Читать книгу The British Woodlice - Wilfred Mark Webb - Страница 3
ОглавлениеPREFACE.
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of Natural History;" these are, however, scattered, and contain but few figures, while other literature that exists is out of date.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.
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).
FIG. 1.—PARTS OF THE BODY. (Oniscus asellus.)
FIG. 2.—THE FIRST ANTENNA. (Oniscus asellus.)
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.
FIG. 3.—THE SECOND ANTENNA. (Oniscus asellus.)
FIG. 4.—THE UNDERSIDE OF THE HEAD. (Oniscus asellus.)
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.
FIG. 5.—THE MANDIBLES. (Oniscus asellus.)
FIG. 6.—THE FIRST MAXILLAE. (Oniscus asellus.)
FIG. 7.—THE SECOND MAXILLAE. (Oniscus asellus.)
FIG. 8.—THE FUSED MAXILLIPEDS. (Oniscus asellus.)
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.
FIG. 9.—THE "UPPER LIP. (Oniscus asellus.) FIG. 10.—THE "LOWER LIP. (Oniscus asellus.)
FIG. 11.—A TYPICAL THORACIC SEGMENT. (Oniscus asellus.)
FIG. 12.—THE FIFTH THORACIC SEGMENT OF A FEMALE. (Oniscus asellus.)
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.
FIG. 13.—THE UNDERSIDE OF THE ABDOMEN OF A FEMALE. (Oniscus asellus.)]
FIG. 15.—THE FIRST ABDOMINAL APPENDAGE OF THE MALE. (Oniscus asellus.)
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.
