Morphology and action of the hind leg joints controlling jumping in froghopper insects

SUMMARY The morphology and movements of key joints of the hind legs that generate the rapid jumping of froghoppers were analysed. The movements of an individual hind leg during a jump occur in three phases. First, the trochanter is slowly levated about the coxa so that the femur moves anteriorly and engages with a lateral protrusion on the coxa. Second, both hind legs are held in this fully levated (cocked) position without moving for a few seconds. Third, both hind legs depress and extend completely in less than 1 ms. The critical, power-generating movement underlying a jump is the rapid and simultaneous depression of the trochantera about the coxae. The lever arm of the hind trochanteral depressor muscle is smallest at the cocked position, but does not appear to go over the centre of the pivot. It then increases to a maximum after some 80° of depression movement. By contrast, the lever arm of the trochanteral levator tendon is similar over the range of joint movements and is exceeded by that of the depressor only after 40° of depression. Three prominent arrays of hairs on the trochantin, coxa and trochanter are appropriately positioned to act as proprioceptors signalling key movements in jumping. In the fully levated position, a protrusion on the dorsal, proximal surface of a hind femur engages with a protrusion from the ventral and lateral part of a coxa. These structures are not present on the front and middle legs. Both protrusions are covered with a dense array of small projections (microtrichia) that both increase the surface area and may interlock with each other. To depress rapidly in a jump these protrusions must disengage. If the hind leg of a dead froghopper is forcibly levated, it will lock in its cocked position, from which it can depress rapidly by movement of the coxo-trochanteral joint and disengagement of the femoral and coxal protrusions. A prominent click sound occurs at the start of a jump that results either from the initial movements of the coxo-trochanteral joint, or from the disengagement of the microtrichia on the coxa and femur. Larval Philaenus, which do not jump, lack a femoral protrusion and have no microtrichia in equivalent positions on either the coxa or femur.