Mechanical and optical properties of the femoral chordotonal organ in beetles (Coleoptera)

Abstract

Femoral chordotonal organ (FCO) in beetles differs from that in orthopterids in origin of a FCO apodeme: directly from the tibia in the latter, amidst the tendon of the extensor muscle in the former. In many beetles, the apodeme pops up from the tendon as a short sclerite (arculum). It turns distally upon bending of the tibia. The turn of the arculum is several times more then the turn of the tibia. The arculum is connected with the FCO. This system behaves as a high pass filter with a time constant close to the step period. The arculum was studied at various aspects: shape of the arculum in various taxa, biomechanics, matched neural activity in the FCO, evolutionary aspects. Results, published in 1985-2003 in Russian, are unknown for foreign readers. Original texts and the list of studied species (>350) are now available on line. Recently, we minimized the system to few components: proximal tibial ledge-tendon-arculum. The elastic tendon contains resilin. In four model species, the arculum readily turned upon stretch of the tendon. Turning was video recorded. The force about 0.5 G, applied to a tendon of about ¼ mm in size, is enough for the utmost turn of the arculum. The arculum turned also upon local deformations close to its base. Ability to turn vanished after incision between the arculum and the distal part of the extensor apodeme. A mechanical model of an amplifier is proposed. The apodeme includes optically active structures, which behave differently in polarized light.