Jumping in a wingless stick insect,Timema chumash(Phasmatodea,Timematodea, Timematidae)

Abstract

SUMMARYThe stick insect Timema chumash belongs to a sub-order of the phasmids that is thought to have diverged early from other stick insects, and which is restricted to the southwest of North America. It jumps by rapidly extending the tibiae of both its hind legs simultaneously from an initially fully flexed position, unlike any other stick insect that has been described. The hind legs are 1.5 times longer than the front and middle legs, but still represent only half the length of its body, and the femoro-tibial joints show few specialisations for jumping. In its best jumps, the wingless body is accelerated in 12 ms to a take-off velocity of 0.9 m s–1 and experiences an acceleration of 75 m s–2, the equivalent of 8 g. This performance requires an energy expenditure of 19 μJ,generates a power output of 1.6 mW and exerts a force of 3.6 mN. The jump propels the body forward a distance of 80 mm from a mean take-off angle of 39°. Heights of 20 mm were also achieved. Elevation of the jump was controlled by the initial position of the hind legs; when the hind tibiae and femora projected above the dorsal outline of the body the jump was forwards,when parallel with the long axis of the body the jump was backwards and could result in somersaulting. The jumping movements would appear to displace Timema in different directions away from a potential predator.