Slugs are hermaphroditic, possessing both male and female organs; most species mate, however, with one slug pursuing the slimy trail of another. If a slug is in mating mode, there is a chemical that is present in its slime that conveys this information to other slugs. When two receptive slugs first encounter each other, there can be extensive interaction prior to mating. The pursuer often mouths the tip of the tail of the slug it’s pursuing (see photograph) to confirm that it’s receptive. The pursued slug may shake its tail vigorously to signal that it’s not interested, in which case they go their separate ways. If the leading slug is receptive, however, mating eventually takes place, with sperm being transferred from each slug to the other through penises that extend half the length of their bodies. During this process, the sexual organs are entwined; occasionally, in some species, the organs get stuck. If this happens, one slug gnaws off the other’s penis in a process called apophallation. The penis is not replaced and the slug lives the rest of its life as a female. (The opening you see on the side of the slug is its respiratory opening, or pneumostome.)
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