thumb|upright=1.5|The external anatomy of a spider, with the epigyne labeled
thumb|upright=1.75|The internal anatomy of a spider, with the reproductive system (purple) reaching the exoskeleton at the epigyne
The epigyne or epigynum is the external genital structure of female spiders. As the epigyne varies greatly in form in different species, even in closely related ones, it often provides the most distinctive characteristic for recognizing species. It consists of a small, hardened portion of the exoskeleton located on the underside of the abdomen, in front of the epigastric furrow and between the epigastric plates.
Functions
The primary function of the epigyne is to receive and direct the palpal organ of the male during copulation. The various specific forms of epigynes are correlated, in each case, with corresponding specific differences in the palpus of the male. This specialization prevents individuals of different species from mating. The epigyne covers or accompanies the openings of the spermathecae, which are pouches for receiving and retaining sperm. Frequently, the openings of the spermathecae are on the outer face of the epigyne and can be easily seen. In many cases the guide extends laterally on each side at its posterior end. This is true to a slight extent in the epigyne of Trabeops, but more markedly so in that of many species of Geolycosa, where the lateral expansions often conceal the openings of the spermathecae, as in the epigyne of Geolycosa pikei.