clear directional sensitivity wasfound, as it responds to bending in the anterior and posterior direction as well. Insome of these experiments the eyelash was glued to a glass rod which was mounted ona loudspeaker driven by a function generator. Hair stimulated in this way gave thesame responses, with no clear directional sensitivity. All the hairs tested respondedphasically.

In order to define more clearly the stimulus which was delivered by hand, thefollowing parameters were measured on 49 sensory spike bursts: (i) the duration of asingle burst (Fig. 3 a), (ii) the half time of the frequency distribution within a sensoryspike burst (intervals between single spikes were measured, Fig. 36) and (iii) the

dumber of spikes in a single burst (Fig. 3c).

168 H.-J. PFLUGER

150° -

(c)

100 Frames(2) (sec)

Fig. 4. (a) Sequence of movements during the avoidance behaviour (= lifting of the hind leg).(b) Angular measurements taken from films, analysed frame to frame. The following angles ofthe joints were measured, a: the angle between the long axes of the femur and the animal'sbody; it includes the angle of two joints, the coxal-thoracic and the coxal-trocbanteral jointas to distinguish the movements of these two joints was impossible from the films (*±s.D. =i8'7±8'7°, range: 8-45°); /?: the angle between femur and tibia (#±S.D. = ii'o. ±8-3°,range: 3-25°); y: the angle between tibia and tarsus (*±s.D. = 23-2 ± 10-9°, range: 10-50°).(c) Diagram showing three examples from three different operated animals. As the tibial hairswere stimulated by hand the duration of the stimulus was different in all three examples, asindicated by the arrows.

Response of intact animals

If, after touching 4-10 tibial hairs with an eyelash, the behavioural responseoccurred, it was as follows: the ipsilateral hind leg was lifted, and the joints of theleg flexed (see Fig. 4). The contralateral hind leg was never observed to show anavoidance response at the same time. It either remained in its position not moving atall, or it was lowered in such a way that the tarsus could contact the ground.

In intact, restrained animals touching some hairs (4-10) is not a very powerfulstimulus for inducing the avoidance response of the hind leg. Ten animals weretested and only after hairs were touched several (4-6) times did four of them show

Function of hair sensilla on locust's leg 169avoidance movement. A slight touch to the cuticle, to stimulate hairs, possibly cam-paniform sensilla (see Discussion) and other receptors (proprioceptors, subgenualorgan) was a better stimulus to induce the avoidance response.

In the four animals, the leg movement was prevented if the tarsus was in contactwith the ground. Then only very strong tactile stimulation to the cuticle was effective,such as brushing all the hairs in the rows on the ventral side of the tibia. Such strongstimuli necessarily excited some campaniform sensilla. In order to abolish any visualinputs to motoneurones of the legs, which may inhibit such avoidance responses, theeyes of three animals were blackened with Scutan. The avoidance movement of thehind leg was shown more frequently than in animals without the eyes covered andall three animals responded. Here too, a slight touch to the cuticle was a better stimulusthan touching the hairs alone.

Response of operated animals {partially-isolated metathoracic ganglion)

In all the 20 animals which were tested the avoidance response of the hind legcould be evoked easily after stimulation of 4-10 tibial hairs simultaneously or in rapidsuccession. In a few animals the hind leg movement occurred even after bending justone hair (see Fig. 5 b, arrow shows the beginning of the sensory spike burst).

(1) Cini film analysisTen of the test animals were filmed and by a frame to frame analysis of the films

the entire sequence of leg movements was measured (Fig. 4a). In Fig. 4c three examplesfrom three different operated animals are given. In all three animals, the movementsare roughly the same and last for about 1 s during which time each joint is flexed andthen extended again. This extension is often incomplete, so that the leg remains halfway between the initial extended and later flexed position. The amplitudes of themovements are very small (see Fig. 4).

After repeated stimulation (5-10) the avoidance response habituated.

(2) Myograms

In all 20 operated animals, stimulating the tibial hairs always excites the (i) levatorof the tarsus, (ii) the flexor of the tibia, and (iii) the promotor of the coxa. Also some ofthe muscles functioning in flight and walking (bifunctional muscles: Wilson, 1962;Eisner, 1974) are activated. For example, the subalar muscle (M 129), one function ofwhich is to act as an elevator of the coxa, is activated by touching the tibial hairs. Allmuscles are activated at about the same time (Fig. 5). The muscular response to astimulation can be different if the experimental condition is different. Thus, stimula-tion of tibial hairs of animals in which the hind leg is rigidly fixed causes a 'weaker'muscular response; that is, (i) the frequency of muscle potentials is lower, and (ii)some of the motor units may be absent. ' Stronger' muscular responses were obtainedwhen the hind leg was free to move (Fig. 5 a).

Fig. 5 c shows the sort of response one gets if in an intact animal the cuticle istouched lightly near the tibial-tarsal joint, thus stimulating a few campaniform sensillaand hairs simultaneously. In this preparation the hind leg was rigidly fixed, and theeyes were blackened; therefore no other sensory structures should have been stimu-lated. It can be seen from this figure that the levator muscle of the trochanter (big

170 H.-J. PFLOGER

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Fig. 5. Myograms of the hind leg lifting shown by operated animals (a, 6). The arrows showwhen the stimulus was set (see Method). Calibration: 100 ms. (a) The hind leg was free tomove and stimulation of 4—5 hairs was controlled through the binocular microscope. Extra-cellular recordings from various muscles (see text). The lowest trace gives the beginning of thestimulus. The duration of the stimulus was different because of stimulation by hand but did notexceed 60 ms. The fourth trace of the second Fig. s(a) shows a recording of M 133 (smallspikes) and M 132 (big spikes). (6) The hind leg was rigidly fixed and one hair of the anteriorside of the tibia near the tibial—tarsal joint was stimulated by slowly bending it into the proximaldirection with an eyelash. (Duration of the stimulus 120 msec.) Extracellular recording fromthe anterior branch of the tibial nerve. The beginning of the stimulus is shown by the sensoryspikes (arrow, third trace). The big spikes are motor spikes from the levator muscle of thetarsus, (c) The hind leg of an intact animal was rigidly fixed and several hairs on the posteriorside of the tibia near the tibial-tarsal joint and some few campaniform sensilla were stimulatedby a slight touch. Stimulation was controlled through the binocular microscope. Intracellularrecording from the motoneurone of the depressor of the trochanter (MN 133, first trace).

Function of hair sensilla on locust's leg 171

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Fig. 6. Histogram of the delay times between the beginning of the stimulation of tibial hairsand the beginning of the electrical muscular activity. The arrows show the median values.

spike in second trace) and the levator muscle of the tarsus (big motor spikes in thirdtrace) are the first ones activated after stimulation of the tibial hairs (arrow andstimulus on fourth trace). During activation of these two muscles the depressor of thetrochanter is inhibited, as the intracellular recording from the motoneurone shows(first trace).

The delay times between the stimulus marker and the first observable muscle

Extracellular recording from the depressor of the trochanter (M 133) in the coxa. The bigspikes are cross-talk from the levator of the trochanter (M 132, second trace). Extracellularrecording from the anterior branch of the tibial nerve. The big spikes are motor spikesof the levator of the tarsus (third trace). The fourth trace gives the beginning of thestimulus (arrow).

172 H.-J. PFLUGER

potential are extremely variable. The median values lie between 150 and 200 ms. Fig. 6shows the measurements from individual muscles. The median values for the delaysare in fairly good agreement with each other. The actual delay time was certainly lessthan these values, as the stimulus marker signal generally was generated some 20-40 msafter the first sensory spike was recorded in the nerve. As flexion and lifting of thehind leg is the response to stimulation, few data were obtained from recordings of theextensor tibiae muscle. If muscle potentials occurred at all they arose from the slowextensor tibiae motoneurone.

DISCUSSION

The results clearly demonstrate that a complex 'avoidance movement' involvingmore than one joint of the leg can be organized at the level of the metathoracicganglion; isolating this ganglion from the rest of the CNS in fact improves theresponsiveness of the avoidance response.

That responsiveness of a reflex can be improved by decerebration is known fromstudies on praying mantids by Roeder (1937). Pringle (1940) also found that cock-roaches, with one of their thoracic ganglia isolated from the rest of the CNS, appearedto be in a 'hypersensitive condition', which meant that the responsiveness of leg re-flexes was increased. Rowell (1969), by making different cuts at the level of the thoracicganglia, was able to show that the best responses of the grooming reflex occurred afterisolating the prothoracic ganglion from the rest of the central nervous system. Heeven found that in intact animals the reflex could not be elicited.

The sense organs which are able to mediate the avoidance response described hereare the long trichoid sensilla of the tibia. This is clearly shown by the various experi-ments where touching these hairs alone strongly excites the promoters and the levatorof the coxa, the levator of the trochanter, the flexor of the tibia and the levator of thetarsus. Although the best muscular responses are obtained by stimulation of the hairsnear the tibial-tarsal joint, tactile stimuli to other hairs of the same kind on the othersegments can have effects. In general, however, stimulation of these hairs does notactivate so many muscles. For example, touching the coxal hairs strongly excitesvarious muscles in the coxa, but has no effects on muscles in the tibia.

As there are so many hairs on the tibia one might suppose that several (4-10) ofthem have to be stimulated before a muscular response occurs. This was found to betrue for most of the operated and the four responding intact animals. In order torelease the foreleg-grooming reflex, Rowell (1969) had to stimulate 'a few well-definedsensilla'. Similarly, Runion & Usherwood (1968) found that in order to excite theslow excitory motoneurone to the extensor tibiae muscle and the common inhibitor,it was necessary to stimulate ate least four tarsal trichoid sensilla, either simultaneouslyor in close succession. Nevertheless, in 3 out of 20 operated animals (Fig. 56) stimu-lation of only one hair can be sufficient to activate a muscle. This observation, too, isconsistent with the results of Runion & Usherwood (1968), which show that stimu-lation of one tarsal trichoid sensillum can excite the fast extensor tibiae motoneurone.As in cockroaches (Pringle, 1940) and stick insects (Bassler, 1977) a clear directionalsensitivity of these hairs was not found.

The muscular responses may be different depending on the experimental con-ditions, perhaps for the following reason. If all the joints of the hind leg are rigidtffixed, tonic stimuli from various receptors (campaniform sensilla, 'joint' receptor^"

Function of hair sensilla on locust's leg 173.etc.) may provide the CNS with continuous sensory information. If, for example,some of these receptors make inhibitory connexions with some of the neurones in-volved in the leg movement, the 'net' response will be weak. When the leg is un-restrained these receptors may either be inactive, or active only in certain phases, sothat the response will be strong.

Slight touches to the cuticle, which were found to be good stimuli to induce legmovements in cockroaches (Pringle, 1940) may stimulate the following receptors:hairs, campaniform sensilla, proprioceptors and other receptors like the subgenualorgan. As far as the subgenual organ is concerned it is true that it may have beenstimulated even if the leg is rigidly fixed. Experiments with animals on which thesubgenual organs are partially destroyed (H.-J. Pfliiger unpublished) suggest that ifany influences exist they are weak, as the responsiveness of the avoidance movementis not changed. Rigidly fixing the leg seems to be sufficient to prevent any phasicinput from chordotonal organs and other proprioceptors. Does stimulation of thetibial hairs by a slight touch to the cuticle also excite campaniform sensilla (CS)? Incockroaches it is known that the campaniform sensilla which respond to cuticularstrain are widely distributed over the leg (Pringle, 1938). In locusts a rough count ofthe CS viewed in the binocular microscope yielded the following numbers: totalnumber from coxa tc tarsus, foreleg 90, middle leg 100, hind leg 130 (including theCS-nelds).

Given the large numbers of campaniform sensilla found on the tibia of the hindleg, it is clear that some would be stimulated even by slight touches to a fairly smallarea of the cuticle. As such slight touches are always the best stimuli for releasing thehind leg lifting, it is concluded that stimulation of the CS can have the same effect asstimulation of the hairs. Therefore it is likely that hairs, campaniform sensilla andperhaps other not yet identified sensory structures (e.g. 'free-nerve-endings') mayrepresent a sort of sensory system of the cuticular surface similar to the 'skin sense'of vertebrates, in order to inform the insect of the many stimuli acting on its surface.From the long duration and the variability of the delay times between the stimulusand the muscular response (Fig. 6), it is concluded that the afferent fibres from thehairs do not synapse directly onto the motoneurones, although the measurements ofthe delay times due to stimulation by hand are not as precise as if the hairs werestimulated by a mechanical device. The likelihood of polysynaptic pathways is under-lined by the results of cobalt stains, which show the anatomical projections of trichoidsensilla from the coxa and trochanter into the CNS (Pfliiger unpublished). Thesensory axons enter the ganglion through the main leg nerve (n5) and have an ex-tended branching area near the midline of the ganglion. This neuropilar region islocated near the ventral surface of the ganglion (ventro-medial). From the point ofentering the ganglion to their neuropile they do not have any side branches, thereforebranching is limited to a very distinct area. From drawings of the known motoneurones(Burrows & Hoyle, 1973; Wilson & Hoyle, 1978), it appears likely that the area ofoverlap between the sensory neuropile of the hair receptors and the neuropile ofmotoneurones is negligible. Therefore from this finding, it is very likely that inter-neurones are involved. These interneurones are thought to be within the ipsilateralhalf of the ganglion, as the hind leg lifting has no contralateral effect in animals with

partially isolated metathoracic ganglion.

174 H.-J. PFLUGER

Behavioural significance

It is thought that in intact animals the influences from hairs on motoneurones areweak and that those influences are masked by other activity. Only if this other activityis reduced, for example by partially isolating the ganglion, can these weak influencesbe detected readily. Some behavioural experiments may give clues as to when infor-mation from the trichoid sensilla is needed. If in a freely walking animal the hind legpushes against an obstacle during protraction it is lifted immediately. During theseexperiments the hairs as well as the campaniform sensilla are massively stimulated,because a separate stimulation of the two receptors is hardly possible. When the legis first lifted away from the obstacle the tarsus is levated. If as the animal movesforward the numerous sense organs on the underside of the tarsus come in contactwith the obstacle, the tarsus grasps the object and the whole leg takes part in walkingagain. Similar observations were made in cockroaches by Pringle (1940). That tarsalcontact can force an uncoupled leg into motor behaviour was described for stickinsects (walking: Bassler, 1967; rocking: Pfluger, 1977). Also a motor behaviour suchas flight can be stopped by tarsal contact (Fraenkel, 1932).

Thinking of the many different stimuli impinging upon the surface of an insect'scuticle, such as, for example, those from twigs and leaves encountered duringclimbing in a tree, one may get some ideas as to how the hair sensilla may be of use.

I am greatly indebted to Dr M. V. S. Siegler and Dr M. Burrows for discussions,suggestions and help with the English manuscript. Also thanks for valuable sugges-tions to Prof. Dr H. Cruse and Prof. Dr P. Gorner. Thanks to Mrs B. Gast, Mr P.Meyer and all the members of the mechanical workshop for technical help.

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BASSLER, U. (1977). Sense organs in the femur of the stick insect and their relevance to the control ofposition of the femur-tibia joint. J. comp. Physiol. lai, 99—113.

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DETHIER, V. G. (1963). The Physiology of Insect Senses. London: Methuen.DIRSH, V. M. (1974). Genus Schistocerca (Acridomorpha, Insecta). The Hague: Dr W. Junk B.V.ELSNER, N. (1974). Neural economy: Bifunctional muscles and common central pattern elements in

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