Microscopic analysis of mechanosensory system monitoring the dynamic claw actions in the tenebrionid beetle Zophobas atratus

Many insects have a pair of claws on each leg. The distribution of mechanoreceptors that monitor claw actions was examined in the tenebrionid beetle Zophobas atratus. Each claw has 25–45 campaniform sensilla (CS) that detect the claw’s deformation due to substrate engagement. Five CS clusters are observed around the end of the 5th tarsomere (Ta5) in a concave, socket-like structure. The 1st cluster, containing 2–5 CS, is embedded in the unguifer to which the claws are articulated. The symmetrical 2nd and 3rd clusters, each containing two CS, are located bilaterally in the ventrolateral grooves of the sidewall of the socket, into which the unguis retractor plate slides. The 4th and 5th clusters, containing 1–2 CS with two hair sensilla, are localized near the ventrolateral ridges of the socket into which the basal portion of the claw is pressed during maximal claw flexion. In addition, Ta5 has a chordotonal organ of six sensory cells to monitor claw extension. These results suggest that the mechanoreceptor system may directly monitor the precise mechanical states of individual claws and provide the central nervous system with the sensory information required for fine feedback control of movements of the pretarsus and other leg segments for locomotion and other purposes.