In front of its first ring (a transversal segment) you see a little black head, with two lateral bead-like eyes, and a couple of antennæ. The latter are each composed of three joints, which are extremely mobile; their base is covered by the edges of the sloping head. The most conspicuous rings of the body are seven in number; their lateral borders are pointed in front, and rounded behind. But, if you look closely, you will see some other rings, a little less projecting than the former; they circumscribe the abdominal region, the belly, properly so called, in which the intestines are lodged. These rings, or abdominal segments, are six in number; but they have not all the same form. The one which occupies the tail is triangular, pointed, and surrounded by four (caudal) appendages. The three next segments, counting from the front to the rear, are prolonged laterally in a very marked manner; the two anterior, on the contrary, have no such distinction. As for the caudal appendages, the two outer ones are very strong, conical, and composed of two articulations, while the inner, situated above the former, are frail, cylindrical, and terminated by a tuft of hairs, whence issues a viscous liquid. (See Fig. 35, a.)
Fig. 35. The Wood-Louse.
An enumeration of these characteristics is tedious, but necessary for the determination of the genus and the species. They belong to the Oniscus asellus, or common wood-louse. But why, you ask, why such a strange conjunction of names,—one Greek, ὀνίσκος, the other Latin, asellus? Both carry the same meaning: why not, then, have called our tiny crustacean an ass-ass (if such a compound be possible)? Why, neither close at hand nor at a distance, has it the slightest resemblance to an ass; and to say that we have only borrowed these names from the ancients, is neither an explanation nor a justification.
But we have not yet done with the wood-lice. Are these interesting little creatures (they are interesting, are they not?) oviparous or viviparous? I defy you to show me anywhere a single wood-louse's egg. Have the patience to observe our crustaceans more nearly. Among the crowd, you will remark some—they are the females—with a kind of membraneous pouch underneath the body, stretching from the head to the fifth pair of legs. The pellicle which forms this pouch is so thin, and so transparent, that you can distinguish the eggs within it. These eggs, instead of being expelled for incubation, remain in the mother's pouch until they are hatched. At that felicitous moment, the membranous bag splits cross-wise, longitudinally and transversally, to permit the emergence of the young wood-lice. The latter are extremely small, and in form resemble nothing in the world so much as a little white line (Fig. 35, b). They differ from their parents only in having one pair of feet, and one ring less than they have. They undergo no metamorphoses. After their birth, the little ones, which have proportionally very large antennæ, do not immediately separate from their mother. By a wonderful act of forethought on the part of Nature,[56] they keep themselves concealed in the middle of the respiratory laminæ, which garnish the under part of the tail.
The specific characters of the Oniscus asellus are tolerably well defined. By its rings of dark gray, a little lighter at the edges, which form for it an articulated, glossy carapace, marked with white spots, longitudinally arranged; by the uniform pale gray colour of its belly and its legs, covered with scattered hairs; and, particularly, by its habits, our wood-louse, which the Germans call cellar-louse (Kellerlaus), is distinguished from its kindred species, of which naturalists have made distinct genera. Thus, the asellus found generally under stones, which counterfeits death by rolling itself up in a ball like a hedgehog, and will rather suffer itself to be crushed than unfold, is the Oniscus armadillo, which some naturalists transform into the Armadillo vulgaris. (See Fig. 36.) This species prefers the solitude of the field to inhabited places. Its body is considerably expanded, and its rings do not terminate in a point on their lateral and posterior edges.
Fig. 36. Oniscus armadillo.
Another species, equally common underneath stones, has its head and tail covered over with granulations; its antennæ are composed of seven joints, of which the fourth and fifth are perceptibly situated lengthwise. This is the Oniscus granulatus of some entomologists; others have designated it the Porcellio scaber. Why not simplify the study of species?
The wood-lice seem to live upon decomposed vegetable matter. But in default of other food, they devour their own kind; in this respect resembling beings who are supposed to rank much higher in the animal hierarchy.