THE BRITISH WOODLICE.

This monograph first appeared in the "Essex Naturalist"
(Volume XIV., 1905-6) and has been republished by special
arrangement with the Council of the Essex Field Club.

THE BRITISH WOODLICE.

BEING

A MONOGRAPH OF THE TERRESTRIAL ISOPOD CRUSTACEA

OCCURRING IN THE BRITISH ISLANDS.

BY

WILFRED MARK WEBB, F.L.S.,

Lecturer on Biology and Nature Study to the Surrey County Council,
Honorary Secretary of the Selborne Society,
Sometime, Senior Assistant Lecturer on Biology to the Essex County Council,
and Editor of the Journal of Malacology,
Joint Author of  "Eton Nature Study and Observational Lessons."

AND

CHARLES SILLEM.

With Twenty-Five Plates and Fifty-Nine Figures in the Text.

LONDON:

DUCKWORTH & CO.,

3, HENRIETTA STREET, COVENT GARDEN.

1906.

PREFACE.

In Professor Sars' "Crustacea of Norway," quite a number of the British species of woodlice are figured in detail and described in English, but few copies of this fine work are to be met with in our country. The Rev. Canon Norman has from time to time published notes on the British species in "The Annals and Magazine of Natural History;" these are, however, scattered, and contain but few figures, while other literature that exists is out of date. Under these circumstances, we have thought that the following account and figures of all the British species would be useful to those anxious to work at the woodlice, and might also encourage others to pay attention to the distribution and habits of the interesting tribe to which they belong.

The writers would welcome any corrections or additions in view of a second edition.

W.M.W.
C.S.

Odstock, Hanwell, December, 1905.

CONTENTS.

LIST OF FIGURES.

PLATES I.-XXV. with a List, will be found at the end of the Book.

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).

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.

Each of the seven joints of the thorax bears a pair of walking legs (fig. 11), and in the female at the time when the eggs are laid, a pair of plates (fig. 12) arises on segments II. to V. These plates together form a brood pouch, in which the eggs are carried (fig. 12) until they are hatched, and in which the young ones remain for some time afterwards.

When we examine the abdomen, we find that the appendages are plate-like, with the exception of the last pair (fig. 13), and they all agree in having two divisions, an arrangement which would prove awkward in limbs used for walking or feeling.

The inner plate (or endopodite) is in structure a gill, but the blood that passes through it, is enabled to take up oxygen from moist air, while the outer division (or exopodite) acts as a protecting cover (fig. 14). In Porcellio, air-tubes (tracheae) may be present (see below).

In the male, the first two pairs of abdominal appendages are specially modified, the inner divisions (endopodites) being long and pointed (figs. 15 and 16). The last pair, or tail appendages, in the male are often considerably larger than in the female, and the form of these structures is sometimes of value in classification.

Alimentary canal.—The main portion of the alimentary system is, practically speaking, a straight tube (fig. 17). Its first part (not shown in the figure) is a narrow gullet, which after passing through the nerve collar dilates to form a sort of stomach. Into this the secretion of four digestive glands is poured by two ducts. These glands have a somewhat striking appearance, being yellow tubes spirally coiled, and they end blindly. From the stomach the intestine runs to the hinder end of the body and passes under the heart.

Circulatory system.—The blood being aërated in the abdominal appendages, we find that the heart is situated towards the hinder end of the body (fig. 18). Three main arteries supply the thorax and head, while the blood is brought from the gills to the heart.

Excretory system.—The excretory organs consist of a (a) pair of so-called "shell glands," which are considered to be the equivalents of the excretory tubes or nephridia of annelid worms. In the woodlouse these excretory organs open on the second pair of maxillae. They are composed of a tube (sacculus) closed at one end and more or less bent upon itself (5, p. 261) which communicates with a labyrinth that is provided with an excretory orifice. Matters are eliminated by the epithelial cells [the histology has been described and figured in Ligidium hypnorum (66)], which are very large in Ligia oceanica.

(b) Masses of cellules in the head, very greatly developed in Ligia oceanica (but numbering scarcely more than ten in Oniscus asellus), which have no external opening. They also function as excretory organs (5, p. 263), and have been called "cephalic nephrocytes."

(c) Other "branchial nephrocytes" are situated on the dorsal surface between the last thoracic and the first abdominal segments, as well as between those that follow, with the exception of the last two; they are in distinct patches, one on each of the middle line in Ligia, but more or less continuous in Oniscus (5, p. 265).

(d) The digestive glands have also been shown to be excretory (5, p. 270).

Nervous system.—The nervous system consists of paired ganglia in the head, above the alimentary canal which send off nerves (commissures) that meet below, to form a double nerve cord with ganglia at intervals (see fig. 19).

Reproductive organs.—In the female there are a pair of ovaries in the positions shewn in fig. 20; and ducts run to the underside of the fifth thoracic segment.

The openings are very difficult to identify, and Lereboullet (39, p. 113) was unable to find them. It is obvious that the openings must be underneath the plates that form the egg pouch, and as a change of skin is required to set these free, it would appear that at ordinary seasons the ducts from the ovaries are closed.