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

Abstract

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. K E Y W O R D Sanatomy, Arachnida, chelicerae, homology

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

“…Compared with those of other spiders, the cheliceral bases in mecysmaucheniids are surrounded by relatively large amounts of membranous tissue and are not close enough to other sclerites to restrict movement. Complicating this, in mecysmaucheniids and other spiders, there are no less than nine different muscles that articulate the basal section of the chelicerae (Wood and Parkinson, 2019).…”

“…from Wood and Parkinson (2019), different cheliceral/cephalothorax structures were digitally labeled ('segmented') and converted to a surface mesh with the purpose of homologizing the cheliceral muscles that operate the paturon within Palpimanoidea. In the current study, I follow the naming conventions and color-coding of Wood and Parkinson (2019) for each of the nine homologous cheliceral muscles (Table 1, Fig. 2; and see figs 4 and 5 of Wood and Parkinson , 2019).…”

“…For simplicity, whenever an individual muscle is discussed, it will be followed in parentheses with the color code used in Table 1 and the figures. Using Avizo software, to quantify morphological differences the following measurements were taken, and for paired structures, measurements were averages of both the left and right side: carapace Wood and Parkinson (2019).…”

“…Next, I asked whether differences in functional performance correspond with differences in the internal morphology of the muscles. Synchrotron-based micro-computed tomography (µCT) scanning allows the visualization of tiny internal structures and was recently done across Palpimanoidea, including both Aotearoa and Zearchaea, with the goal of homologizing cheliceral musculature (Wood and Parkinson, 2019). The 3D surfaces from these µCT scans were used to compare muscle and sclerite morphologies in Aotearoa and Zearchaea.…”

Do exaggerated chelicerae function as weapons or genitalia in a long‐jawed spider? Functional allometric analysis yields an answer

“…Less is known about the impact of cheliceral form and function in spiders, although new research has uncovered some incredible adaptations (Wood, Parkinson, Griswold, Gillespie, & Elias, 2016). In several species of the trap-jawed spiders (Archaeidae), both males and females have extremely long chelicerae, suggesting that natural selection shaped the elongated chelicerae to enhance prey capture (Wood, Griswold, & Gillespie, 2012;Wood & Parkinson, 2019). Yet, in the sexually dimorphic jumping spider Myrmarachne lupata (L. Koch 1879, Salticidae) only males bear the long chelicerae, which function as weapons in intraspecific interactions, suggesting sexual selection has lengthened male chelicerae (Jackson, 1982).…”