The vibrational startle response of the desert locust Schistocerca gregaria

Substratum vibrations elicit a fast startle response in unrestrained quiesc ent desert locusts (Schistocerca gregaria). The response is graded with sti mulus intensity and consists of a small, rapid but conspicuous movement of the legs and body, but it does not result in any positional change of the a nimal. With stimuli just above threshold, it begins with a fast twitch of t he hindlegs generated by movements of the coxa-trochanter and femur-tibia j oints. With increasing stimulus intensity, a rapid movement of all legs may follow, resulting in an up-down movement of the whole body. The magnitude of both the hindleg movement and electromyographic recordings from hindleg extensor and flexor tibiae muscles increases with stimulus am plitude and reaches a plateau at vibration accelerations above 20 m s(-2) ( peak-to-peak). Hindleg extensor and flexor tibiae muscles in unrestrained a nimals are co-activated with a mean latency of 30 ms. Behavioural threshold s are as low as 0.47 ms(-2) (peak-to-peak) at frequencies below 100 Hz but rise steeply above 200 Hz. The response habituates rapidly, and inter-stimu lus intervals of 2 min or more are necessary to evoke maximal reactions. Intracellular recordings in fixed (upside-down) locusts also revealed co-ac tivation of both flexor and extensor motor neurones with latencies of appro ximately 25 ms. This shows that the neuronal network underlying the startle movement is functional in a restrained preparation and can therefore be st udied in great detail at the level of identified neurones.