Ultrastructure of the ejaculatory duct of Terrobittacus implicatus (Mecoptera: Bittacidae)

The unique mating behavior of Bittacidae has been extensively studied, yet the mechanisms underlying internal sperm transport and temporary storage before mating remain enigmatic. Herein, we aim to elucidate these mechanisms by investigating the fine structure of the ejaculatory duct, which serves for sperm transport and temporary storage. The ultrastructure of the ejaculatory duct of Terrobittacus implicatus (Mecoptera: Bittacidae) was examined by light and transmission electron microscopy for the first time in this study. The ejaculatory duct is composed of a median duct and a pair of symmetrical accessory sacs. In the proximal fifth portion, the two accessory sacs encompass the median duct in two loose layers. In the remaining distal portion, the median duct remains centrally positioned, and two accessory sacs symmetrically enclose its lateral and ventral surfaces. The distal median duct consists of a basal lamina, an unevenly arranged epithelium, a large subcuticular cavity, and a narrow inner cuticle. The distal accessory sac can be divided into three areas with distinct ultrastructural features. The ejaculatory duct exhibits conspicuous secretory activity, and given the absence of an ectodermal accessory gland in males, it is possible that the ejaculatory duct may fulfill additional glandular function. The narrow lumen and the reduced muscular sheath of the ejaculatory duct may be associated with the unique mechanism of ejaculation and mating.