antenna of Cylisticus convexus

39 56. Flagellum and last peduncular joint of the antenna of Armadillidium nasatum 40 57. Flagellum and last peduncular joint of the antenna of Armadillidium vulgare 41 58. Flagellum and last peduncular joint of the antenna of Armadillidium pulchellum 42 59. Flagellum and last peduncular joint of the antenna of Armadillidium depressum 43

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THE BRITISH WOODLICE.

Introduction.—Having finished a somewhat exhaustive list of the land and fresh-water molluscs of Essex, [1] one of the present writers felt that if he were to make any further contributions of importance to a knowledge of the fauna of that interesting county, he must turn his attention to some other group of animals. It seemed most fitting that some creatures should be chosen which are commonly met with during the search for molluscs. Centipedes, millepedes, and woodlice fulfilled these conditions, and all were collected, but as only seventeen species of woodlice had at the time been found in England, it was deemed advisable to study these in detail to begin with. The present contribution is the result of the undertaking, and we have thought that a general consideration of the British Woodlice, with careful drawings from nature of all the species now known from this country, ought to lead to a more general study of these interesting creatures and their habits.

Position in the scheme of classification.—The Woodlice belong to an immense group of invertebrate animals known as the Arthropoda, the bodies of which are segmented and provided with jointed appendages for purposes of walking, swimming, and feeding. Of this group, two large divisions are recognized. The first contains the forms which breathe by means of air-tubes, such as the Insects; and the second has been constituted for Crustacea, which breathe by means of gills. The latter are, of course, adapted more especially for a life in water, but here and there we come across examples so modified that they can exist in air. The land-crabs are a case in point, and so are the Woodlice. These belong to an order which contains many fresh-water and marine species, known as the Isopoda.

Geological history.—The known history of the order is a long one, for remains occur in the Old Red Sandstone (Devonian) of Herefordshire, and in the Coal Measures. (79)[2]. A form which has been named Archæoniscus brodiei, and is said to be referable to the recent family Aegidae which is found in some numbers in the Purbeck Beds (Upper Jurassic), of this country (47). Fossil Isopods have also been recorded from the Oolite and from the Oligocene (Isle of Wight).

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.