Comparative female genital morphology in Stegodyphus spiders (Araneae: Eresidae)

Abstract

The anatomy of the female genitalia sets the arena for sperm competition in species in which females mate multiply and store sperm. In spiders, females possess cuticular internal structures that have evolved into diverse sperm storage sites. Here, we investigate the female genital morphology of seven eresid spider species. We used X-ray micro-computed tomography for 3D reconstruction of the anatomy of the female genital system in the social Stegodyphus dumicola, S. mimosarum and S. sarasinorum, and the subsocial S lineatus, S. pacificus, S. tentoriicola as well as Eresus sandaliatus. We used histology to assess the sites of sperm storage in two selected species S. lineatus and S. dumicola. Our results show that the internal genitalia of the Stegodyphus species consist of two bilateral folds instead of closed ducts as often reported for entelegyne spiders. Along each fold, three regions are discriminable that differ in their specific morphology but are all surrounded by glandular tissue. Between species, the regions differ in shape and dimension. In virgin females, the lumina of all regions are filled with secretion. In mated females, spermatozoa were found in all three regions; however, only those sperm that are stored in the posterior region seem to become activated. Sperm found in anterior regions are embedded in a substance that might act as a mating plug. Our data suggest that the regions of the female genital tract differ in accessibility by the male as well as in their potential use for ejection and manipulation of sperm by the female.