ONISCUS, in natural history, a genus of insects of the order Aptera. Jaw trun cate denticulate ; lip bifid ; antenna from two to four, setaceous ; body oval, consist ing of about fourteen transverse seg ments ; fourteen legs. These insects feed on animal and vegetable matter; and they cast their skin. There are nearly fifty species divided into sections. A. with out feelers; four antenna, sessile. B. feel ers unequal, the hind-ones longer ; an tenna filiform. The most common spe cies is the 0. asellus, or common wood louse, found in great quantities under the bark of decayed trees, beneath stones in damp situations. It preys on minuter in sects. 0. armadillo, the medical wood louse, is of a darker colour than the for mer, but found in similar situations. When suddenly disturbed or touched, it rolls itself up into a round form in the manlier of the armadillos ; frequently re maining in that state for a considerable length of time. This insect was former
ly considered as a specific in many disor ders, but is now rarely used. Among marine insects of this genus, is the 0. guadeloupensis, measuring about one inch and three quarters in length ; an tenna very short, compressed, inferior pair rather longer; abdomen covered with six scales, beneath ; tail somewhat ovate, flat, furnished with a lateral style on each side, the last joint of which is bi fid -; the five segments of the body before the tail are much narrowed, and destitute of feet. The female protects her young, for a considerable time after their exclu sion, under the abdominal scales ; in this respect resembling the opossum amongst the quadrupeds. They are often found in the mouth of the Clupea menhaden or mossbanker, attached to the palate, and partaking of its form.