THE VENOM OF THE COCKROACH-HUNTING WASP AMPULEX-COMPRESSA CHANGES MOTOR THRESHOLDS - A NOVEL TOOL FOR STUDYING THE NEURAL CONTROL OF AROUSAL

1. The wasp Ampulex compressa hunts for cockroaches Periplaneta americ ana which are stung twice, first in the prothorax and then in the neck . Cockroaches stung only in the prothorax recover after a few minutes. Cockroaches stung in both the prothorax and the neck become unrespons ive to various mechanical stimuli. The maximal effect is observed roug hly 20 minutes after completion of the second sting. Most animals reco ver fully from the behavioural deficits within about 15 days. 2. Stung cockroaches show little spontaneous movement although they are able t o walk or fly. Wind stimuli directed at the cerci, which normally prod uce strong escape responses, are no longer effective in stung cockroac hes. For wind-evoked flying behaviour, the threshold of flight initiat ion increases. The wingbeat frequency, however, remains unchanged. 3. Intracellular recordings from neurons of the flight and escape circuit ry show that the venom does not affect signal transmission between the wind afferents and the ascending giant interneurons (GIs). The wind-e voked response of the GIs is also comparable to that of controls. The typical flight motor pattern can be induced in control and stung cockr oaches by driving specific giant interneurons. However, in stung anima ls more action potentials from Gis are required to elicit flight. This indicates that the strength of the connection between the GIs and tho racic interneurons (TIs) and/or TIs to motorneurons is depressed by th e venom. 4. The effect of the venom on the initiation of locomotory be haviour is compared with the known effects of various neuromodulators which may control the arousal state of insects.