Epigynum: the external female genital structure of spiders

Structure and parts

Externally, the epigynum may be a simple slit, a complex sclerotized plate, or display grooves and lobes. Internally it connects to copulatory ducts and paired storage organs called spermathecae, where the female keeps received sperm until eggs are ready. Key components often cited by anatomists include:

• the epigynal plate or atrium,
• copulatory openings,
• internal ducts and spermathecae.

Function and mating

During courtship the male deposits sperm on a small web and charges his pedipalps with it; he then inserts his pedipalps into the female's epigynum to transfer sperm. The epigynum thus guides sperm to internal storage without itself producing gametes. Stored sperm can fertilize eggs days, weeks, or longer after mating, depending on the species.

Variation, identification and significance

Epigynal morphology is often species-specific, making it an important character for taxonomists and field identifiers. Differences in shape, position and internal duct configuration are used to distinguish closely related species and sexes. Behavioral and evolutionary studies also examine epigynal traits in the context of sexual selection, mate choice and mechanisms like mating plugs or sperm precedence.

Notes and terminology

The term derives from Greek roots referring to a female organ and is used in arachnology to denote the external genital region. While this article focuses on adult females, juvenile spiders lack a fully formed epigynum until maturity. For general information about spider anatomy and reproductive behavior see resources on spider biology and comparative genital morphology at sources such as taxonomic guides and review texts on reproductive anatomy. For further reading on mating mechanics see introductions to pedipalp function and studies of sperm storage and use in spiders.