The ultrastructure of trichobothria (Tm2, Tr2, Tr4) of the elytra has been studied in the troglobitic Coleoptera Geotrechus vulcanus and Aphaenops cerberus. Two bipolar neurons innervate these trichobothria. The first ends at the level of the hair base, and its distal segment contains a tubular body, characteristic of mechanoreceptive bristles. The other does not possess a tubular body, and its distal segment ends in the bristle canal; its function is unknown. The trichobothria possess one glial enveloping cell, one trichogen cell, and one tormogen cell; the latter two show an apical, common, large, receptor lymph cavity. The small trichobothria are innervated by a large mechanoreceptor neuron, and by 4 smaller neurons; its function is unknown. The trichobothria of blind Trechinae are highly specialized. A cuticular cup enshrines the hair base; hair and cup move together. The large amplitude swaying movements of the hair are controlled by a spongious tissue around the cup. The trichobothria of Trechinae and Periplaneta have the same type of cuticular dome-shaped structure, and differ from trichobothria that arise from a cavity in the cuticle.