THE TRAP-JAW MECHANISM IN THE DACETINE ANTS DACETON ARMIGERUM AND STRUMIGENYS SP

Ants of three different subfamilies, among them the tribe Dacetini, ha ve evolved very fast snapping mandibles called trap-jaws, The two dace tine genera examined, the large Daceton and the small Strumigenys, emp loy the same mechanism for their mandible strike, Video analysis revea ls that, in Strumigenys sp., the strike takes less than 2.5 ms, It is released within 5 ms by contact of trigger hairs on the labrum, The an ts employ a catapult mechanism to generate such a fast movement. Befor e the strike, the mandibles are opened wide and locked in the open pos ition by the labrum, which functions as a latch. They stay open even w hen the large slow closer muscles contract, Upon trigger hair stimulat ion, the labrum is pulled backwards by a small, fast trigger muscle. T he mandibles are thus freed from the catch and close rapidly. This ref lex is controlled by giant sensory and motor neurones in the labral ne uromere that are probably monosynaptically coupled. The short latency of the reflex thus results from the combination of a catapult mechanis m, fast trigger muscles, high neuronal conduction velocities and small synaptic delays. Comparison with the trap-jaw mechanism of the ant ge nus Odontomachus reveals a remarkable example of convergent evolution.