Equilibrium receptors of cockroach cerci: Anatomy of cerci and tricholiths

Pendulous sensilla, seemingly related to trichobothria, on the ventral surface of the cerci of the burrowing cockroach Arenivaga are shown physiologically to be equilibrium receptors (Walthall, 1978). These modified sensilla, arranged in two rows on each cercus, are termed tricholiths. The plumb bob-like shape of the tricholiths is ideally suited for position reception. Each tricholith is composed of a dense sphere located distally on a slender shaft, the shaft being elliptical in cross section. The sensilla, keeping a constant relationship with respect to the gravitational force vector, occupy a null or non-stimulating position when the animal is at its primary orientation. Upon the cockroach's movement away from primary orientation, the tricholiths are deflected stimulating innervating sensory cells. Only one buttress hinges each tricholith, a condition which allows more universal movement of the tricholiths. However, direction of movement is restricted by the eccentric placement of the shaft in the socket. This affords movement of each row of tricholiths away from the midline of the cercus. There is a strong positive relationship between the structure and function of this equilibrium receptor system. Four interneurons are responsive to equilibrium information, each interneuron being driven by afferents from a specific row of tricholiths (Walthall, 1978). Polar plots of the interneurons' responses mimic the quadrants of movement for the rows of tricholiths. The sensitivity differences seen for the interneurons is explained by the rotation of the cercus from the horizontal body axis.