Mecysmaucheniidae spiders have evolved ultra-fast cheliceral strikes four 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 examines two different sister-lineages: Zearchaea has ultra-fast cheliceral strikes and Aotearoa, based on external morphology, is hypothesized to have slower strikes. Using high-speed videography, I gather kinematic data on each taxon and test the hypothesis that external morphology predicts cheliceral strike performance. Then, using histology and data from &[mu]-Computed-Technology scanning I ask whether internal muscle morphologies also correspond to performance differences. Results from high-speed video analysis reveal that Zearchaea sp. achieves peak angular velocities of 25.0 &[plusmn] 4.8 x 103 rad s-1 (mean &[plusmn] standard deviation) in durations of 0.0843 &[plusmn] 0.017 ms. The fastest recorded strike had a peak angular and linear velocity of 30.8 x 103 rad s-1 and 18.2 m s-1, respectively. The slower striking sister-species, Aotearoa magna, was three orders of magnitude slower in velocity and longer in duration. Histology revealed sarcomere length differences, with some muscles specialized to be slow and forceful, and others to be fast and non-forceful. 3D printed models reveal structural differences that explain how the chelicerae hinge open and closed. 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 and result in increased rotation in joints and increased muscle specialization.

Data were collected using a high-speed video camera. Cheliceral strikes from individual videos were tracked using the 'point tool' in Fiji. Cheliceral displacement was fitted with a quintic spline and kinematic variables were calculated.