Identifying Wisconsin slugs is not that difficult at the family level, with the most-easily observed traits being those relating to the morphology of the mantle covering some or all of the animal's back.
Most slugs encountered in Wisconsin are non-native species belonging to the families Agriolimacidae, Arionidae, and Limacidae, all three having a mantle that covers half or less of the anterior portion of the body. The Agriolimacidae and Limacidae have a respiratory pore located laterally in the posterior half of the right side of the mantle, while the Arionidae have it in the anterior half. Fine sculptural patterning on the dorsal surface of the mantle is centered to the right of the mid-line in the Agriolimacidae but centered at the mid-line in the Limacidae.
Native slugs of the family Philomycidae have a mantle which extends nearly the entire length of the body.