The
vagina of a jumping
spider. It is a hardened plate on the underside of her
abdomen, up near her waist, into which the males put
sperm using their
palpi. The male places his palpi near the epigynum, it swells up, enters, and then just fires away like this: doo doo do dodododododo dooo. It's also used for identification of the spider's sex.
spiders.arizona.edu