Turning to the Woodlice proper, we find that they first make their appearance in the Miocene (of Oenigen and Baden), and occur also in amber (79); while examples of genera, such as Oniscus and Porcellio, have been discovered in late Tertiary deposits (47).
External structure and appendages.—Woodlice agree in being of a somewhat oval form, and their bodies are arched, the curve varying in different genera and species. A head is to be distinguished; behind this comes the thorax of seven segments which are often considerably broader than the six succeeding ones which form the abdomen (see fig. 1.)
The head carries two large antennae (fig. 3) which are very evident, and a careful search with a lens will reveal a second and minute pair (the smaller antennae) situated between the base of the others, and really anterior to them. (figs. 2 and 4.)
The larger antennae are customarily bent at certain points, and we can distinguish a terminal part, or flagellum, and a basal part, the peduncle (fig. 3). The number of joints in these structures, which varies in different genera and species, forms a useful classificatory character, and the relative length of the component parts is of considerable value in distinguishing species.
There are four pairs of mouth appendages—namely the jaws or mandibles (fig. 5), the first maxillae (fig. 6), the second maxilla (fig. 7), and the maxillipeds (fig. 8). When the head is examined from the underside the last of these organs will be seen first, covering in the others.
A small median plate attached to the front of the head has been called "the upper lip" (fig. 9), while inside the mouth appendages is a little bilobed structure "the lower lip" (fig. 10).
Before leaving the external features of the head, we must allude to the pair of eyes which are usually present, though never raised on stalks. In the Common Woodlouse (Oniscus asellus, from which all our figures to illustrate structure have been made), as in many other species, the eyes are compound (fig. 4), but in some forms these are simple.
