Comparative morphology of cheliceral muscles using high-resolution X-ray microcomputed-tomography in palpimanoid spiders (Araneae, Palpimanoidea).

Spiders are important predators in terrestrial ecosystems, yet we know very little about the principal feeding structures of spiders, the chelicerae, which are functionally equivalent to "jaws" or "mandibles" and are an extremely important aspect of spider biology. In particular, members of Palpimanoidea have evolved highly unusual cheliceral morphologies and functions, including high-speed, ballistic movements in mecysmaucheniid spiders, the fastest arachnid movements known thus far, and the elongated, highly maneuverable chelicerae of archaeids that use an attack-at-a-distance strategy. Here, using micro-Computed-Tomography scanning techniques, we perform a comparative study to examine cheliceral muscle morphology in six different spider specimens representing five palpimanoid families. We provide a hypothesis for homology in palpimanoid cheliceral muscles and then compare and contrast these findings with previous studies on other non-palpimanoid spiders. We document and discuss two sets of cheliceral muscles in palpimanoids that have not been previously observed in other spiders or which may represent a position shift compared to other spiders. In the palpimanoids, Palpimanus sp., Huttonia sp., and Colopea sp. showed similar cheliceral muscle anatomy. In Eriauchenius ranavalona, which has highly maneuverable chelicerae, some of the muscles have a more horizontal orientation, and there is a greater degree of cheliceral muscle divergence. In Zearchaea sp. and Aotearoa magna, some muscles have also shifted to a more horizontal orientation, and in Zearchaea sp., a species with a ballistic, high-speed predatory strike, there is a loss of cheliceral muscles. This research is a first step toward understanding cheliceral form and function across spiders.