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

The femoral chordotonal organ (FCO) in beetles differs from that in orthopterids in the origin of its apodeme: it originates directly from the tibia in the latter, but 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 than the turn of the tibia, and the arculum is connected to the FCO. This system behaves as a high-pass filter with a time constant close to the step period. Various aspects of the arculum have been studied previously, including its shape in various taxa, its biomechanics, matched neural activity in the FCO, as well as evolutionary aspects. The results of previous studies, published in 1985-2003 in Russian, are inaccessible to most foreign readers. However, original texts and the list of studied species (>350) are now available online. Recently, we minimized the system to three components: the proximal tibial ledge, the tendon and the arculum. The elastic tendon contains resilin. In four model species, the arculum readily turned upon stretching of the tendon. Turning was video recorded. The force of approximately 0.005 N, applied to a tendon approximately 0.25 mm in size, is enough for the utmost turn of the arculum. The arculum turned also upon local deformations close to its base. The 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.