The effect of leg length on jumping performance of short- and long-legged leafhopper insects

Abstract

To assess the effect of leg length on jumping ability in small insects, the jumping movements and performance of a sub-family of leafhopper insects (Hemiptera, Auchenorrhyncha, Cicadellidae, Ulopinae) with short hind legs were analysed and compared with other long-legged cicadellids (Hemiptera, Auchenorrhyncha, Cicadellidae). Two species with the same jumping characteristics but distinctively different body shapes were analysed: Ulopa, which had an average body length of 3 mm and was squat, and Cephalelus, which had an average body length of 13 mm with an elongated body and head. In both, the hind legs were only 1.4 times longer than the front legs compared with 1.9-2.3 times in other cicadellid leafhoppers. When the length of the hind legs was normalised relative to the cube root of their body mass, their hind legs had a value of 1-1.1 compared with 1.6-2.3 in other cicadellids. The hind legs of Cephalelus were only 20% of the body length. The propulsion for a jump was delivered by rapid and synchronous rotation of the hind legs about their coxo-trochanteral joints in a three-phase movement, as revealed by high-speed sequences of images captured at rates of 5000 s(-1). The hind tarsi were initially placed outside the lateral margins of the body and not apposed to each other beneath the body as in long-legged leafhoppers. The hind legs were accelerated in 1.5 ms (Ulopa) and 2 ms (Cephalelus) and thus more quickly than in the long-legged cicadellids. In their best jumps these movements propelled Ulopa to a take-off velocity of 2.3 m s(-1) and Cephalelus to 2 m s(-1), which matches that of the long-legged cicadellids. Both short-legged species had the same mean take-off angle of 56 degrees but Cephalelus adopted a lower angle of the body relative to the ground (mean 15 degrees) than Ulopa (mean 56 degrees). Once airborne, Cephalelus pitched slowly and rolled quickly about its long axis and Ulopa rotated quickly about both axes. To achieve their best performances Ulopa expended 7 microJ of energy, generated a power output of 7 mW, and exerted a force of 6 mN; Cephalelus expended 23 microJ of energy, generated a power output of 12 mW and exerted a force of 11 mN. There was no correlation between leg length and take-off velocity in the long- and short-legged species, but longer legged leafhoppers had longer take-off times and generated lower ground reaction forces than short-legged leafhoppers, possibly allowing the longer legged leafhoppers to jump from less stiff substrates.