THE HYDRAULIC MECHANISM OF THE PREDATORY STRIKE IN DRAGONFLY LARVAE

1. 1. Aeschna larvae catch prey with a fast-moving elongated labium. The mechanism of this movement was analysed by high-speed cinematographs and by hydrostatic and electrophysiological measurements. 2. 2. The strike movement consists of an initial, mid and late phase. The angular acceleration of the joints of the labium is 2·6 × 105 and 7·8 × 105 deg·s−2 during the initial and mid phase respectively. The torque necessary for the acceleration was calculated to be 1·3 × 10−5 and 4·0 × 10−5 N.m for the initial and mid-phases respectively. 3. 3. The relation between the pressure applied to the labium and the extension torque at the joints has been established. No torque develops about the postmentum-prementum joint as long as the click of the flexed labium is engaged. 4. 4. The power production of the extensor muscles is less than the power output of the mid phase. The power for the mid phase is derived from the internal body pressure developed by the contraction of the abdominal dorsoventral muscles. The required pressure for the mid phase is about 60 cmH2O if the resistance is neglected and 80 cmH2O when the resistance is considered. 5. 5. Abdominal dorso-ventral muscles contract 110–500 ms before the onset of the strike and the body pressure of the animal increases to a peak of 40–120 cmH2O at the onset of the strike. 6. 6. The geometry of the labial joints gives the primary flexor muscles of the labium a large mechanical advantage over the extensor muscles in the fully flexed labium, and allows the extensor muscles to contract almost isometrically. 7. 7. The extensor muscles and the primary flexor muscles co-contract for 75–100 ms before the strike. The strike movement begins when the flexor muscles relax. The stored energy in the extensor system is released suddenly and disengages the click producing the initial phase. Once the click is disengaged the internal pressure produces the large torque to move the labium with great acceleration during the mid phase.