The mechanics and neural control of the prey capture strike in the mantid shrimps Squilla and Hemisquilla

Abstract

1. The prey capture strike of Squilla and Hemisquilla has been studied by high speed motion picture photography and by electromyography from the limb muscles of the unrestrained animal. 2. The strike consists of a rapid unfolding of the dactyl, propus and carpus about the merus of the raptorial, 2nd thoracic limb. The prey may be struck by the folded propus-dactyl joint, pierced by the extended dactyl or grasped between the open propus and dactyl. 3. Contact is made with the prey in 4–8 ms, when the animal is underwater, at which time the propus is moving about the merus at an angular velocity of 20,000°·s−1 or at a linear velocity of 10 m·s−1. 4. Acceleration is applied to the limb in less than 1.5 ms and an energy requirement of 1.25×105 ergs is necessary for the strike. This could not be produced by a single muscle twitch. 5. Electrophysiology and experiments in which the strike has been simulated have shown that the strike is produced by the co-contraction of flexor and extensor muscles in the merus operating a ‘click’ joint. The flexor muscles begin to contract first, often 1 s before the strike so pulling a sclerite over a stop on the ventral wall of the merus. This gives them a 900∶1 mechanical advantage over the extensor muscles and allows the extensor muscles to contract almost isometrically and time to reach tetanic tension. The flexor muscles relax just prior to the strike so that the sclerite then snaps over its stop on the merus allowing the stored energy of the extensor muscles to be delivered suddenly.