An anatomical atlas of the spinneret muscle system with insights into the evolution of the spinning apparatus of spiders (Araneae).

Silk production is a characteristic trait of spiders, with silk extruded through spigots on the spinnerets-articulated appendages at the rear of the abdomen. The number, shape, and length of spinnerets, as well as the number of spinneret articles vary in different evolutionary lineages of spiders. Recent research has revealed that, despite their small size, spider spinnerets possess complex and varied musculature, enabling precise movements for producing diverse silk materials. Nevertheless, the detailed muscular configuration of the spinning apparatus remains poorly understood, and it is unclear whether there are distinct apomorphic muscular conditions in different lineages or notable variation within clades due to ecological adaptations. This study addresses these questions by examining the musculature of the spinning apparatus in representatives of all major spider lineages using micro-computed tomography. We found that the core musculature of the anterior lateral spinnerets is highly conserved across all lineages surveyed. In contrast, the posterior median spinnerets display some consistent patterns but generally exhibit greater variability in their muscular arrangement than the anterior lateral spinnerets. This difference may relate to the anterior lateral spinnerets' role in producing dragline silk from major ampullate glands, which is vital for safety lines and is associated with conserved spinning behaviours. Meanwhile, the posterior lateral spinnerets are more prone to specialized behavioural adaptations. Overall, these findings enhance our understanding of spider evolutionary morphology and provide important insights into the diversity of spinning behaviours across the spider tree of life.