Wind-sensitive interneurones in the terminal ganglion of praying mantids

Abstract

1. Twelve types of wind-sensitive neurone have been identified in the terminal ganglion of the mantids,Archimantis sobrina andA. latistylus (Fig. 1). Nine have conspicuously larger axons than the others in the connective of the ventral nerve cord and are termed ‘giant’ interneurones (Figs. 1, 2, 3), although they are small by comparison with those of other orthopteroid insects (Fig. 15). 2. Transverse sections of connectives reveal nine large axon profiles whose spatial relationship changes along the ventral nerve cord (Fig. 4). 3. Transverse sections of ganglia containing stained giant cells show that major arborizations are found in neuropilar regions containing the terminals of cereal afferents (Fig. 5). 4. Recorded cells could be divided into two groups depending on whether their responses to wind stimuli were purely excitatory (Figs. 6, 7), or contained an inhibitory component (El responses, Fig. 8). Electrical stimulation of cercal afferents confirmed the response patterns evoked by wind (Figs. 7, 8 and 9). 5. Dendritic arborizations are more strongly developed on the side of major synaptic input to each giant cell (Figs. 1, 2, 5) as established by electrical and wind stimulation of afferents in left and right cerci (Fig. 10). 6. Conduction velocities in the giant cells ranged from 2.5–3.1 m/s (Fig. 11, Table 1).These neurones are thus amongst the slowest conducting insect giant neurones, consistent with their small diameter relative to those of other insects (Fig. 15). 7. The short response latencies and the 1∶1 nature of their response to high frequency (100 Hz) electrical stimulation of the cereal nerve indicated that at least neurone Types 5, 8 and 11 (Fig. 11, Table 1) probably have direct connections with cereal receptors. 8. Several cells had high rates of spontaneous activity and in one (Type 5, Fig. 12A), injection of hyperpolarizing current produced a rhythmical bursting at approximately half the spontaneous rate. The interburst interval could be altered by phasic stimulation of the cereal nerve (Fig. 12 B). 9. Behavioural experiments with a tethered mantid showed responses in leg- and flight-motor pathways to wind stimulation of the cerci. In a dissected preparation, electrical stimulation of abdominal connectives and wind stimulation of the cerci both evoked responses in four neurones of the metathoracic ganglion: one spiking local interneurone and three motoneurones (Figs. 13, 14).