Jumping and kicking in the false stick insect Prosarthria teretrirostris: kinematics and motor control

Abstract

The false stick insect Prosarthria teretrirostris looks and behaves like a real stick insect but can jump and kick rapidly and powerfully like a locust, to which it is more closely related. It has an elongated body with slender hind legs that are some 2.5 times longer than the front and middle legs. A male with a body 67 mm long and weighing 0.28 g can jump 90 cm with a take-off angle of 40 degrees and velocity of 2.5 ms(-1), requiring an energy expenditure of 850 microJ. The body is accelerated at 165 ms(-2) for only 30 ms. The larger and heavier females (mean body length 104 mm and weighing 1.5 g) can jump on average a distance of 49 cm. During jumping, the tibiae of the hind legs are extended in 30 ms with maximum rotational velocities of 11.5 degrees per ms, but during kicking, when there is no body weight to support, extension is complete in 7 ms with rotational velocities as high as 48 degrees per ms. The short time available to accelerate the body indicates that the movements are not powered by direct muscle contractions and that there must be storage of elastic energy in advance. The motor patterns responsible for generating the necessary forces in the hind legs for jumping and kicking are similar and consist of three phases; an initial flexion of the tibia is followed by a co-contraction of the small flexor and large extensor tibiae muscles lasting several hundred milliseconds while the tibia remains fully flexed. Finally, the flexor motor neurons stop spiking so that the tibia is able to extend rapidly. The small semi-lunar processes at the femoro-tibial joints are not distorted, so that they cannot act as energy stores. Some 7% of the energy is stored transiently by bending the thin tibiae during the initial acceleration phase of a jump and releasing it just before take-off. The jumping and kicking mechanisms of Prosarthria teretrirostris have features in common with those used by locusts but also have their own characteristics. The evolution of jumping in Orthoptera is discussed in this context.