Distribution of mechanoreceptive sensilla and their functions in the defensive behavior of tenebrionid beetle pupae

Abstract

correspondence: Toshio Ichikawa Department of Biology, Faculty of sciences, Kyushu University, Fukuoka 812-8581, Japan Tel +81 92 642 2612 Fax +81 92 642 2315 email tichiscb@kyuhsu-u.org Background: Pupae of the tenebrionid beetles exhibit different defensive behaviors in response to tactile stimulation of different parts of their bodies. Little is known about the mechanosensory mechanisms that initiate these defensive behaviors. To understand the mechanisms, we examined the distribution and function of the mechanoreceptive sensilla on the pupal body of Zophobas atratus. Methods: The sensilla on the surface of the pupal cuticle were observed using light and scanning electron microscopy. The tactile sensitivity of various parts of the pupal body was tested using three probes with different bending forces. The function of particular sensilla was examined by stimulating them electrically. Results: Hair (trichoid) sensilla were classified according to their length, ie, long hair sensilla (80–200 μm) were mostly located on the lateral regions of each body segment and short hairs (5–50 μm) were distributed across almost the entire surface of the pupal body, except for the elytra and intersegmental membrane of the abdomen. In addition to the trichoid sensilla, almost all parts of the pupal body, including the elytra and the intersegmental membrane, had campaniform sensilla (CS) with a dome-like structure in the cuticular layer; these sensilla detected strains in the cuticle. The CS were concentrated on the articulation of segmented appendages and the lateral region of the intersegmental membrane. The defensive response (abdominal rotation) to electrical stimulation was greatly reduced when afferent nerves from CS clustered on the apical region of the tarsus were transected. Conclusion: CS may be responsible for the high sensitivity of pupal appendages to tactile stimuli. The CS located on the appendages and abdominal intersegmental membrane may trigger specific and effective defensive behaviors. Both types of mechanoreceptive sensilla may enable pupal perception of the external environment, including the location of potentially harmful stimuli, and are involved in the control of defensive posture and movement.