Comparative anatomy of the spinneret musculature in cribellate and ecribellate spiders (Araneae)

Silk production is a prominent characteristic of spiders. The silk is extruded through spigots located on the spinnerets, which are single- to multimembered paired appendages at the end of the abdomen. Most extant spiders have three pairs of spinnerets, and in between either a cribellum (spinning plate) or a colulus (defunct vestigial organ), dividing these spiders into cribellate and ecribellate species. Previous research has shown that cribellate and ecribellate spiders differ not only in the composition of their spinning apparatus but also in the movements of their spinnerets during silk spinning. The objective of this study was to determine whether the differences in spinneret movements are solely due to variations in spinneret shape or whether they are based on differences in muscular anatomy. This was accomplished by analyzing microcomputed tomography scans of the posterior abdomen of each three cribellate and ecribellate species. It was found that the number of muscles did not generally differ between cribellate and ecribellate species, but varied considerably between the species within each of these two groups. Muscle thickness, particularly of the posterior median spinneret, varied slightly between groups, with cribellate spiders exhibiting more robust muscles, possibly to aid in the combing process during cribellar thread production. Interestingly, the vestigial colulus still possesses muscles, that can be homologized with those of the cribellum. This exploration into spinneret anatomy using microcomputed tomography data reveals that despite being small appendages, the spider spinnerets are equipped with a complex musculature that enables them to perform fine-scaled maneuvers to construct different fiber-based materials.