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dayglo - Slugs are hermaphroditic: having both female and male reproductive organs. Once a slug has located a mate they encircle each other and sperm is exchanged through their protruding genitalia. A few days later hundreds of eggs are laid in holes in the ground. Although some species hibernate over the winter in temperate climates, in most species the adults die in the autumn.

A commonly seen practice among many slugs is apophallation, when one or both of the slugs chew off the other's penis. The penis of these species is curled like a cork-screw and often becomes entangled in their mate's genitalia in the process of exchanging sperm. Apophallation allows the slugs to separate themselves. It is possible that the damaged genitalia regenerate, much like the "optical tentacles."

Various species of slug can also reproduce via tiny "darts" of sperm which they fling in the direction of their mate's genitalia.

Slug genitalia are also some of the most prodigious in the world. Ariolimax dolichophallus, a species of banana slug (dolichophallus meaning "long penis" in Latin) has the largest penis-to-body length ratio of any animal. The record-holding specimen had a body length of 6 inches, with a phallus length of 32.5 inches, well over five times the body length.