Morphological comparison of the cephalothoracic endoskeleton of the infraorder Mygalomorphae

The morphology of dorsal apodemes and endosternites was examined in nine female spider species belonging to eight genera and five families of the infraorder Mygalomorphae in Japan.Dorsal apodemes and endosternites are endoskeletal structures in the cephalothorax that serve as attachment sites for the muscles of chelicerae, pedipalps, legs, and pedicel.Dorsal apodemes are formed by an invagination of the ectoderm, which is externally visible as the fovea, and endosternites are endodermal in origin and possess tendons in some groups, such as Mesothelae and Mygalomorphae.These tendons connect to the sternum, with their attachment sites externally recognizable as sigilla.The morphology of dorsal apodemes was classified into the following four types based on the frontal sectional shape: (1) horizontally plate-shaped, (2) longitudinally teardrop-shaped, (3) longitudinally T-shaped, and (4) longitudinally Y-shaped.The morphology of endosternites was classified into the following two types based on the number of tendons: (1) endosternites with four pairs of tendons and (2) endosternites with two pairs of tendons.This study revealed that differences in the morphology of dorsal apodemes and endosternites correspond to differences in the characteristics of the attached muscles.Moreover, the morphology of the endosternite and its associated muscle attachment patterns exhibited consistency within families and genera that included multiple observed species.Therefore, these structures may serve as fundamental taxonomic traits for establishing higher taxonomic classifications such as families and genera.