“Trap-jaw” spiders are tiny (about 2 mm), ground-dwelling spiders that rely on hunting rather than web-building to capture prey. Hannah Wood, curator of spiders at the Smithsonian’s National Museum of Natural History, was curious as to why these spiders’ carapaces (their front sections) seemed almost neck-like and their chelicerae (mandibles or jaws) were longer, more maneuverable, and snapped shut faster compared to other spiders.
Because of their tiny size, customary methods for examining the spiders’ internal musculature and anatomy (such as dissection or conventional radiography) are impractical. So, Wood turned to synchrotron x-ray microtomography at ALS Beamline 8.3.2—essentially a CT scan but on a microscopic scale—to examine about 30 species of trap-jaw spiders and their relatives. She identified several morphological innovations that directly relate to cheliceral function, such as a highly modified carapace with horizontally oriented cheliceral muscles. Such structural innovations may have set the stage for the parallel evolution of power-amplified predatory strikes, in which movements can surpass the maximal power output of muscles by releasing slowly stored energy almost instantaneously.
The microtomography, combined with high-speed video studies of the kinematics of the chelicerae and molecular phylogenetic analysis (computer-aided reconstructions of evolutionary genetic relationships), led to the conclusion that power-amplified predatory strikes had evolved four times independently, once the basic trap-jaw body plan was in place. Understanding how these movements are achieved in spiders—the fastest movements observed in arachnids to date—could be useful in understanding basic principles of high-speed movements across all life forms.
Work performed at ALS Beamline 8.3.2.
H.M. Wood, D.Y. Parkinson, C.E. Griswold, R.G. Gillespie, and D.O. Elias, “Repeated Evolution of Power-Amplified Predatory Strikes in Trap-Jaw Spiders,” Current Biology 26, 1057 (2016).