Kinematics of elongate harvestmen chelicerae: Comparative range of motion analyses in extant Ischyropsalis (Dyspnoi, Opiliones)

Chelicerae, the mouthparts of chelicerates, are essential for food processing. Particularly within harvestmen (Opiliones), some species have greatly elongated their tripartite chelicerae and utilize them for mating behavior, defense, and primarily for predation. We investigated two European species, Ischyropsalis muellneri and Ischyropsalis hellwigii, which occupy different niches (caves, forests), exhibit different feeding ecologies (opportunist, specialist), and first and foremost possess different chelicerae morphologies (long and thin, short and robust). We scanned the specimens using state-of-the-art micro-CT, generated surface reconstructions, and equipped one chelicera of each specimen with artificial joints to explore their Range of Motion in a 3D kinematic approach. For a size-corrected comparison of the two species, we analyzed the Range of Motion in addition to three different settings (original body size, body scaled to 5 mm, chelicerae scaled to 5 mm). Ischyropsalis muellneri reached a higher maximum excursion angle (= single Range of Motion) in all three joints, also exhibiting a greater total Range of Motion in the original body length setting, as well as the scaled body length setting. Only in the third setting, the total Range of Motion of Ischyropsalis hellwigii was slightly higher, although Ischyropsalis muellneri still extended further ventrally. Our results suggest that the sturdier, more massive chelicerae of Ischyropsalis hellwigii, attributable to strong specialization on snails as prey, are associated with reduced Range of Motion. The less food-specialized species Ischyropsalis muellneri apparently requires higher flexibility of its chelicerae for prey capture, likely due to its restriction to cave ecosystems, where food availability is relatively scarce. We could show that virtual Range of Motion analyses in harvestmen chelicerae can play a pivotal role in understanding the theoretical feeding ecology and functional morphology of this group. This approach can be verified by in-vivo observations and measurements or extended to other arachnid taxa and other body parts.