Varieties of filiform hairs: range fractionation by sensory afferents and cereal interneurons of a cricket

Summary

• 1. Wide variations in the size of the cercal filiform hairs inGryllus bimaculatus are described (Figs. 1, 2). The length of the hairs varies from 30 to 1,500 μm, while the diameter varies from 1.5 to 9 μm (Fig. 2). The range of hair length overlaps well with the physical depth of air-motion on a substrate floor. The length dependency of sensory threshold to air-current stimulus is predictable.

• 2. The sensory threshold to the alternating air-current stimulus was measured. The sensory afferent was penetrated at the cereal nerve bundle. The length of the filiform hair of the recorded afferent was identified by needle probe. All sensory afferents showed phase locked responses to each cycle of bursts of sinusoidal air-current (Fig. 3).

• 3. The long filiform hairs are spontaneously active and sensitive to a low frequency stimulus (Figs. 3, 4). They are regarded as velocity sensitive hairs. The short hairs are spontaneously inactive and insensitive to low frequency stimulus. They are acceleration sensitive hairs.

• 4. The selective deprivation of the sensory hairs longer than 500 μm has little effect on the threshold of large interneurons 9-1 (LGI) and 8-1 (MGI) (Fig. 6). Under the same deprivation we were unable to record small-sized interneurons 10-2 and 10-3.

• 5. The threshold curves of the sensory hairs and those of the cereal interneurons are compared (Fig. 7). The conspicuously long cereal filiform hairs converge upon two small sized interneurons 10-2 and 10-3. Large cereal interneurons 9-1 (LGI) and 8-1 (MGI) receive the main excitatory sensory input from the short hairs around 200–300 μm.