Do enlarged hind legs of male thick-legged flower beetles contribute to take-off or mating?

The volume of the hind femora in the adult male flower beetle Oedemera nobilis is 38 times greater than in adult females. To determine what advantage limbs with swollen femora might provide, the behaviour of these insects was analysed with high-speed videography. First, because large hind legs are often associated with jumping and take-off, the performance of this behaviour by the two sexes was determined. Take-off was generated by a series of small-amplitude wing beats followed by larger ones, with the hind legs contributing little or no propulsion. The mean acceleration time to take-off was not significantly different in males (46.2 ms) and females (45.5 ms), but the mean take-off velocity of males was 10% higher than in females. Second, to determine if enlarged hind legs were critical in specifically male behaviour, interactions between males and females, and between males were videoed. The male mounted a female and then encircled her abdomen between the enlarged femora and tibiae of both his hind legs. The joint between these leg parts acted like a mole wrench (vice grip) so that when the tibia was fully flexed, a triangular space of 0.3 mm(2) remained, in which a female abdomen (cross-sectional area 0.9 mm(2)) could be compressed and restrained firmly without inflicting damage. The flexor tibiae muscle in a male hind femur was 5.9 times larger than the extensor. In interactions between males, attempts to achieve a similar entrapment were frequently thwarted by the pursued male extending his hind legs vertically.