Montana Field Guide

A medium to large slug, reaching 70 mm extended, but often smaller. Dorsal base color is brownish-yellow to pale orange, the tentacles dark. The mantle is granulose, about 1/3 the body length, and the same color as the tail, with a lighter region along the margin and an indistinct pair of brown lateral bands. 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 lateral bands extending to the posterior end, lacks a mid-dorsal keel, and is covered dorsally with many fine tubercles in a series of rows. The sole is pale, undivided (lacking two longitudinal furrows); the mucous is yellowish to orangish-yellow (McDonnell et al. 2009, Hendricks 2012, Burke 2013). Internal anatomy is described by Pilsbry (1948).

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 large size, pale to darker orangish color, indistinct bands on sides, lack of mantle speckles, pale yellow sole, and production of yellowish to orange-yellow mucous.

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

Eggs are globular and a dull yellow color, occurring in clusters of 10-20 (Hendricks 2012).