Morphology and performance of the 'trap-jaw' cheliceral strikes in spiders (Araneae, Mecysmaucheniidae).

Mecysmaucheniidae spiders have evolved ultra-fast cheliceral strikes 4 times independently. The mechanism for producing these high-speed strikes is likely due to a latch/spring system that allows for stored energy to be rapidly released. This study examined two different sister lineages: has ultra-fast cheliceral strikes and , based on external morphology of the clypeus, is hypothesized to have slower strikes. Using high-speed videography, I first gathered kinematic data on each taxon. Then, using histology and data from micro-computed tomography scanning, I examined internal cheliceral muscle morphology to test whether shifts in muscle anatomy correspond to performance differences in cheliceral strike. Results from high-speed video analysis revealed that achieves peak angular velocities of 25.0×10±4.8×10 rad s (mean±s.d.) in durations of 0.0843±0.017 ms. The fastest recorded strike had a peak angular and linear velocity of 30.8×10 rad s and 18.2 m s, respectively. The slower striking sister species, , was three orders of magnitude slower in velocity and longer in duration. Histology revealed sarcomere length differences, with some muscles optimized for force, and other muscles for speed. 3D printed models revealed structural differences that explain how the chelicerae hinge open and close. Combining all of this evidence, I put forth a hypothesis for the ultra-fast trap-jaw mechanism. This research documents the morphological shifts that accompany ultra-fast movements that result in increased rotation in joints and increased muscle specialization.