Fig 2. Mandible diversity and trap-mechanism of Strumigenys ants.

Strumigenys is a pantropical, hyperdiverse genus of leaf litter predators. (A) Many species have normal mandibles which they use to grip and hold their prey. Some species have power-amplified “trap-jaw” mandibles which they use to strike and stun prey, which comes in short-mandibled (S-TRAP) and long-mandibled (L-TRAP) forms. (B) The trap-jaw mechanism (seen here in a segmented X-ray micro-CT image) involves the modification of the labrum and basal mandibular process into a latch which locks the mandibles in an open position, with labrum muscle triggering release and mandible closure. The trap mechanism is associated with the reorganization of the muscle fibers, a transition from fast to slow muscle fiber orientation, and other changes to the head design (described further in S1 Text). (C) With trap-jaw mandibles in the locked position, the labrum and basal mandibular processes form a latch mechanism, controlled by rotation of the labrum in the sagittal plane.