Here are a couple of mating banana slugs. Banana slugs are the second largest terrestrial slug in the world, and can reach lengths of up to 25cm (A slug form Europe is the largest and can reach lengths up to 30cm). Banana slugs are hermaphrodites, meaning they poses both male and female reproductive organs. When they mate each slug will transfer to the other a package of sperm known as a spermatophore. If an individual can not find a mate, they have the ability to mate with themselves.