Montana Field Guide

A very large slug, reaching 150 mm or more when extended, although many individuals are smaller. Dorsal base color is various shades of black (the typical color), brown, or orange, juveniles more variable than adults; generally lacking dorsal patterning (bands, stripes, spots). The mantle is the same color as the tail and covers about 1/3 of the body. The pneumostome is in the anterior half of the mantle margin on the right side, posterior to the mantle cleft. The tail lacks a keel but is covered dorsally by many irregular, coarse and long tubercles arranged in many oblique rows. The sole is pale and undivided (lacking two longitudinal furrows); the mucous is clear (McDonnell et al. 2009, Hendricks 2012, Burke 2013). Internal anatomy is described by Pilsbry (1948) and Forsyth (2004).

Arion lack an elevated visceral hump, possess a mantle about 1/3 the length of the animal with the pneumostome in the anterior half of the mantle, have an undivided foot, lack a dorsal keel on the tail, have a relatively broad tail covered with rows of prominent turbercles and with a caudal mucous pit at the tip where the pedal furrows meet. A. rufus differs from other members of the genus by a combination of it's very large size, black color (although rusty red and brown forms found in other states), lack of bands on mantle and tail, and production of a clear mucous.

Occupies moist to wet sites (stream-sides, fens, marshy areas) within forested or shrubby locations, usually near areas of human activity, such as campgrounds and residences. Canopy species include western redcedar, western larch, Douglas-fir, paper and water birch, and black cottonwood; secondary canopy species include willow and dogwood (Hendricks 2012).