The cerci and abdominal giant fibres of the house cricket, Acheta domesticus . I. Anatomy and physiology of normal adults

The abdominal cerci of crickets are clothed with approximately 3400 hair and campaniform sensilla. Their axons form functional connexions in the terminal abdominal ganglion with a group of ten giant interneurons. The two largest interneurons (l.g.i. and m.g.i.) are powerfully excited by ipsilateral sound and air puff stimulation, weakly excited by substrate vibration via non-cercal receptors, and inhibited by contralateral cercal stimulation. These patterns of connexion are used to assay the precision of synapse formation in animals whose cercal development has been experimentally manipulated. The cercal sensilla comprise: about 300 singly innervated filiform hairs and 65 morphologically related clavate hairs; 2400 multiply innervated appressed trichoid sensilla of two subtypes; and about 600 campaniform sensilla associated with the sockets of the filiform hairs. The cercal nerve carries a corresponding number of axons with the majority in the 0.1 to 0.5 $\mu $m diameter range. The appressed hairs are rather uniformly distributed over the cercal surface. In contrast, the clavate hairs occur as a single patch on the medial aspect of the base of each cercus. The structure of the socket causes each filiform hair to vibrate in a single plane. The majority vibrate in the longitudinal axis of the cercus, but well defined mid-dorsal and mid-ventral rows vibrate transversely. Only a few obliquely oriented hairs are found. The axons of the cercal nerve degenerate very rapidly after removal of the cercus, and degeneration figures indicate the distribution of axonal terminals in the ganglion. All terminals in normal animals are ipsilateral, arranged in two loosely organized glomeruli. The l.g.i. and m.g.i. respond to sound stimuli of 2000 Hz and less (best frequency - 600 Hz), to air streams of velocities about 0.5 m/s and above, and to substrate vibration in a narrow band of frequencies centred on about 2000 Hz. Thresholds have not been measured carefully, but are 70 dB or less for sound in an open laboratory, and 10$^{-6}$ cm or less for substrate vibration. The l.g.i. and m.g.i. show a marked directional sensitivity to sound, presumably related to the vibration planes of the filiform hairs; differences of 5 degrees in speaker location appear to be discriminable.