The role of contact chemoreception in the egg-laying behaviour of the desert locust, Schistocerca gregaria

Chemoreception plays an important role in the egg-laying behaviour of locusts. Chemicals that represented the basic taste classes were added to sandy substrates at various concentrations and their effect on egg-laying analysed. Concentration dependent reductions in egg-pod number were observed for all chemicals, with the highest concentrations of all chemicals preventing egg-laying. Similarly, locusts spent less time egg-laying in substrates containing higher chemical concentrations, compared to low chemical concentrations. The concentration at which chemicals acted aversively depended on the type of chemical. Isolation of the locust abdomen from the thorax produces fictive digging movements of the ovipositor valves that are required for successful egg-laying. Chemical stimulation of the ovipositor valves of rhythmically active preparations resulted in a cessation of the digging rhythm. High chemical concentrations stopped the digging rhythm for significantly longer durations than low chemical concentrations and distilled water controls. The neuromodulator, nitric oxide (NO) was found to modulate the oviposition digging rhythm. Increasing and decreasing NO and its known molecular targets in the part of the nervous system that produces the digging movements of the values, the terminal abdominal ganglion, significantly increased and decreased the frequency of the digging rhythm respectively. Moreover, NO was also found to significantly alter the effects of chemical stimulation of the valves. Increasing NO levels in the terminal abdominal ganglion resulted in a significant decrease in the duration of cessation of the digging rhythm when the valves were chemically stimulated, whereas decreasing NO levels resulted in a significant increase in the cessation of the digging rhythm.