Mandibular premotor interneurons of larval Manduca sexta

Manduca sexta caterpillars parasitized by the wasp Cotesia congregata stop chewing despite an intact motor system

The parasitic wasp Cotesia congregata suppresses feeding in its host, the caterpillar Manduca sexta, during specific periods of wasp development. We examined both feeding behaviour and the neurophysiology of the mandibular closer muscle in parasitized and unparasitized control M. sexta to determine how the wasp may accomplish this. To test whether the wasps activated a pre-existing host mechanism for feeding cessation, we examined the microstructure of feeding behaviour in caterpillars that stopped feeding due to illness-induced anorexia or an impending moult. These microstructures were compared with that shown by parasitized caterpillars. While there were overall differences between parasitized and unparasitized caterpillars, the groups showed similar progression in feeding microstructure as feeding ended, suggesting a common pattern for terminating a meal. Parasitized caterpillars also consumed less leaf area in 100 bites than control caterpillars at around the same time their feeding microstructure changed. The decline in food consumption was accompanied by fewer spikes per burst and shorter burst durations in chewing muscle electromyograms. Similar extracellular results were obtained from the motorneuron of the mandibular closer muscle. However, chewing was dramatically re-activated in non-feeding parasitized caterpillars if the connectives posterior to the suboesophageal ganglion were severed. The same result was observed in unparasitized caterpillars given the same treatment. Our results suggest that the reduced feeding in parasitized caterpillars is not due to damage to the central pattern generator (CPG) for chewing, motor nerves or chewing muscles, but is more likely to be due to a suppression of chewing CPG activity by ascending or descending inputs.

Analyses of the mouthpart kinematics in Periplaneta americana (Blattodea, Blattidae) using synchrotron-based X-ray cineradiography

The kinematics of the biting and chewing mouthparts of insects is a complex interaction of various components forming multiple jointed chains. The non-invasive technique of in vivo cineradiography by means of synchrotron radiation was employed to elucidate the motion cycles of the mouthparts in the cockroach Periplaneta americana. Digital X-ray footage sequences were used in order to calculate pre-defined angles and distances, each representing characteristic aspects of the movement pattern. We were able to analyze the interactions of the mouthpart components and to generate a functional model of maxillary movement by integrating kinematic results, morphological dissections and fluorescence microscopy. During the opening and closing cycles, which take about 450-500 ms on average, we found strong correlations between the measured maxillary and mandibular angles, indicating a strong neural coordination of these movements. This is manifested by strong antiphasic courses of the maxillae and the mandibles, antiphasic patterns of the rotation of the cardo about its basic articulation at the head and by the deflection between the cardo and stipes. In our functional model of the maxilla, its movement pattern is explained by the antagonistic activity of four adductor-promotor muscles and two abductor-remotor muscles. However, beyond the observed intersegmental and bilateral stereotypy, certain amounts of variation across subsequent cycles within a sequence were observed with respect to the degree of correlation between the various mouthparts, the maximum, minimum and time course of the angular movements. Although generally correlated with the movement pattern of the mandibles and the maxillary cardo-stipes complex, such plastic behaviour was especially observed in the maxillary palpi and the labium.

Different physiological properties in a pool of mandibular closer motor neurons in a caterpillar, Bombyx mori

To determine whether a large pool of mandibular closer motor neurons in an insect consists of different functional groups, the physiological properties of the motor neurons were investigated by intracellular recording techniques. The mandibular closer muscle of a caterpillar, Bombyx mori, was innervated by at least 11 motor neurons with large somata and 2-3 neurons with small somata. The large 11 motor neurons could be divided into slow and fast types on the basis of thresholds of neurons and frequency of soma spikes by current injections. The fast motor neurons usually evoked relatively larger muscle potentials, whereas the slow neurons evoked smaller potentials. The slow neurons spiked while the mandible was held in a fully closed position, whereas the fast neurons did not. The slow neurons did not spike while the mandible was free to move. These results suggest different physiological properties among closer motor neurons, supporting the hypothesis that the closer motor neurons in a large neuron pool can be divided into different functional groups.

Chemosensory tuning to a host recognition cue in the facultative specialist larvae of the mothManduca sexta

Larvae of Manduca sexta are facultative specialists on plants in the family Solanaceae. Larvae reared on solanaceous foliage develop a strong preference for their host; otherwise, they remain polyphagous. The host-specific recognition cue in potato foliage for Manduca larvae is the steroidal glycoside, indioside D. Two pairs of galeal taste sensilla, the lateral and medial sensilla styloconica, are both necessary and sufficient for the feeding preferences of host-restricted larvae. We conducted electrophysiological tip recordings from sensilla of solanaceous or wheat germ diet-reared larvae. For each animal, recordings of the responses to indioside D, glucose, tomatine and KCl were compared. All responses included both phasic and tonic portions. The sensilla styloconica of solanaceous-reared larvae were tuned to indioside D, defined as maintaining a high sensitivity to indioside D, while showing lower sensitivity to other plant compounds. Half of the sensillar neurons of solanaceous-reared larvae were `tuned' to indioside D,whereas those of wheat germ diet-reared larvae were not. The different responses between the two types of animals were a result of changes of individual receptor cells' responses in the sensilla. Feeding on solanaceous foliage therefore appears to result in a modification of the physiological responses of individual taste receptor cells that causes them to be tuned to the host-recognition cue indioside D. We propose that this tuning is the basis for the host-restricted larvae's strong behavioral preferences for solanaceous foliage.

Temporally patterned activity recorded from mandibular nerves of the isolated subesophageal ganglion of Manduca

We recorded bursts of motor neuron activity from closer and opener mandibular nerves of isolated subesophageal ganglia (SOG) and compared them with the feeding motor pattern of intact Manduca larvae. Closer bursts recorded from isolated SOG lasted from 1 to 4s, interburst interval durations lasted from 2 to 49s, and within- and between-animal variability was great. In contrast, motor activity bursts (EMGs) measured from mandibular closer muscles of intact, feeding animals lasted 0.08 to 0.24s with interburst intervals of 0.26 to 0.57s. Variability both within and between animals was small. Bath application of 10(-4)M octopamine to the isolated SOG tended to increase frequency and reduce the duration of bursts, so that they became more like those recorded during feeding.