Performance, morphology and control of power-amplified mandibles in the trap-jaw ant (Hymenoptera: Formicidae).

Trap-jaw ants are characterized by high-speed mandibles used for prey capture and defense. Power-amplified mandibles have independently evolved at least four times among ants, with each lineage using different structures as a latch, spring and trigger. We examined two species from the genus (subfamily Formicinae), whose morphology is unique among trap-jaw ant lineages, and describe the performance characteristics, spring-loading mechanism and neuronal control of strikes. Like other trap-jaw ants, latch their jaws open while the large closer muscle loads potential energy in a spring. The latch differs from other lineages and is likely formed by the co-contraction of the mandible opener and closer muscles. The cuticle of the posterior margin of the head serves as a spring, and is deformed by approximately 6% prior to a strike. The mandibles are likely unlatched by a subgroup of closer muscle fibers with particularly short sarcomeres. These fast fibers are controlled by two large motor neurons whose dendrites overlap with terminals of large sensory neurons originating from labral trigger hairs. Upon stimulation of the trigger hairs, the mandibles shut in as little as 0.5 ms and at peak velocities that are comparable with other trap-jaw ants, but with much slower acceleration. The estimated power output of the mandible strike (21 kW kg) confirms that jaws are indeed power amplified. However, the power output of mandibles is significantly lower than distantly related trap-jaw ants using different spring-loading mechanisms, indicating a relationship between power-amplification mechanism and performance.