A sowbug (Trachelipus rathkei) at left and a pillbug (Armadillidium vulgare) on right with inset showing pillbug rolled up in defense posture.

Appearance:

Not more than 3/4 inch long; thorax composed of seven hard overlapping plates with seven pairs of legs; only pillbugs are able to roll up into a ball for protection, when distrubed.
Both sowbugs and pillbugs are terrestrial members of the crustacean group Isopoda, more closely related to crabs and shrimp than to insects. Most isopods are aquatic or marine, and many are parasites of fish. Characteristically isopods have seven pairs of legs and the body consists of a head, with antennae, and a series of armored body plates, ending in a tail-like telson.

Habits & Diet:

Prefer moist locations; found under objects on damp ground; mostly nocturnal; sometimes found in basements and ground levels of structures.
Sowbugs and pillbugs are major pests in gardens, nurseries and glasshouses along the Coast and other areas of high rainfall or fog drip. They feed on decayed vegetable matter and tender young plant growth, attacking young plants of all kinds, including strawberries, vegetables, forage crops and even mushrooms.
They are nocturnal and are most active during the rainy season, or in damp foggy conditions.

Reproduction:

Both sowbugs and pill bugs mate throughout the year, with most activity in the spring. The female carries the eggs, numbering from 7 to 200, in a brood pouch on the underside of her body. Eggs hatch in three to seven weeks and the young are white-colored. They remain in the brood pouch for six to eight weeks until they are able to take care of themselves. There may be one to two generations per year, with individuals living up to three years depending on weather conditions.

Other Information:

Armadillidium, the pillbug, is actually an introduced pest species from Europe. Controlling these creatures is difficult and generally involves eliminating damp, dark places where they can shelter, and the use of pesticides.