Goblin spiders without distinct receptacula seminis (Arachnida: Araneae: Oonopidae)

The mechanism of sperm transfer in spiders is unique among arthropods. Males use their modified pedipalps to transmit sperm which is received by females of most spider groups in specialized organs, the receptacula seminis. Oonopidae are a diverse spider family belonging to the Haplogynae. Female haplogynes have supposedly simple genitalia where the receptacula derive from the uterus externus, which is the duct passed by the eggs. The purpose of the present study is to describe the unusual female genitalia of the oonopids Xyphinus sp. and Ischnothyreus sp. by combining three methods of investigation: histological serial sections, scanning electron microscopy, and X-ray ultra microscopy, and to compare the results with previous findings on haplogynes. Furthermore, the male palps are briefly described. The female genitalia of both investigated species do not correspond to the general description of haplogyne genitalia given in the literature. Distinct receptacula are lacking in both species. In Xyphinus sp., sperm are deposited into the uterus internus. A sclerite with attached muscles in the uterus wall might serve females to lock the uterus during copulation in order to prevent sperm from getting into it. The male palp of Xyphinus sp. bears complicated structures indicating that males could use the palps as copulatory courtship devices. The ventral scutum of female Ischnothyreus sp. forms a large depression which was always empty in the examined specimens. Sperm could be stored in a large fold of the uterus externus. As in Xyphinus sp., spermatozoa inside the uterus internus of Ischnothyreus sp. indicate that fertilization takes place in the uterus or the ovary. The function of a sclerotized squiggled tube in females of Ischnothyreus sp. remains unknown. Surrounding glandular tissue suggests the involvement of the tube in glandular activity. The male palp shows a simple embolus accompanied by a small conductor.