Tight turns in stick insects

We investigated insects Carausius morosus walking whilst hanging upside down along a narrow 3 mm horizontal beam. At the end of the beam, the animal takes a 180° turn. This is a difficult situation because substrate area is small and moves relative to the body during the turn. We investigated how leg movements are organised during this turn. A non-contact of either front leg appears to indicate the end of the beam. However, a turn can only begin if the hind legs stand in an appropriate position relative to each other; the outer hind leg must not be placed posterior to the inner hind leg. When starting the turn, both front legs are lifted and usually held in a relatively stable position and then the inner middle leg performs a swing-and-search movement: The leg begins a swing, which is continued by a searching movement to the side and to the rear, and eventually grasps the beam. At the same time the body is turned usually being supported by the outer middle leg and both hind legs. Then front legs followed by the outer middle leg reach the beam. A scheme describing the turns based on a few simple behavioural elements is proposed.     

Response characteristics of single trochanteral campaniform sensilla in the stick insect, Cuniculina impigra

ABSTRACT. The campaniform sensilla on the trochanter of the stick insect, Cuniculina impigra Redtenbacher, were stimulated by slightly bending the leg in the horizontal plane. Single sensory units in the nerve were recorded using glass microelectrodes. These units can be classified into tonic and phasic-tonic receptors. In both cases there were units which increased their discharge frequency during forward movement of the femur, and units which responded to backward movement. No purely phasic receptors were found.     

Cuticular microstructures turn specular black into matt black in a stick insect

The stick insect Peruphasma schultei stands out from other insects by its deep matt black cuticle. We tested whether the appearance of P. schultei is due to microstructures of the cuticle, a phenomenon that has recently been described for the velvet black scales of the Gaboon viper. The shiny black stick insect Anisomorpha paromalus served as a control. We found that the P. schultei cuticle is characterised by two different types of microstructures, tall elevations with a maximum size of 18 μm and small structures with a height of 4 μm. Other than in the snake, P. schultei microstructures do not bear nanostructures. The microstructures scatter light independently of the viewing angle. This causes the matt appearance of the cuticle, whereas pigments are responsible for the black colouration, resulting in a maximum reflectance of 2.8% percent. The microstructures also cause the hydrophobic properties of the cuticle with contact angles near 130°. Resin replicas and bleaching of the cuticle strongly support these results. Moreover, the matt black cuticle has a higher heat absorption compared to the control. We discuss the selective benefit of the matt black appearance of P. schultei in the context of behaviour, ecology and phylogeny.     

Physiology of vibration-sensitive afferents in the femoral chordotonal organ of the stick insect

The femoral chordotonal organ in orthopterans signals proprioceptive sensory information concerning the femur-tibia joint to the central nervous system. In the stick insect, 80 out of 500 afferents sense tibial position, velocity, or acceleration. It has been assumed that the other sensory cells in the chordotonal organ would serve as vibration detectors. Extracellular recordings from the femoral chordotonal organ nerve in fact revealed a sensitivity of the sense organ for vibrations with frequencies ranging from 10 Hz to 4 kHz, with a maximum sensitivity between 200 and 800 Hz. Single vibration-sensitive afferents responded to the same range of frequencies. Their spike activity depended on acceleration amplitude and displacement amplitude of the vibration stimulus. Additionally, 80% of the vibration-sensitive afferents received indirect presynaptic inputs from themselves or from other afferents of the femoral chordotonal organ, the amplitude of which depended on stimulus frequency and displacement amplitude. They were associated with a decrease of input resistance in the afferent terminal. From the present investigation we conclude that the femoral chordotonal organ of the stick insect is a bifunctional sensory organ that, on the one hand, measures position and movement of the tibia and, on the other hand, detects vibration of the tibia. Accepted: 6 November 1998     

Sensorimotor pathways processing vibratory signals from the femoral chordotonal organ of the stick insect

The femoral chordotonal organ of stick insects senses position and velocity of movements in the femur-tibia joint, as well as tibial vibration. While sensory information about large-scale tibial movements is processed by a well-known neuronal network and elicits resistance reflexes in extensor and flexor tibiae motoneurons, it is not yet known how sensory information about vibration of the tibia is processed. We investigated the transmission of vibration stimuli to tibial extensor motoneurons and their premotor interneurons. Vibration stimuli applied to the femoral chordotonal organ evoked responses in tibial extensor and flexor muscles. During ongoing vibration this response adapted rapidly. This adaptation had no effect on the motoneuronal response to large-scale tibial movements. Recording from premotor interneurons revealed that vibratory signals were processed in part by the same interneuronal pathways as (large-scale) velocity and position information. While only certain parts of the interneuronal reflex pathways showed little or no response during vibration stimuli, most neurons responded to both position or velocity stimuli and vibration at the femoral chordotonal organ. We conclude that sensory information about vibration of the tibia shares part of the interneuronal pathways that transmit sensory information about large-scale tibial movements to the motoneurons. Accepted: 25 April 1999     

A dynamic model of thoracic differentiation for the control of turning in the stick insect

Leg movements of stick insects (Carausius morosus) making turns towards visual targets are examined in detail, and a dynamic model of this behaviour is proposed. Initial results suggest that front legs shape most of the body trajectory, while the middle and hind legs just follow external forces (Rosano H, Webb B, in The control of turning in real and simulated stick insects, vol. 4095, pp 145–156, 2006). However, some limitations of this explanation and dissimilarities in the turning behaviour of the insect and the model were found. A second set of behavioural experiments was made by blocking front tarsi to further investigate the active role of the other legs for the control of turning. The results indicate that it is necessary to have different roles for each pair of legs to replicate insect behaviour. We demonstrate that the rear legs actively rotate the body while the middle legs move sideways tangentially to the hind inner leg. Furthermore, we show that on average the middle inner and hind outer leg contribute to turning while the middle outer leg and hind inner leg oppose body rotation. These behavioural results are incorporated into a 3D dynamic robot simulation. We show that the simulation can now replicate more precisely the turns made by the stick insect. This work was supported by CONACYT México and the European Commission under project FP6-2003-IST2-004690 SPARK.     

Independent suboesophageal neuronal innervation of the defense gland and longitudinal muscles in the stick insect (Peruphasma schultei) prothorax

This study investigates the neuroanatomy of the defense gland and a related muscle in the stick insect Peruphasma schultei with axonal tracing and histological sections. The gland is innervated by three neurons through the Nervus anterior of the suboesophageal ganglion (SOG), the ipsilateral neuron (ILN), the contralateral neuron (CLN) and the prothoracic intersegmental neuron (PIN). The ILN has a large soma which is typical for motoneurons that cause fast contraction of large muscles and its dendrites are located in motor-sensory and sensory neuropile areas of the SOG. The CLN might be involved in the coordination of bilateral or unilateral discharge as its neurites are closely associated to the ILN of the contralateral gland. Close to the ejaculatory duct of the gland lies a dorsal longitudinal neck muscle, musculus pronoto-occipitalis (Idlm2), which is likely indirectly involved in gland discharge by controlling neck movements and, therefore, the direction of discharge. This muscle is innervated by three ventral median neurons (VMN). Thus, three neuron types (ILN, CLN, and PIN) innervate the gland muscle directly, and the VMNs could aid secretion indirectly. The cytoanatomy of motorneurons innervating the defense gland and neck muscle are discussed regarding the structure and functions of the neuropile in the SOG. As a basis for the neuroanatomical study on the defense gland we assembled a map of the SOG in Phasmatodea.     

Response of moist-air receptor on antenna of the stick insect,Carausius morosus,to step changes in temperature

Summary The moist-air sensory cell in the antennal mound-shaped sensillum of Carausius responds to changes in relative humidity brought about by changing either the temperature (T) of the air or the partial pressure of water vapor (Pw) (Figs. 1, 5). When changes in either parameter cause relative humidity (Hr) to rise between roughly 5% and 55%, the same Hr-changes elicit very close to the same responses, no matter how Hr is changed (Figs. 2, 3). Even the resolving power for upward Hr-steps produced by lowering T is very close to that produced by raising Pw: 7.4% vs. 6.3%. Thus upward transients in impulse frequency (F) may be read off directly as quantitative rises in Hr. Whether the rise in Hr as signalled by the moist-air cell is the result of a rise in Pw or of a drop in T could well be indicated by the simultaneous reactions of the cold and dry-air cells (Figs. 1, 5). The moist-air cell also reacts to downward step changes in Hr, but very differently, depending on how they are brought about. Upward steps in T have a much larger effect on F than corresponding downward steps in Pw (Fig. 4). This result demonstrates that changes in relative humidity do not suffice to explain the changes in the activity of the moist-air cell. The receptor may be better construed as bimodal, reacting to changes in T and Pw with independent sensitivities and well matched to relative humidity when changes in either lead to increases in this parameter. The moistair cell's responses to T-steps at Pw = 0 (Figs. 1, 5) offer some support for this interpretation.Abbreviations F impulse frequency in impulses/s (imp/s) - Hr relative humidity in % - Ps saturation pressure of water vapor in torr - Pw partial pressure of water vapor in torr - r correlation coefficient - T temperature in °C     

Adaptive control for insect leg position: controller properties depend on substrate compliance

This paper concentrates on the system that controls the femur-tibia joint in the legs of the stick insect, Carausius morosus. Earlier investigations have shown that this joint is subject to a mixture of proportional and differential control whereby the differential part plays a prominent role. Experiments presented here suggest another interpretation: single legs of a stick insect were systematically perturbed using devices of different compliance and compensatory forces and movements monitored. When the compliance is high (soft spring), forces are generated that return the leg close to its original position. When the compliance is low (stiff spring), larger forces are generated but sustained changes in position occur that are proportional to the force that is applied. Selective ablation of leg sense organs showed that the leg did not maintain its position after elimination of afferents of the femoral chordotonal organ. Ablation of leg campaniform sensilla had no effect. These data support the idea that different control strategies are used, depending upon substrate compliance. In particular, what we and other authors have called a differential controller, is now considered as an integral controller that intelligently gives up when the correlation between motor output and movement of the leg is low.We would like to dedicate this article to Prof. Dr. Ulrich Bässler. Starting in the 1960s, his seminal work stimulated a long series of fruitful studies that, even today, reveal exciting insights into motor control.