Leg movements of stick insects walking with five legs on a treadwheel and with one leg on a motor-driven belt

Stick insects walking with five legs on a self-propelled treadwheel and with the left hindleg (L3) on a motor-driven belt may move the "belt" leg L3 and the "wheel" legs with different frequencies. When L3 made less steps than L2, that step of L2, which was performed during the swing phase of L3, is prolonged. The time interval between the end of swing phase of L3 and the onset of the following swing phase of L2 was remarkably constant. When L3 made more steps than L2, that step of L3, which was performed during the swing phase of L2, is prolonged. Again, the time interval between the end of swing phase of L3 preceding a L2 swing phase and the onset of the L2 swing phase was relatively constant. For both kinds of walking situations phase response curves were drawn. They show that two types of coordinating channels exist: An anteriorly directed type is more dependent on absolute time than on phase. A posteriorly directed type is phase-dependent. Both inhibit the transition from stance to swing for some time. The results are compared with the existing coordination models.     

Timing and shaping influences on the motor output for walking in stick insects

An examination of the literature on walking stick insects shows: sensory influences on the transition from stance to swing phase (timing influences) also influence the shape of the stance phase motor output, as far as this was tested. Therefore, the distinction between timing and shaping influences seems to be artificial in this case. The results suggest that the walking pattern generator is a relaxation oscillator.     

Leg movements of stick insects walking with five legs on a treadwheel and with one leg on a motor-driven belt

Five legs of a fixed stick insect walked on a double treadwheel. The left hindleg (L3) walked on a motor-driven belt. When the belt was slower than the wheels L3 made less steps than the other legs and when the belt was faster than the wheels it made more steps than the other legs. In the case of slowlier stepping of the belt-leg, the motor neurons of the retractor coxae muscle of this leg showed a high activity when the leg was pulled backwards by the belt. This activity was modulated in the step rhythm of the wheel-legs. When all legs showed the same stepping frequency (1:1-coordination) the protraction duration of L3 was almost independent of step-period, as well as the lag between onset of protraction of L3 and that of L2. In some cases only L3 could be made to step on the belt even when all other legs did not walk.     

Control of flexor motoneuron activity during single leg walking of the stick insect on an electronically controlled treadwheel

In the present study, motoneurons innervating the flexor tibiae muscle of the stick insect (Cuniculina impigra) middle leg were recorded intracellularly while the single leg performed walking-like movements on a treadwheel. Different levels of belt friction (equivalent to a change in load) were used to study the control of activity of flexor motoneurons. During slow leg movements no fast motoneurons were active, but a recruitment of these neurons could be observed during faster leg movements. The firing rate of slow and fast motoneurons increased with incremented belt friction. Also, the force applied to the treadwheel at different frictional levels was adapted closely to the friction of the treadwheel to be overcome. The motoneurons innervating the flexor tibiae were recruited progressively during the stance phase, with the slow motoneurons being active earlier than the fast (half-maximal spike frequency after 10-15% and 50-60% of the stance phase, respectively). The resting membrane potential was more hyperpolarized in fast motoneurons (64.6 +/- 6.5 mV) than in slow motoneurons (-52.9 +/- 5.4 mV). However, the threshold for the initiation of action potentials was not statistically significantly different in both types of flexor motoneurons. Therefore, action potentials were generated in fast motoneurons after a longer period of depolarization and thus later during the stance phase than in slow motoneurons. We show that motoneurons of the flexor tibiae receive substantial common excitatory inputs during the stance phase and that the difference in resting membrane potential between slow and fast motoneurons is likely to play a crucial role in their consecutive recruitment.     

Reflex modulation in locusts walking on a treadwheel intracellular recordings from motoneurons

Summary In locusts (Locusta migratoria) walking on a treadwheel, afferents of tarsal hair sensilla were stimulated via chronically implanted hook electrodes (Fig. 1). Stimuli applied to the middle leg tarsus elicited avoidance reflexes (Fig. 2). In quiescent animals, the leg was lifted off the ground and the femur adducted. In walking locusts, the response was phase-dependent. During the stance phase, no reaction was observed except occasional, premature triggering of swing movements; stimuli applied near the end of the swing phase were able to elicit an additional, short leg protraction.Central nervous correlates of phase-dependent reflex modulation were observed by recording intracellularly from motoneuron somata in walking animals. As a rule, motoneurons recruited during the swing phase showed excitatory stimulus-related responses around the end of the swing movement, correlated to the triggering of additional leg protractions (Figs. 3, 4, 5). Motoneurons active during the stance phase were often inhibited by tarsal stimulation, some showed only weak responses (Figs. 8, 9, 10). Common inhibitory motoneuron 1 was excited by tarsal stimulation during all phases of the leg movement (Figs. 6, 7). In one type of flexor tibiae motoneuron, a complex response pattern was observed, involving the inversion of stimulus-related synaptic potentials from excitatory, recorded during rest, to inhibitory, observed during long-lasting stance phases (Figs. 11, 12).The results demonstrate how reflex modulation is represented on the level of synaptic input to motoneurons. They further suggest independent gain control in parallel, antagonistic pathways converging onto the same motoneuron as a mechanism for reflex reversal during locomotion.Abbreviations CI ₁ common inhibitory motoneuron (1) - EMG electromyogram - Feti fast extensor muscle of the tibia     

Motor nerve topology reflects myocyte morphology in hamster retractor and epitrochlearis muscles

Neuromuscular activation is a primary determinant of metabolic demand and oxygen transport. The m. retractor and m. epitrochlearis are model systems for studying metabolic control and oxygen transport; however, the organization of muscle fibers and motor nerves in these muscles is unknown. We tested whether the topology of motor innervation was related to the morphology of muscle fibers in m. retractor and m. epitrochlearis of male hamsters ( approximately 100 g). Respective muscles averaged 47 and 12 mm in length 100 and 35 mg in mass. Staining for acetylcholinesterase revealed neuromuscular junctions arranged in clusters throughout m. retractor and as a central band across m. epitrochlearis, suggesting differences in fiber morphology. For both muscles, complete cross-sections contained approximately 1,700 fibers. Fiber cross-sectional areas were distributed nearly normal in m. epitrochlearis (mean = 1,559 +/- 17 microm(2)) and skewed left (P < 0.05) in m. retractor (mean = 973 +/- 15 microm(2)). Single fiber length (Lf) spanned muscle length (Lm) in m. epitrochlearis, while fibers tapered to terminate within m. retractor (Lf/Lm = 0.43 +/- 0. 02). With myelin staining, a single branch of ulnar nerve projected axons across the midregion of m. epitrochlearis. For m. retractor, the spinal accessory nerve branched to give rise to proximal and distal regions of innervation, with intermingling of axons between nerve branches. Nerve bundle cross-sections stained for acetylcholinesterase indicate that each motor axon projects to an average of 65 muscle fibers in m. epitrochlearis and 100 in m. retractor. Differences in fiber morphology, innervation topology, and neuromuscular organization may contribute to the heterogeneity of metabolic demand and oxygen supply in skeletal muscle.     

Interaction of central and peripheral mechanisms during walking in first instar stick insects, Extatosoma tiaratum

ABSTRACT. Individual stick insects were studied walking on a tread-wheel. When a leg caught hold of a small fixed rod beside the wheel it: (a) applied a rhythmically modulated backward directed force (with the modulation frequency identical to the stepping frequency of the other legs); or (b) regularly lifted off the stick during the phase of minimum force, and then returned to the stick; or (c) it stepped onto the wheel. If the wheel was stopped, the applied force increased. It is deduced that this behaviour is driven by central rhythmicity of unknown origin. All the known sensory inputs to this behaviour are superimposed on this central oscillation. A hypothesis is discussed which qualitatively fits all the experimental results on the effect of these sensory influences on the timing of an individual leg's movements in stick insects.     

A key to the eggs of stick and leaf insects (Phasmida)

Abstract. An illustrated key is provided for the identification of the eggs of fifty-eight genera of Phasmida.     

Mass-induced oscillations in the femur-tibia control system of stick insects

Attaching an inert mass to a freely moving tibia of an otherwise fixed stick insect Carausius morosus, induces undamped oscillations of the tibia. We describe the use of a rotational pendulum to observe these oscillations applying various amounts of inertia. The dependence of the frequency of these oscillations on the moment of inertia is similar to that of a purely mechanical system. The sequence of the oscillatory behavior can be separated into 3 distinct behavioural states. The transitions between some of these states could be elicited by external stimuli and partly showed characteristics of habituation and dishabituation. With a rotational pendulum on each middle leg, simultaneous oscillations of both legs were measured to investigate coupling effects between the neural control systems of the two legs. In some cases, significant coupling effects could be observed in phase and frequency. In many other cases, no coupling was found. The habituation and dishabituation effects were not transferred between the middle legs.     

Ecological niche dimensionality and the evolutionary diversification of stick insects

The degree of phenotypic divergence and reproductive isolation between taxon pairs can vary quantitatively, and often increases as evolutionary divergence proceeds through various stages, from polymorphism to population differentiation, ecotype and race formation, speciation, and post-speciational divergence. Although divergent natural selection promotes divergence, it does not always result in strong differentiation. For example, divergent selection can fail to complete speciation, and distinct species pairs sometimes collapse ('speciation in reverse'). Widely-discussed explanations for this variability concern genetic architecture, and the geographic arrangement of populations. A less-explored possibility is that the degree of phenotypic and reproductive divergence between taxon pairs is positively related to the number of ecological niche dimensions (i.e., traits) subject to divergent selection. Some data supporting this idea stem from laboratory experimental evolution studies using Drosophila, but tests from nature are lacking. Here we report results from manipulative field experiments in natural populations of herbivorous Timema stick insects that are consistent with this 'niche dimensionality' hypothesis. In such insects, divergent selection between host plants might occur for cryptic colouration (camouflage to evade visual predation), physiology (to detoxify plant chemicals), or both of these niche dimensions. We show that divergent selection on the single niche dimension of cryptic colouration can result in ecotype formation and intermediate levels of phenotypic and reproductive divergence between populations feeding on different hosts. However, greater divergence between a species pair involved divergent selection on both niche dimensions. Although further replication of the trends reported here is required, the results suggest that dimensionality of selection may complement genetic and geographic explanations for the degree of diversification in nature.     

Information processing in the femur-tibia control loop of stick insects

The complicated response characteristics of the identified nonspiking interneuron type E4 upon elongation stimuli to the femoral chordotonal organ (fCO) can be obtained by a computer simulation using the neuronal network simulator BioSim, if the following assumptions were introduced: (1) The interneurons receive direct excitatory input from position- and velocity-sensitive fCO afferents but also, in parallel delayed inhibition from the same velocity-sensitive afferents. (2) Position-sensitive afferents in part show adaptation with a rather long time-constant. A subsequent experimental analysis demonstrated that all these assumptions fit the reality: (1) Interneurons of type E4 receive direct excitatory input from fCO afferents. (2) Interneurons of type E4 are affected by velocity dependent delayed inhibitory inputs from the fCO. (3) The fCO does contain adapting position-sensitive sensory neurons, which have not been described before. The described principle of the information processing is also able to generate the response in interneurons of type E6 with less steep amplitude-velocity characteristic due to a different weighting of the direct excitation and delayed inhibition.Abbreviations EPSP excitatory postsynaptic potential - FETi fast extensor tibiae motor neuron - fCO femoral chordotonal organ - FT-control loop femur-tibia control loop - IPSP inhibitory postsynaptic potential - SETi slow extensor tibiae motor neuron     

The mitochondrial genome of Bacillus stick insects (Phasmatodea) and the phylogeny of orthopteroid insects

The Order Phasmatodea (stick and leaf insects) includes many well-known species of cryptic phytophagous insects. In this work, we sequenced the almost complete mitochondrial genomes of two stick insect species of the genus Bacillus. Phasmatodea pertain to the Polyneoptera, and represent one of the major clades of heterometabolous insects. Orthopteroid insect lineages arose through rapid evolutionary radiation events, which likely blurred the phylogenetic reconstructions obtained so far; we therefore performed a phylogenetic analysis to resolve and date all major splits of orthopteroid phylogeny, including the relationships between Phasmatodea and other polyneopterans. We explored several molecular models, with special reference to data partitioning, to correctly detect any phylogenetic signal lying in rough data. Phylogenetic Informativeness analysis showed that the maximum resolving power on the orthopteroid mtDNA dataset is expected for the Upper Cretaceous, about 80millionyears ago (Mya), but at least 70% of the maximum informativeness is also expected for the 150-200 Mya timespan, which makes mtDNA a suitable marker to study orthopteroid splits. A complete chronological calibration has also been computed following a Penalized Likelihood method. In summary, our analysis confirmed the monophyly of Phasmatodea, Dictyoptera and Orthoptera, and retrieved Mantophasmatodea as sister group of Phasmatodea. The origin of orthopteroid insects was also estimated to be in the Middle Triassic, while the order Phasmatodea seems to appear in the Upper Jurassic. The obtained results evidenced that mtDNA is a suitable marker to unravel the ancient splits leading to the orthopteroid orders, given a proper methodological approach.     

Modelling of the active reaction of stick insects by a network of neuromimes

Depending on the activity status of the animal, the control system of the femur-tibia joint in stick insects exhibits either a resistance reflex, or the active reaction, a totally different action pattern (Bässler 1988a). Using analog electronic neuron models, several different neuronal circuits are explored that model the active reaction with all its features. The models differ in complexity, redundance and the robustness against small variations of network parameters (e.g. coupling strengths). The circuit with the highest robustness and redundancy requires interneurons with special features, such as found in real animals. When inserted into a closed loop modeling movement and sensory feedback from the periphery, this circuit produces oscillations similar to searching movements found in the real animal. In addition to intracellular recording methods, the authors propose modeling with realistic neuromimes as a complementary method in the investigation of neuronal networks which have well documented input-output relationships.     

Identified nonspiking interneurons in leg reflexes and during walking in the stick insect

In the stick insect Carausius morosus identified nonspiking interneurons (type E4) were investigated in the mesothoracic ganglion during intraand intersegmental reflexes and during searching and walking.In the standing and in the actively moving animal interneurons of type E4 drive the excitatory extensor tibiae motoneurons, up to four excitatory protractor coxae motoneurons, and the common inhibitor 1 motoneuron (Figs. 1–4).In the standing animal a depolarization of this type of interneuron is induced by tactile stimuli to the tarsi of the ipsilateral front, middle and hind legs (Fig. 5). This response precedes and accompanies the observed activation of the affected middle leg motoneurons. The same is true when compensatory leg placement reflexes are elicited by tactile stimuli given to the tarsi of the legs (Fig. 6).During forward walking the membrane potential of interneurons of type E4 is strongly modulated in the step-cycle (Figs.8–10). The peak depolarization occurs at the transition from stance to swing. The oscillations in membrane potential are correlated with the activity profile of the extensor motoneurons and the common inhibitor 1 (Fig. 9).The described properties of interneuron type E4 in the actively behaving animal show that these interneurons are involved in the organization and coordination of the motor output of the proximal leg joints during reflex movements and during walking.Abbreviations CLP reflex, compensatory leg placement reflex - CI1 common inhibitor I motoneuron - fCO femoral chordotonal organ - FETi fast extensor tibiae motoneuron - FT femur-tibia - SETi slow extensor tibiae motoneuron     

Multiple direct transitions from sexual reproduction to apomictic parthenogenesis in Timema stick insects

Transitions from sexual reproduction to parthenogenesis may occur along multiple evolutionary pathways and involve various cytological mechanisms to produce diploid eggs. Here, we investigate routes to parthenogenesis in Timema stick insects, a genus comprising five obligate parthenogens. By combining information from microsatellites and karyotypes with a previously published mitochondrial phylogeny, we show that all five parthenogens likely evolved spontaneously from sexually reproducing species, and that the sexual ancestor of one of the five parthenogens was probably of hybrid origin. The complete maintenance of heterozygosity between generations in the five parthenogens strongly suggests that eggs are produced by apomixis. Virgin females of the sexual species were also able to produce parthenogenetic offspring, but these females produced eggs by automixis. High heterozygosity levels stemming from conserved ancestral alleles in the parthenogens suggest, however, that automixis has not generated the current parthenogenetic Timema lineages but that apomixis appeared abruptly in several sexual species. A direct transition from sexual reproduction to (at least functional) apomixis results in a relatively high level of allelic diversity and high efficiency for parthenogenesis. Because both of these traits should positively affect the demographic success of asexual lineages, spontaneous apomixis may have contributed to the origin and maintenance of asexuality in Timema .