Montana Field Guide

A very large slug, adults reaching 100 mm or more when extended; one of the largest slugs in Montana. Base color is yellowish-gray to brown, with many black spots on the mantle and tail, sometimes merging on the tail into two or three pairs of broken bands; head and sole are paler, tentacles darker. The mantle is 1/4 to 1/3 of the body length, and lacks a notch on the posterior margin. Pneumostome is in the posterior half of the mantle on the right side, and posterior to the mantle cleft. The sole is tripartite (having two longitudinal furrows), the tail is keeled only near the posterior end; the mucous is clear (McDonnell et al. 2009, Hendricks 2012, Burke 2013). Internal anatomy is described by Pilsbry (1948) and Forsyth (2004).

A combination of large size, lack of visceral mantle hump, tripartite sole, lack of keel on tail, and body markings (body with two or three dark lines or rows broken into short stripes or spots; mantle irregularly marked but not striped) separate this species from all others in Montana.

Inhabits gardens and city parks near water and human habitation, sometimes out of town in campgrounds and valley-bottoms (for example, along Avalanche Creek in Glacier National Park and on Wild Horse Island in Flathead Lake). Forest canopy includes western redcedar, western hemlock, grand fir, Douglas-fir, paper birch, black cottonwood, secondary canopy of alder, willow, dogwood, and snowberry. Most often found under woody debris and leaf litter, sometimes in downed rotten wood or wood piles (Hendricks 2012).

Eats mainly fungus and decaying plant matter, not green plants.

Mates while suspended from a mucous thread.