1. The failure to record receptor potentials from cut ends of the large tactile spines of the legs of the cockroach Periplaneta americana has prompted a histological and electrophysiological search for the sensory unit of these tactile spines.

2. Histological sections, made with a cuticle-softening fixative containing tetrahydrofuran, and cleared whole-cuticle preparations reveal a single campaniform sensillum in the thick cuticular wall of the spine at its junction with the soft cuticle of its socket.

3. Punctate stimulation in the same region reveals a highly mechanosensitive region similar in size, shape, orientation and location to the campaniform sensillum.

4. Movement of the spine during after-discharges which sometimes follow punctate stimulation does not produce a second impulse train, but modulates the after-discharge.

5. It is concluded that the campaniform sensillum is the sensory structure of the tactile spines of the cockroach, and is responsible for the normal discharge that occurs on contact with the spine.

6. Strong stimulation causes a failure of the afferent discharge, resembling the ‘overstretch’ phenomena seen with other mechanoreceptors.

7. Spike wave-form analysis suggests that impulse initiation may occur at more than one site on the sensory neuron or in more than one manner, and includes the possibility of a delayed, non-propagated discharge of the soma of the sensory neuron.

Supported by the Medical Research Council of Canada.