Montana Field Guide

A small slug, reaching 15-25 mm extended. Dorsal base color usually pale yellowish-gray, the head and tentacles darker, the mantle more yellow in the anterior half. The mantle is about 1/3 the body length, and covered with a series of small tubercles that form sharp translucent points when the animal is contracted; lateral bands are very pale or absent. The pneumostome is in the anterior half of the mantle on the right side, posterior to the mantle cleft. There is no mid-dorsal keel on the tail, lateral bands on the tail are pale or absent. The sole is pale yellow to pale orange; the mucous is yellowish to orangish-yellow. Because of small adult size, it can be confused with juveniles of other Arion species (McDonnell et al. 2009, Hendricks 2012, Burke 2013). Internal anatomy is described by Pilsbry (1948) and Cadiz and Gallardo (2007).

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. intermedius differs from other members of the genus by a combination of it's small size (usually less than 20 mm), pale gray to pale yellowish color, indistinct bands on sides, lack of mantle speckles, dark head and tentacles, pale yellow sole, and production of lemon-yellow mucous; when contracted, points form on the turbercles.

Occupies moist areas within forested or shrubby locations, such as stream-sides and marshy areas and lake shore near areas of human activity, such as campgrounds and other disturbed sites, also gardens and parks in residential areas. Forest canopy in Montana includes western redcedar, Engelmann spruce, and western larch, with a secondary canopy of alder. Most often found under woody debris and leaf litter (Hendricks 2012).