Physiological properties of afferents from tactile hairs on the hindlegs of the locust.

1. The spatial distribution and physiological properties of the tactile hairs and their sensory afferents of the hindlegs of the locust Schistocerca gregaria are described. Hairs occur in a consistent position from animal to animal. Hairs on the dorsal tibia are of a greater mean length, 403.1 +/- 17.9 microns, than hairs on the ventral tibia, 265.2 +/- 10.44 microns. Hairs on the dorsal and ventral femur are approximately 310 microns long and hairs on the tarsus are shorter, approximately 200 microns. 2. Based on their threshold responses to sinusoidal mechanical stimulation, the tactile hairs are of two basic physiological types. High-threshold hairs occur only on the proximal dorsal tibia and at the proximal base of each tibial spine, whereas low-threshold hairs are found on the femur, tibia and tarsus. 3. High-threshold hair afferents respond phasically to imposed deflections of the hair shaft and have mean velocity thresholds of 21.1 +/- 4.2 degrees s-1. They adapt rapidly to repetitive stimulation, after as few as 11 cycles at 5 Hz. Low-threshold hairs respond in a more phaso-tonic manner to imposed deflections and have velocity thresholds of less than 3 degrees s-1. They adapt to repetitive stimulation at a similar rate to the high-threshold hairs, but continue to respond to stimulation after 200 cycles at 5 Hz. 4. All the hairs are directionally sensitive, with the preferred direction being oriented proximally and medially (i.e. proximally and towards the body); they are least sensitive to deflections in the opposite direction.