The gray garden slug is about 5 cm (2 in.) long when fully grown (fig. 2). It is variably colored, ranging from cream colored with irregular gray spots to dark brown with dark spots. The mantle is situated forward, near the head, and the respiratory pore is behind the mid-point of the mantle. The keel is located only at the tail. The gray garden slug produces a milky slime when disturbed (fig. 3); this slime distinguishes it from other species, many of which produce clear slime.
Slugs need a certain amount of dampness or humidity to survive. They are usually active at night or when it rains, and spend sunny days under logs, sticks, stones, or crop debris such as corn stalks. Many slugs are omnivorous; they may eat a variety of foods such as fungi, dead worms, or dead insects, in addition to green plants.