Montana Field Guide

A small slug, animals may reach 30 mm or more, but are often smaller. Body and mantle base-color is cream, head and lateral grooves (7-8) on the body are dark to black, the latter highlighting a mid-dorsal tawny strip on the tail. The posterior margin of the mantle is deeply notched, similar to Zacoleus, and the pneumostome is present above or anterior to the mantle cleft. The mantle covers less than half the body, and has blackish lateral lines on the posterior half that converge towards the posterior margin, with small black spots on the anterior half of the mantle vaguely delineating a grid pattern. The right mantle line is sometimes more sinuous than the left, and the two converging lines form a lyreshaped symbol—hence the species name, lyrata. The back has a prominent keel especially noticeable in contracted individuals. The sole is tripartite (having two longitudinal furrows); the mucous is clear (Hendricks 2012, Burke 2013). Internal anatomy is described by Webb (1959) and Russell and Webb (1980).

There may be two subspecies in Montana. U. l. lyrata: from specimens collected 4.0 km west of the crest of Lolo Pass, Clearwater County, Idaho (Webb 1959); U. l. russelli: from specimens collected 1.6 km west of Potomac, Missoula County, Montana (Russell and Webb 1980). The taxonomic validity of these subspecies needs reevaluation.

A combination of size, coloration, mantle markings, and overall morphology readily distinguish this species from other slugs. The sole of the foot is tripartite (having two longitudinal furrows best seen when the animal crawls on a clear surface), a strongly keeled back (most prominent when the animal is more or less contracted), a small notch on the right posterior mantle margin, a pneumostome (breething pore) above or anterior to the mantle cleft. The similar sheathed slug (Zacoleus idahoensis) shares these traits, but Udosarx has a bluish-gray to cream base color with a contrasting blackish head and dark grooves on the upper surface of the tail demarcating rows of tubercles, and a pair of dark lateral lines on the mantle that converge posteriorly.

Mostly mesic mixed conifer forest and riparian woodlands, sometimes with talus, also at higher elevation in drier habitat where snow banks and seeps keep soil moister. Canopy species include Engelmann spruce, subalpine fir, whitebark pine, Douglas-fir, ponderosa pine, lodgepole pine, western larch, western hemlock, western redcedar, black cottonwood and paper birch, secondary canopy includes alder, willow, mountain maple, and dogwood. Usually found under rocks and woody debris, sometimes within rotten logs (Hendricks 2012).

Courtship and copulation have been observed in captive animals in November (Russell and Webb 1980).