Leg kinematics and muscle activity during treadmill running in the cockroach, Blaberus discoidalis : I. Slow running

We have combined high-speed video motion analysis of leg movements with electromyogram (EMG) recordings from leg muscles in cockroaches running on a treadmill. The mesothoracic (T2) and metathoracic (T3) legs have different kinematics. While in each leg the coxa-femur (CF) joint moves in unison with the femur-tibia (FT) joint, the relative joint excursions differ between T2 and T3 legs. In T3 legs, the two joints move through approximately the same excursion. In T2 legs, the FT joint moves through a narrower range of angles than the CF joint. In spite of these differences in motion, no differences between the T2 and T3 legs were seen in timing or qualitative patterns of depressor coxa and extensor tibia activity. The average firing frequencies of slow depressor coxa (Ds) and slow extensor tibia (SETi) motor neurons are directly proportional to the average angular velocity of their joints during stance. The average Ds and SETi firing frequency appears to be modulated on a cycle-by-cycle basis to control running speed and orientation. In contrast, while the frequency variations within Ds and SETi bursts were consistent across cycles, the variations within each burst did not parallel variations in the velocity of the relevant joints. Accepted: 24 May 1997     

Vibration signals from the FT joint can induce phase transitions in both directions in motoneuron pools of the stick insect walking system

The influence of vibratory signals from the femoral chordotonal organ fCO on the activities of muscles and motoneurons in the three main leg joints of the stick insect leg, i.e., the thoraco-coxal (TC) joint, the coxa-trochanteral (CT) joint, and the femur-tibia (FT) joint, was investigated when the animal was in the active behavioral state. Vibration stimuli induced a switch in motor activity (phase transition), for example, in the FT joint motor activity switched from flexor tibiae to extensor tibiae or vice versa. Similarly, fCO vibration induced phase transitions in both directions between the motoneuron pools of the TC joint and the CT joint. There was no correlation between the directions of phase transition in different joints. Vibration stimuli presented during simultaneous fCO elongation terminated the reflex reversal motor pattern in the FT joint prematurely by activating extensor and inactivating flexor tibiae motoneurons. In legs with freely moving tibia, fCO vibration promoted phase transitions in tibial movement. Furthermore, ground vibration promoted stance-swing transitions as long as the leg was not close to its anterior extreme position during stepping. Our results provide evidence that, in the active behavioral state of the stick insect, vibration signals can access the rhythm generating or bistable networks of the three main leg joints and can promote phase transitions in motor activity in both directions. The results substantiate earlier findings on the modular structure of the single-leg walking pattern generator and indicate a new mechanism of how sensory influence can contribute to the synchronization of phase transitions in adjacent leg joints independent of the walking direction.     

Kinematics and motor activity during tethered walking and turning in the cockroach, <Emphasis Type="Italic">Blaberus discoidalis</Emphasis>

When insects turn from walking straight, their legs have to follow different motor patterns. In order to examine such pattern change precisely, we stimulated single antenna of an insect, thereby initiating its turning behavior, tethered over a lightly oiled glass plate. The resulting behavior included asymmetrical movements of prothoracic and mesothoracic legs. The mesothoracic leg on the inside of the turn (in the apparent direction of turning) extended the coxa-trochanter and femur-tibia joints during swing rather than during stance as in walking, while the outside mesothoracic leg kept a slow walking pattern. Electromyograms in mesothoracic legs revealed consistent changes in the motor neuron activity controlling extension of the coxa-trochanter and femur-tibia joints. In tethered walking, depressor trochanter activity consistently preceded slow extensor tibia activity. This pattern was reversed in the inside mesothoracic leg during turning. Also for turning, extensor and depressor motor neurons of the inside legs were activated in swing phase instead of stance. Turning was also examined in free ranging animals. Although more variable, some trials resembled the pattern generated by tethered animals. The distinct inter-joint and inter-leg coordination between tethered turning and walking, therefore, provides a good model to further study the neural control of changing locomotion patterns.     

Leg Regrowth in Blaberus discoidalis (Discoid Cockroach) following Limb Autotomy versus Limb Severance and Relevance to Neurophysiology Experiments

Background

Many insects can regenerate limbs, but less is known about the regrowth process with regard to limb injury type. As part of our neurophysiology education experiments involving the removal of a cockroach leg, 1) the ability of Blaberus discoidalis cockroaches to regenerate a metathoracic leg was examined following autotomy at the femur/trochanter joint versus severance via a transverse coxa-cut, and 2) the neurophysiology of the detached legs with regard to leg removal type was studied by measuring spike firing rate and microstimulation movement thresholds.

Leg Regrowth Results

First appearance of leg regrowth was after 5 weeks in the autotomy group and 12 weeks in the coxa-cut group. Moreover, regenerated legs in the autotomy group were 72% of full size on first appearance, significantly larger (p<0.05) than coxa-cut legs (29% of full size at first appearance). Regenerated legs in both groups grew in size with each subsequent molt; the autotomy-removed legs grew to full size within 18 weeks, whereas coxa-cut legs took longer than 28 weeks to regrow. Removal of the metathoracic leg in both conditions did not have an effect on mortality compared to matched controls with unmolested legs.

Neurophysiology Results

Autotomy-removed legs had lower spontaneous firing rates, similar marked increased firing rates upon tactile manipulation of tibial barbs, and a 10% higher electrical microstimulation threshold for movement.

Summary

It is recommended that neurophysiology experiments on cockroach legs remove the limb at autotomy joints instead of coxa cuts, as the leg regenerates significantly faster when autotomized and does not detract from the neurophysiology educational content.     

Regeneration and autotomy exhibited by the Black Widow spider,Latrodectus variolus walckenaer

Summary Crustaceans, insects and to a lesser degree arachnids have been employed in regeneration studies. Amputation and ligation of the legs was used to determine occurring in a Black Widow spider complied with the developmental gradient model of regeneration. The occurrence of autotomy in this species was also documented. Amputation indicated the most proximal point from which regeneration of the leg could occur was the femoral mid-point. Amputation proximal to that area did not result in leg regeneration. Autotomy following amputation was not observed. Ligation of the legs resulted in autotomy when applied at and proximal to the mid-point of the tibia, increasing in frequency as more proximal segments were ligated. Autotomy always occurred at the trochanter-coxa joint. Autotomized legs did not regenerate. The regeneration observed complied with the developmental gradient model.     

The investigation of control mechanisms of stepping rhythm in human in the air-stepping conditions during passive and voluntary leg movements

In unloading condition the degree of activation of the central stepping program was investigated during passive leg movements in healthy subjects, as well as the excitability of spinal motoneurons during passive and voluntary stepping movement. Passive stepping movements with characteristics maximally approximated to those during voluntary stepping were accomplished by experimenter. The comparison of the muscle activity bursts during voluntary and imposed movements was made. In addition to that the influence of artificially created loading onto the foot to the leg movement characteristics was analyzed. Spinal motoneuron excitability was estimated by means of evaluation of amplitude modulation of the soleus H-reflex. The changes of H-reflexes under the fixation of knee or hip joints were also studied. In majority of subjects the passive movements were accompanied by bursts of EMG activity of hip muscles (and sometimes of knee muscles), which timing during step cycle was coincided with burst timing of voluntary step cycle. In many cases the bursts of EMG activity during passive movements exceeded activity in homonymous muscles during voluntary stepping. The foot loading imitation exerted essential influence on distal parts of moving extremity during voluntary as well passive movements, that was expressed in the appearance of movements in the ankle joint and accompanied by emergence and increasing of phasic EMG activity of shank muscles. The excitability of motoneurons during passive movements was greater then during voluntary ones. The changes and modulation of H-reflex throughout the step cycle without restriction of joint mobility and during exclusion of hip joint mobility were similar. The knee joint fixation exerted the greater influence. It is supposed that imposed movements activate the same mechanisms of rhythm generation as a supraspinal commands during voluntary movements. In the conditions of passive movements the presynaptic inhibition depend on afferent influences from moving leg in the most degree then on central commands. It seems that afferent inputs from pressure receptors of foot in the condition of "air-stepping" actively interact with central program of stepping and, irrespective of type of the performing movements (voluntary or passive), form the final pattern activity.     

Thoracic leg motoneurons in the isolated CNS of adult Manduca produce patterned activity in response to pilocarpine,which is distinct from that produced in larvae

In the hawkmoth, Manduca sexta, thoracic leg motoneurons survive the degeneration of the larval leg muscles to innervate new muscles of the adult legs. The same motoneurons, therefore, participate in the very different modes of terrestrial locomotion that are used by larvae (crawling) and adults (walking). Consequently, changes in locomotor behavior may reflect changes in both the CNS and periphery. The present study was undertaken to determine whether motor patterns produced by the isolated CNS of adult Manduca, in the absence of sensory feedback, would resemble adult specific patterns of coordination. Pilocarpine, which evokes a fictive crawling motor pattern from the isolated larval CNS, also evoked robust patterned activity from leg motoneurons in the isolated adult CNS. As in the larva, levator and depressor motoneurons innervating the same leg were active in antiphase. Unlike fictive crawling, however, bursts of activity in levator or depressor motoneurons of one leg alternated with bursts in the homologous motoneurons innervating the opposite leg of the same segment and the leg on the same side in the adjacent segment. The most common mode of intersegmental activity generated by the isolated adult CNS resembled an alternating tripod gait, which is displayed, albeit infrequently, during walking in intact adult Manduca. A detailed analysis revealed specific differences between the patterned motor activity that is evoked from the isolated adult CNS and activity patterns observed during walking in intact animals, perhaps indicating an important role for sensory feedback. Nevertheless, the basic similarity to adult walking and clear distinctions from the larval fictive crawling pattern suggest that changes within the CNS contribute to alterations in locomotor activity during metamorphosis. Electronic Publication     

Avoidance reflexes mediated by contact chemoreceptors on the legs of locusts

Taste receptors, or basiconic sensilla, are distributed over the legs of the locust and respond to direct contact with chemical stimulants. The same chemosensory neurones that responded to contact with salt solutions also responded to particular acidic odours. Odours of food and other chemicals had no effect on the chemosensory neurones. In locusts free to move, an acid odour presented to the tarsus of a hind leg evoked a rapid avoidance movement in which the tarsus was levated, the tibia flexed and the femur levated. Intracellular recordings from motor neurones that innervate muscles of the hind leg showed that when an acid odour was directed towards basiconic sensilla on the leg there was a reciprocal activation of antagonistic motor pools that move the leg segments about each joint. Thus an extensor tibiae motor neurone was inhibited while a flexor tibiae motor neurone was excited, and the tarsal depressor and retractor unguis motor neurones were inhibited while the tarsal levator motor neurone was excited. This method of odour stimulation of taste receptors generates less adaptation than direct contact with chemicals, and therefore represents an ideal method for stimulating taste receptors for further studies on the central pathways processing taste signals. Accepted: 2 June 1998     

Interlimb interactions during cyclic in-phase and antiphase movements of arms and legs and their dependence on afferent influences

Coordinated arm and leg movements imply neural interactions between the rhythmic generators of the upper and lower extremities. In ten healthy subjects in the lying position, activity of the muscles of the upper and lower extremities was recorded during separate and joint cyclic movements of the arms and legs with different phase relationships between the movements of the limbs and under various conditions of the motor task. Antiphase active arm movements were characterized by higher muscle activity than during the inphase mode. The muscle activity during passive arm movements imposed by the experimentalist was significantly lower than muscle activity during passive arm movements imposed by the other arm. When loading one arm, the muscle activity in the other, passively moving, arm increased independently from the synergy of arm movements. During a motor task implementing joint antiphase movements of both upper and lower extremities, compared to a motor task implementing their joint in-phase movements, we observed a significant increase in activity in the biceps brahii muscle, the tibialis anterior muscle, and the biceps femoris muscle. Loading of arms in these motor tasks has been accompanied by increased activity in some leg muscles. An increase in the frequency of rhythmic movements resulted in a significant growth of the muscle activity of the arms and legs during their cooperative movements with a greater rate of rise in the flexor muscle activity of the arms and legs during joint antiphase movements. Thus, both the spatial organization of movements and the type of afferent influences are significant factors of interlimb interactions, which, in turn, determine the type of neural interconnections that are involved in movement regulation.     

Control of climbing behavior in the cockroach, Blaberus discoidalis. II. Motor activities associated with joint movement

Deathhead cockroaches employ characteristic postural strategies for surmounting barriers. These include rotation of middle legs to re-direct leg extension and drive the animal upward. However, during climbing the excursions of the joints that play major roles in leg extension are not significantly altered from those seen during running movements. To determine if the motor activity associated with these actions is also unchanged, we examined the electromyogram activity produced by the slow trochanteral extensor and slow tibial extensor motor neurons as deathhead cockroaches climbed over obstacles of two different heights. As they climbed, activity in the slow trochanteral extensor produced a lower extension velocity of the coxal-trochanteral joint than the same frequency of slow trochanteral extensor activity produces during horizontal running. Moreover, the pattern of activity within specific leg cycles was altered. During running, the slow trochanteral extensor generates a high-frequency burst prior to foot set-down. This activity declines through the remainder of the stance phase. During climbing, motor neuron frequency no longer decreased after foot set-down, suggesting that reflex adjustments were made. This conclusion was further supported by the observation that front leg amputees generated even stronger slow trochanteral extensor activity in the middle leg during climbing movements.     

Activation of interlimb interactions increases the motor output in legs of healthy subjects: Study under the conditions of arm and leg unloading

The effect of arm movements and movements of individual arm joints on the electrophysiological and kinematic characteristics of voluntary and vibration-triggered stepping-like leg movements was studied under the conditions of horizontal support of the upper and lower limbs. The horizontal support of arms provided a significant increase in the rate of activation of locomotor automatism by noninvasive impact on tonic sensory inputs. The addition of active arm movements during involuntary stepping-like leg movements led to an increase in the EMG activity of hip muscles and was accompanied by an increase in the amplitude of hip and shin movements. The movement of the shoulder joints led to an increase in the activity of hip muscles and was accompanied by an increase in the amplitude of hip and shin movements. Passive arm movements had the same effect on induced leg movements. The movement of the shoulder joints led to an increase in the activity of hip muscles and an increase in the amplitude of movements of knee and hip joints. At the same time, the movement of forearms and wrists had a similar facilitating effect on the physiological and kinematic characteristics of rhythmic stepping-like movements, but influenced the distal segments of legs to a greater extent. Under the conditions of subthreshold vibration of leg muscles, voluntary arm movements led to activation of involuntary rhythmic stepping movements. During voluntary leg movements, the addition of arm movements had a significantly smaller impact on the parameters of rhythmic stepping than during involuntary leg movements. Thus, the simultaneous movements of the upper and lower limbs are an effective method of activation of neural networks connecting the rhythm generators of arms and legs. Under the conditions of arm and leg unloading, the interactions between the cervical and lumbosacral segments of the spinal cord seem to play the major role in the impact of arm movements on the patterns of leg movements. The described methods of activation of interlimb interactions can be used in the rehabilitation of post-stroke patients and patients with spinal cord injuries, Parkinson’s disease, and other neurological diseases.     

The investigation of locomotor activity control system in humans under the air-stepping conditions during passive and voluntary leg movements

The degree of activation of the central stepping program during passive leg movement was studied in healthy subjects under unloading conditions; the excitability of spinal motoneurons was studied during passive and voluntary stepping movements. Passive stepping movements with characteristics maximally close to those during voluntary stepping were accomplished by the experimenter. The bursts of muscular activity during voluntary and imposed stepping movements were compared. In addition, the influence on the leg movement of artificially created loading onto the foot was studied. The excitability of spinal motoneurons was estimated by the amplitude of modulation of the m. soleus H reflex. Changes in the H reflex (Hoffmann’s reflex) after fixation of the knee and hip joints were also studied. In most subjects, passive movements were accompanied by bursts of electromyographic (EMG) activity in the hip muscles (sometimes in shank muscles); the timing of the EMG burst during the step cycle coincided with the burst’s timing during voluntary stepping. In many cases, the bursts in EMG activity exceeded the activity of homonymous muscles during voluntary stepping. Simulation of foot loading influenced significantly the distal part of the moving extremity during both voluntary and passive movements, which was expressed in the appearance of movements in the ankle joint and an increase in the phasic EMG activity of the shank muscles. The excitability of motoneurons during passive movements was higher than during voluntary movements. Changes and modulation of the H reflex throughout the step cycle were similar without restriction of joint mobility and without hip joint mobility. Fixation of the knee joint was of great importance. It is supposed that imposed movements activate the same mechanisms of rhythm generation as supraspinal commands during voluntary movements. During passive movements, presynaptic inhibition depends mostly on the afferent influences from the moving leg rather than on the central commands. Under the conditions of “air-stepping,” the afferent influences from the foot pressure receptors are likely to interact actively with the central program of stepping and to determine the final activity pattern irrespective of the movement type (voluntary or passive).     

Insect rope-walkers: kinematics of walking on thin rods in a bug, Graphosoma italicum (Heteroptera, Pentatomidae)

Leg positions during walking on a plane and on thin rods were recorded by photography, videorecording, and videokymography. Joint angles were reconstructed from the tibia-ending position, using a 3-D model of the body. Participation of leg joints in propulsion was analysed by calculating the partial derivatives of tibia end-point position on different joint angles. Adjustment to walking with a narrow ground base is achieved by additional femur depression and flexion of the tibia in the stance phase. In the swing phase, the leg is raised by the same amount as when walking on a plane, but not to the same superior position, as on a plane. The contribution of the subcoxal joint to body propulsion is 64-94% in fore-and middle legs and 22-49% in hind legs. The oblique alignment of the coxal pivot within the thorax helps maintain a long stride for variable ground bases. In Graphosoma , it is close to the optimal position: according to several criteria, the angle between the coxal axis and the body vertical shall be arctan π/2, or ∼ 57.5°.     

Mutual influences of upper and lower extremities during cyclic movements

The possibility for the activation of muscles in a passive arm during its cyclic movements imposed by active movements of the contralateral arm or by an experimenter and the effect that the movements of lower extremities have on the activity of the arm muscles have been studied. In addition, the activity of the leg muscles was studied as dependent on the motor task performed by the arms. Ten healthy subjects performed antiphase arm movements with and without stepping-like movements of both legs in the supine position. The experiment was performed under three conditions for the arm movements: (1) both arms performed active movements; (2) one arm performed active movements, and the contralateral arm, being entirely passive, was forced to participate in movements; (3) the movement of the passive arm was caused by an experimenter. Under condition (2), additional loadings of 30 and 60 N were applied to the active arm. Under all conditions, the arm movements were performed with and without leg movements. The possibility for the activation of muscles in the arm performing passive movements has been demonstrated. To a large extent, this is possible due to an increase in the afferent inflow from the muscles of the contralateral arm. The electrical activity was modulated during cyclic arm movements and depended on the level of loading of the active arm. During the combined active movements of the arms and legs, the reduction in the activity of the flexor muscles of the shoulder and forearm was observed. In the case of passive stepping-like movements, the concomitant arm movements increased the magnitude of electromyographic bursts in most of the examined leg muscles. During active leg movements, a similar increase in electromyographic bursts was observed only in the m. biceps femoris (BF) and the anterior tibial muscle. An increase in the loading of one arm caused a significant increase in the EMG activity in most examined muscles of the legs. The data obtained provide additional proof for the existence of a functionally significant neuronal interaction between the arms, as well as between the upper and lower extremities, which is probably due to intraspinal neuronal connections.     

Sexually dimorphic tegumental gland openings in Laniatores (Arachnida,Opiliones), with new data on 23 species

Sexually dimorphic glands often release sexual pheromones both in vertebrates and invertebrates. Species of Laniatores (Arachnida, Opiliones) seem to depend on chemical communication but few studies have addressed this topic. In this study, we review the literature for the Phalangida and present new data for 23 species of Laniatores. In 16 taxa, we found previously undescribed sexually dimorphic glandular openings on the femur, patella, metatarsus, and tarsus of legs I and metatarsus of legs III and IV. For the other species, we provide scanning electron micrographs of previously undescribed sexually dimorphic setae and pegs located on swollen regions of the legs. We also list additional species in which males have swollen regions on the legs, including the tibia, metatarsus, and tarsus of legs I, trochanter and tibia of legs II, femur, metatarsus, and tarsus of legs III, and metatarsus and tarsus of legs IV. The function and biological role of the secretions released by these glands are discussed. J. Morphol., 2010. © 2009 Wiley‐Liss, Inc.     

Motion analysis of leg joints associated with escape turns of the cockroach,Periplaneta americana

Considerable information is now available on the neural organization of the escape system of the American cockroach. To relate these data to the behavior, we need detailed information on the movements made at the principle leg joints that produce the turn. We used motion analysis of high speed video records to acquire such information. Records from both free ranging and tethered animals were analyzed. 1. We analyzed individual joint movements using a tethered preparation. Stimuli from 4 different angles around the animal were used. For all wind angles, the femur-tibia (FT) joint on the mesothoracic leg that is ipsilateral to the wind source extended while the contralateral mesothoracic FT joint flexed. This moved both of these legs laterally toward the wind source. In freely moving animals the FT movements provide forces that turn the animal away from the wind source. 2. The ipsilateral mesothoracic coxa-femur (CF) joint extended for all wind angles. The contralateral mesothoracic CF joint extended in response to most winds from the rear, but switched to flexion in response to wind from the side and front. As a result of these joint movements, rear wind resulted in rearward movements of the contralateral mesothoracic leg, while side and front wind resulted in more forward movements of that leg. 3. The CF and FT joints for both ipsilateral and contralateral metathoracic legs extended to wind from the rear and switched to flexion as the wind was placed at more anterior positions around the animal. In freely moving animals, extension of these joints would push the animal forward. Flexion would pull the animal backward. 4. Several of the joints showed correlations between rate of movement and initial joint angle. That is, joints that were already flexed at the onset of stimulation tended to move at a faster rate to a final position than joints that started at a more extended position. 5. Metathoracic FT and CF joints showed a high degree of positive correlation during the escape movements. Indeed, many curves showing movement of metathoracic FT and CF joints with time were virtually identical.     

Role of local nonspiking interneurons in the generation of rhythmic motor activity in the stick insect

Local nonspiking interneurons in the thoracic ganglia of insects are important premotor elements in posture control and locomotion. It was investigated whether these interneurons are involved in the central neuronal circuits generating the oscillatory motor output of the leg muscle system during rhythmic motor activity. Intracellular recordings from premotor nonspiking interneurons were made in the isolated and completely deafferented mesothoracic ganglion of the stick insect in preparations exhibiting rhythmic motor activity induced by the muscarinic agonist pilocarpine. All interneurons investigated provided synaptic drive to one or more motoneuron pools supplying the three proximal leg joints, that is, the thoraco-coxal joint, the coxa-trochanteral joint and the femur-tibia joint. During rhythmicity in 83% (n=67) of the recorded interneurons, three different kinds of synaptic oscillations in membrane potential were observed: (1) Oscillations were closely correlated with the activity of motoneuron pools affected; (2) membrane potential oscillations reflected only certain aspects of motoneuronal rhythmicity; and (3) membrane potential oscillations were correlated mainly with the occurrence of spontaneous recurrent patterns (SRP) of activity in the motoneuron pools. In individual interneurons membrane potential oscillations were associated with phase-dependent changes in the neuron's membrane conductance. Artificial changes in the interneurons' membrane potential strongly influenced motor activity. Injecting current pulses into individual interneurons caused a reset of rhythmicity in motoneurons. Furthermore, current injection into interneurons influenced shape and probability of occurrence for SRPs. Among others, identified nonspiking interneurons that are involved in posture control of leg joints were found to exhibit the above properties. From these results, the following conclusions on the role of nonspiking interneurons in the generation of rhythmic motor activity, and thus potentially also during locomotion, emerge: (1) During rhythmic motor activity most nonspiking interneurons receive strong synaptic drive from central rhythm-generating networks; and (2) individual nonspiking interneurons some of which underlie sensory-motor pathways in posture control, are elements of central neuronal networks that generate alternating activity in antagonistic leg motoneuron pools. © 1995 John Wiley & Sons, Inc.     

Walking in the American cockroach: the timing of motor activity in the legs during straight walking

The timing of bursts of motor activity in extensor muscles in the coxae of pairs of legs in intact freely walking American cockroaches was studied. The timing of bursts in adjacent and non-adjacent leg pairs generally reflected the common alternating tripod gait of these insects. Detailed study of the timing further revealed two previously unreported features. (1) The timing of extensor bursts in the middle legs relative to bursts in the rear legs was more variable than it was relative to those in the front legs. This difference in variability was statistically significant for the means of bursts when all insects were considered together as well as for bursts in individual insects. An apparent difference in variability of the timing of burst starts compared to burst ends for any one leg pair was not significant. (2) There was a shift in the timing of motor bursts relative to one another when an insect walked fast such that motor bursts in the middle legs tended to lag farther behind those in the front legs, and those in the rear legs tended to lag farther behind those in the middle legs compared to the timing during slow walking. This shift was apparent in both burst starts and burst ends, although more obvious in the former. It occurred in both ipsilateral and contralateral leg pairs, and in both the mean data and the data for individual insects. The implications of these characteristics of the timing data are discussed in terms of the neural organization of insect walking.     

Gimbals in the insect leg

We studied the common kinematic features of the coxa and trochanter in cursorial and raptorial legs, which are the short size of the podomers, predominantly monoaxial joints, and the approximate orthogonality of adjacent joint axes. The chain coxa-trochanter with its short elements and serial orthogonality of joint axes resembles the gimbals which combine versatility and tolerance to external perturbations. The geometry of legs was studied in 23 insect species of 12 orders. Insects with monoaxial joints were selected. The joint between the trochanter and the femur (TFJ) is defined either by two vestigial condyles or by a straight anterior hinge. Direction of the joint axes in the two basal podomers was assessed by 3D measurements or by goniometry in two planes. Length of the coxa is <15% (mostly <8%) of the total length of the cursorial leg, that of the trochanter <10%. Angles between the proximal and distal joint axes in the middle coxa range from 124 to 84 degrees (mean 97+/-14 degrees ), in the trochanter (in all legs studied) from 125 to 72 degrees (mean 90+/-13 degrees ). Vectors of the distal axis in the coxa are concentrated about the normal to the plane defined by the proximal axis and the midpoint between the distal condyles. These vectors in the trochanter lie at various angles to the normal; angles are correlated with the direction of the TFJ relative to the femur. Range of reduction about the TFJ is over 60 degrees in the foreleg of Ranatra linearis, Mantispa lobata and the hind leg in Carabus coriaceus (confirming observations of previous authors), 40-60 degrees in the foreleg of Vespa crabro and in the middle one in Ammophila campestris, 10-30 degrees in other studied specimens. The special role of the trochanter in autotomy and in active propulsion in some insect groups is discussed. The majority of insects possess small trochanters and slightly movable TFJs with the joint axis laying in the femur-tibia plane. We pose the hypothesis that the TFJ damps external forces, the vectors of which lie off the femur-tibia plane, the reductor muscle acting as a spring. Thus the TFJ contributes to dynamic stability of legged locomotion.