(1/97) Role of proprioceptive signals from an insect femur-tibia joint in patterning motoneuronal activity of an adjacent leg joint.
Interjoint reflex function of the insect leg contributes to postural control at rest or to movement control during locomotor movements. In the stick insect (Carausius morosus), we investigated the role that sensory signals from the femoral chordotonal organ (fCO), the transducer of the femur-tibia (FT) joint, play in patterning motoneuronal activity in the adjacent coxa-trochanteral (CT) joint when the joint control networks are in the movement control mode of the active behavioral state. In the active behavioral state, sensory signals from the fCO induced transitions of activity between antagonistic motoneuron pools, i.e., the levator trochanteris and the depressor trochanteris motoneurons. As such, elongation of the fCO, signaling flexion of the FT joint, terminated depressor motoneuron activity and initiated activity in levator motoneurons. Relaxation of the fCO, signaling extension of the FT joint, induced the opposite transition by initiating depressor motoneuron activity and terminating levator motoneuron activity. This interjoint influence of sensory signals from the fCO was independent of the generation of the intrajoint reflex reversal in the FT joint, i.e., the "active reaction," which is released by elongation signals from the fCO. The generation of these transitions in activity of trochanteral motoneurons barely depended on position or velocity signals from the fCO. This contrasts with the situation in the resting behavioral state when interjoint reflex action markedly depends on actual fCO stimulus parameters, i.e., position and velocity signals. In the active behavioral state, movement signals from the fCO obviously trigger or release centrally generated transitions in motoneuron activity, e.g., by affecting central rhythm generating networks driving trochanteral motoneuron pools. This conclusion was tested by stimulating the fCO in "fictive rhythmic" preparations, activated by the muscarinic agonist pilocarpine in the otherwise isolated and deafferented mesothoracic ganglion. In this situation, sensory signals from the fCO did in fact reset and entrain rhythmic activity in trochanteral motoneurons. The results indicate for the first time that when the stick insect locomotor system is active, sensory signals from the proprioceptor of one leg joint, i.e., the fCO, pattern motor activity in an adjacent leg joint, i.e., the CT joint, by affecting the central rhythm generating network driving the motoneurons of the adjacent joint. (+info)
(2/97) An interneurone of unusual morphology is tuned to the female song frequency in the bushcricket Ancistrura nigrovittata (Orthoptera, Phaneropteridae).
The interneurone AN5-AG7 of the duetting bushcricket Ancistrura nigrovittata has its soma in the seventh (penultimate) abdominal ganglion. Its major postsynaptic arborizations with dense thin branches of smooth appearance are found in the prothoracic ganglion. The branches terminate in the auditory neuropile, predominantly at the same location as those auditory receptors that respond best to the female song frequency. Correspondingly, AN5-AG7 responds preferentially to frequencies between 24 and 28 kHz, thereby matching the carrier frequency of the female response song quite well. At frequencies below 24 kHz, AN5-AG7 receives inhibition, which is sometimes seen as clear inhibitory postsynaptic potentials. At these frequencies, thresholds of excitatory postsynaptic potentials are considerably lower than spike thresholds. In contrast, above 20 kHz, the two thresholds match and they correspond to the behavioural threshold. The AN5-AG7 interneurone is more sensitive to soma-contralateral stimuli and it receives predominantly inhibition, but also some excitation, from the soma-ipsilateral ear. Response strength is not greatly affected by stimulus duration but shows prominent habituation. This habituation depends only weakly on intensity and frequency. Some AN5-AG7 interneurones show very small graded potentials and no spiking responses to any acoustic stimuli. (+info)
(3/97) Multimodal convergence of presynaptic afferent inhibition in insect proprioceptors.
In the leg motor system of insects, several proprioceptive sense organs provide the CNS with information about posture and movement. Within one sensory organ, presynaptic inhibition shapes the inflow of sensory information to the CNS. We show here that also different proprioceptive sense organs can exert a presynaptic inhibition on each other. The afferents of one leg proprioceptor in the stick insect, either the position-sensitive femoral chordotonal organ or the load-sensitive campaniform sensilla, receive a primary afferent depolarization (PAD) from two other leg proprioceptors, the campaniform sensilla and/or the coxal hairplate. The reversal potential of this PAD is about -59 mV, and the PAD is associated with a conductance increase. The properties of this presynaptic input support the hypothesis that this PAD acts as presynaptic inhibition. The PAD reduces the amplitude of afferent action potentials and thus likely also afferent transmitter release and synaptic efficacy. These findings imply that PAD mechanisms of arthropod proprioceptors might be as complex as in vertebrates. (+info)
(4/97) Central connections of receptors on rotated and exchanged cerci of crickets.
The cerci of crickets, paired abdominal appendages bearing sound-sensitive filiform hairs, can be removed and grafted back so that their morphological axes acquire various relationships to those of the body. We have studied both the morphogenetic consequences of such surgery and the central connections made by the regenerating axons of the cercal sensory neurons. If a cercus is rotated and grafted back into its own socket, it back-rotates towards its original orientation in succeeding molts. If left and right cerci are exchanged, with or without rotation, back-rotation does not occur and super-numerary cerci are formed in predictable locations. There are two sub-populations of filiform hairs: those that vibrate transversely to the cercal shaft (T-hairs) in dorsal and ventral sectors, and longitudinally vibrating hairs (L-hairs) in lateral and medial sectors. Two giant interneurons are excited by T-hairs of their own side but not by L-hairs. If cerci are grafted so that they assume various orientations relative to the body, a consistent physiological result is obtained: T-hairs always appear to be the source of excitatory input to the giant interneurons, no matter where they are caused to be located by prior surgery. The phenomena of back-rotation, formation of supernumerary cerci, and formation of connections selectively by T-hairs, can be interpreted on the hypothesis of morphogenetic gradients. (+info)
(5/97) Biological microtribology: anisotropy in frictional forces of orthopteran attachment pads reflects the ultrastructure of a highly deformable material.
Evolutionarily optimized frictional devices of insects are usually adapted to attach to a variety of natural surfaces. Orthopteran attachment pads are composed of hexagonal outgrowths with smooth flexible surfaces. The pads are designed to balance the weight of the insect in different positions and on different materials. In a scanning electron microscopy study followed by freezing-substitution experiments, the ultrastructural architecture of the pad material was visualized. In friction experiments, the interaction was measured between the attachment pad and a polished silicon surface. The inner structure of this material contains distally directed rods, branching close to the surface, and spaces filled with fluid. The specific design of the pad material provides a higher frictional force in the distal direction. Frictional anisotropy is more enhanced at higher normal forces and lower sliding velocities. It is concluded that optimal mechanical functionality of biosystems is the result of a combination of surface structuring and material design. (+info)
(6/97) Ultrasound avoidance behaviour in the bushcricket Tettigonia viridissima (Orthoptera: Tettigoniidae).
The responses of female Tettigonia viridissima to simulated bat echolocation calls were examined during tethered flight. The insects responded with three distinct behaviours, which occurred at graded stimulus intensities. At low intensities (threshold 54 dB SPL), T. viridissima responded by steering away from the sound source (negative phonotaxis). At intensities approximately 10 dB higher, beating of the hindwing was interrupted, although the insect remained in the flight posture. A diving response (cessation of the wingbeat, closure of the forewings and alignment of the legs against the body) occurred with a threshold of 76 dB SPL. Considering these thresholds, we estimate that the diving response occurs at approximately the sound amplitude at which many aerial-hawking bats first receive echoes from the insect. The other behaviours probably occur before the bat detects the insect and should therefore be interpreted as early avoidance behaviours. The repertoire of startle responses in T. viridissima, with directional and non-directional components, is similar to those of crickets and moths, but quite different from those described for another bushcricket (Neoconocephalus ensiger), which shows only a non-directional response. This supports the conclusion that bat-evasive behaviours are not conserved within the Tettigoniidae, but instead are shaped by the ecological constraints of the insects. (+info)
(7/97) A biologically inspired controller for hexapod walking: simple solutions by exploiting physical properties.
The locomotor system of slowly walking insects is well suited for coping with highly irregular terrain and therefore might represent a paragon for an artificial six-legged walking machine. Our investigations of the stick insect Carausius morosus indicate that these animals gain their adaptivity and flexibility mainly from the extremely decentralized organization of the control system that generates the leg movements. Neither the movement of a single leg nor the coordination of all six legs (i.e., the gait) appears to be centrally pre-programmed. Thus, instead of using a single, central controller with global knowledge, each leg appears to possess its own controller with only procedural knowledge for the generation of the leg's movement. This is possible because exploiting the physical properties avoids the need for complete information on the geometry of the system that would be a prerequisite for explicitly solving the problems. Hence, production of the gait is an emergent property of the whole system, in which each of the six single-leg controllers obeys a few simple and local rules in processing state-dependent information about its neighbors. (+info)
(8/97) Stand tall and they still get you in your Achilles foot-pad.
The free-living first-instar larvae of Strepsiptera (Insecta) are the infective stage of the parasitoid. They normally enter the host via the abdominal cuticle, and there have also been reports of entry via the egg of the host. The first-instar larvae of Stichotrema dallatorreanum Hofeneder in Papua New Guinea were found to enter the host orthopteran via the tarsi. This is, to my knowledge, the first report of entry of first-instar larvae of Strepsiptera via the attachment pads (euplantulae) of the host. (+info)