Why Study Spider Sex: Special Traits of Spiders Facilitate Studies of Sperm Competition and Cryptic Female Choice

I argue that several unusual aspects of spider sexual biology make them extremely promising subjects for future research on sperm competition and cryptic female choice, and outline promising lines for future research. The important traits include: double, bilaterally symmetrical genitalia (allowing the use of the same animal as experimental and control and thus providing unusually complete controls for experimental manipulations); isolation of male ejaculates in pure form during sperm induction (allowing experimental determination of the effects of sperm and male accessory glands on female reproductive physiology, and separation of their effects on the female from those of copulatory courtship and copulation); frequent venter-up orientation and genitalic meshes in which most of the male genitalia is outside rather than inside the female (allowing unusually complete observations of male genital behavior during copulation); immobile sperm (allowing confident deductions about male and female movement of sperm without complications from motility of the sperm themselves); a huge data set on female as well as male genitalic morphology from previous taxonomic studies (enabling, in combination with studies of the fit between male and female genitalia, studies of the details of how rapid genitalic divergence occurs). Studies of spider sex should be in the forefront of the next generation of studies of sperm competition and cryptic female choice.