Montana Field Guide

A small to medium-sized slug, reaching 25 to 45 mm extended. The dorsal base color is gray to yellowish on the mantle, back, and sides, tentacles dark. The body is bell-shaped when contracted. The mantle is granulose, about 1/3 the body length, the same color as the back and lacking speckling; a pair of brownish lateral bands is present close to the mantle margin. The pneumostome is in the anterior half of the mantle on the right side, posterior to the mantle cleft. The tail has a pair of brownish or darker gray lateral bands extending to near the posterior end; the sides paler below the bands than above. The tail also lacks a mid-dorsal keel and is covered dorsally with a series of rows of fine tubercles; the row down the dorsal midline forms a narrow light stripe. The sole is quite pale, undivided (lacking two longitudinal furrows); the mucous is clear and not sticky (Hendricks 2012, Burke 2013). A recent genetic study suggests A. circumscriptus may be a variety of this species (Geenen et al. 2006). Internal anatomy is described by Pilsbry (1948), Forsyth (2004), and Grimm et al. (2009).

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. subfuscus differs from other members of the genus by a combination of it's small to medium size, grayish color with a pale orangish tinge, distinct brownish bands on low on mantle and sides of tail, lack of mantle speckles, pale sole, and production of a clear mucous.

Occupies moist to wet sites (stream-sides, marshy areas) within forested or shrubby locations, usually near areas of human activity, such as campgrounds, gardens, and other disturbed sites. Canopy includes Engelmann spruce, Douglas-fir, lodgepole pine, ponderosa pine, western larch, black cottonwood, paper birch; secondary canopy species include alder and willow. Most often found under woody debris (Hendricks 2012).