Chapter I: Part 1

Transcriber's Note: Italic text is denoted by _underscores_ and bold text by =equal signs=.

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of Natural History;" these are, however, scattered, and contain but few figures, while other literature that exists is out of date.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.

PAGE

Introduction 1
Geological history 1
External structure and appendages 2
Alimentary canal 6
Circulatory system 7
Excretory system 7
Nervous system 8
Reproductive organs 8
Development 9
Habits and Economic considerations 12
Local names 15
Methods of collections and preservation 16
Classification 17
Scheme of classification and synopsis of generic characters 18
British Species 19
Section--Ligiæ 19
Family--Ligiidæ 19
Genus--Ligia Fabricius 19
_Ligia oceanica_ Linzé 19
Genus--Ligidium Brandt 21
_Ligidium hypnorum_ Cuvier 21
Family--Trichoniscidæ 22
Genus--Trichoniscus Brandt 22
_Trichoniscus pusillus_ Brandt 22
_Trichoniscus vividus_ Koch 23
_Trichoniscus roseus_ Koch 24
Genus--Trichoniscoides, Sars 25
_Trichoniscoides albidus_ Budde-Lund 25
Genus--Haplophthalmus Schobl 26
_Haplophthalmus mengii_ Zaddach 26
_Haplophthalmus danicus_ Budde-Lund 27
Family--Oniscidæ 27
Genus--Oniscus Linné 27
_Oniscus asellus_ Linné 27
Genus--Philoscia Latreille 29
_Philoscia muscorum_ Scopoli 29
_Philoscia couchii_ Kinahan 30
Genus--Platyarthrus Brandt 30
_Platyarthrus hoffmannseggii_ Brandt 30
Genus--Porcellio Latreille 32
_Porcellio scaber_ Latreille 32
_Porcellio pictus_ Brandt and Ratzeburg 33
_Porcellio dilatatus_ Brandt 33
_Porcellio rathkei_ Brandt 34
_Porcellio laevis_ Latreille 35
_Porcellio ratzeburgii_ Brandt 36
Genus--Metoponorthus Budde-Lund 37
_Metoponorthus pruinosus_ Brandt 37
_Metoponorthus cingendus_ Kinahan 38
Genus--Cylisticus Schnitzler 38
_Cylisticus convexus_ De Geer 39
Family--Armadillidiidæ 40
Genus--Armadillidium Brandt 40
_Armadillidium nasatum_ Budde-Lund 40
_Armadillidium vulgare_ Latreille 41
_Armadillidium pulchellum_ Zencker 42
_Armadillidium depressum_ Brandt 43
Distribution of species 43
Conclusion 43
Bibliography 49

LIST OF FIGURES.

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

FIGURE. PAGE.

1. Parts of the body (_Oniscus asellus_) 2
2. The first antenna (_Oniscus asellus_) 2
3. The second antenna (_Oniscus asellus_) 3
4. The underside of the head (_Oniscus asellus_) 3
5. The mandibles (_Oniscus asellus_) 4
6. The first maxillae (_Oniscus asellus_) 4
7. The second maxillae (_Oniscus asellus_) 4
8. The fused maxillipeds (_Oniscus asellus_) 4
9. The "upper lip" (_Oniscus asellus_) 5
10. The "lower lip" (_Oniscus asellus_) 5
11. A typical thoracic segment (_Oniscus asellus_) 5
12. The fifth thoracic segment of a female
(_Oniscus asellus_) 5
13. The underside of the abdomen of a female
(_Oniscus asellus_) 6
14. A typical abdominal appendage
(_Oniscus asellus_) 6
15. The first abdominal appendage of the male
(_Oniscus asellus_) 6
16. The second abdominal appendage of the male
(_Oniscus asellus_) 6
17. The alimentary canal (_Oniscus asellus_) 7
18. The circulatory system (_Oniscus asellus_) 7
19. The nervous system (_Oniscus asellus_) 8
20. Female reproductory organs (_Oniscus asellus_) 8
21. The male reproductory organs (_Oniscus asellus_) 9
22. The fertilized egg (_Porcellio scaber_) after Roule 10
23. The fertilized egg seen in section
(_Porcellio scaber_) after Roule 10
24 to 31. The development of a woodlouse
(_Porcellio scaber_) after Roule 10
32. Embryo of the woodlouse showing the three divisions of the
intestine separately developed (_Porcellio scaber_)
after Roule 11
33. Embryo of the woodlouse showing traces of the segments
(_Porcellio scaber_) after Roule 11
34. An embryo woodlouse ready to be hatched
(_Porcellio scaber_) after Roule 11
35. Flagellum and last peduncular joint of the antenna of
_Ligia oceanica_ 12
36. Flagellum and last peduncular joint of the antenna of
_Ligidium hypnorum_ 13
37. Flagellum and last peduncular joint of the antenna of
_Trichoniscus pusillus_ 23
38. Flagellum and last peduncular joint of the antenna of
_Trichoniscus vividus_ 24
39. Flagellum and last peduncular joint of the antenna of
_Trichoniscus roseus_ 24
40. Flagellum and last peduncular joint of the antenna of
_Trischoniscoides albidus_ 25
41. Flagellum and last peduncular joint of the antenna of
_Haplophthalmus mengii_ 26
42. Flagellum and last peduncular joint of the antenna of
_Haplophthalmus danicus_ 27
43. Flagellum and last peduncular joint of the antenna of
_Oniscus asellus_ 28
44. Flagellum and last peduncular joint of the antenna of
_Philoscia muscorum_ 29
45. Flagellum and last peduncular joint of the antenna of
_Philoscia couchii_ 30
46. Flagellum and last peduncular joint of the antenna of
_Platyarthrus hoffmannseggii_ 31
47. Flagellum and last peduncular joint of the antenna of
_Porcellio scaber_ 32
48. Flagellum and last peduncular joint of the antenna of
_Porcellio pictus_ 33
49. Flagellum and last peduncular joint of the antenna of
_Porcellio dilatatus_ 34
50. Flagellum and last peduncular joint of the antenna of
_Porcellio rathkei_ 35
51. Flagellum and last peduncular joint of the antenna of
_Porcellio laevis_ 35
52. Flagellum and last peduncular joint of the antenna of
_Porcellio ratzeburgii_ 36
53. Flagellum and last peduncular joint of the antenna of
_Metoponorthus pruinosus_ 37
54. Flagellum and last peduncular joint of the antenna of
_Metoponorthus cingendus_ 38
55. Flagellum and last peduncular joint of the 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

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.

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. The writers have been able to determine from external examination of specimens which had moulted and were about to lay eggs, that the oviducts at such time open to the inside of the base of each walking leg on the fifth segment. In similar specimens the oviducts were also followed to the opening from within. The _brood pouch_ has already been described.

The male organs consist of six _testes_ arranged in two pairs, each of which is provided with a _reservoir_ (see fig. 21). The efferent ducts from the two reservoirs unite at the base of the thorax to form a common duct (or "penis").

=Development.=--The eggs, in the common species of woodlice, at least, are laid at the beginning of summer, and are retained in the brood pouch, where they undergo their development. The process has been recently traced with great care by Professor Louis Roule (58) in _Porcellio scaber_ and the description which follows is based upon his researches.

As, practically speaking, the larval stages are passed within the egg, and there is no free embryo differing in form from the parent, it is necessary for the young creatures to be well supplied with nutritive material. In fact, the bulk of the large egg is made up of _food-yolk_, on the outside of which the _formative protoplasm_ is disposed in irregular patches. In the fertilized ovum, one of the latter, which lies in a particular position at the end, is found to be larger than the others (see fig. 22). It contains the nucleus of the egg-cell (see fig. 23) and is called the _cicatricula_. This is the only portion of the egg which divides and produces _nucleated cells_. It is these which gradually spread all over the surface of the food-yolk, forming a layer known as the _blastoderm_, which is at first but one cell thick (see figs. 24, 26, and 28).

Before, however, the food-yolk is quite closed in, a differentiation into two layers--the _pro-ectoderm_ and _pro-endoderm_--takes place (see fig. 25) and rudiments of the first two pairs of _appendages_ appear (see fig. 26). Moreover, the cells of the ectoderm change their shape and begin to multiply at two points to form the beginnings of the cerebral ganglia and the nerve cord respectively.

As the blastoderm closes over the food-yolk, two more appendages arise and these are soon followed by others (see fig. 28). A depression appears at the point where the blastoderm closed and internally the pro-endoderm or inner layer is differentiated into two--the _endoderm proper_ and the _mesoderm_ (see fig. 29). The former begins to grow so that its edges unite to form the middle part of the intestine (see fig. 29) seen from the outside in fig. 30. The depression already mentioned grows deeper, forming a tube which is the hind portion of the _intestine_, while at the anterior end of the embryo the front part of the intestine is similarly formed (see fig. 30). By this time also all the nineteen appendages have made their appearance and the mesoderm, (which has grown considerably, to form the beginnings of the muscles) has sent prolongations into each of them. About this time, spaces (see fig. 31) are formed in the muscular mesoderm which are all that remain of the _true body cavity_ characteristic of animals above the level of the jelly fish, and in these spaces the blood ultimately circulates.

_SURFACE VIEWS._

_OPTICAL SECTIONS._

THE DEVELOPMENT OF A WOODLOUSE (_Porcellio scaber_), AFTER ROULE.

Figs. 24, 26, 28, 30, are Surface Views, and figs. 25, 27, 29, 31, which indicate slightly later stages respectively than the others, are of egg seen in Optical Section.

The body next alters somewhat in shape and the three divisions of the intestine approach one another (see fig. 32) previous to their junction. As may be imagined during these processes the food-yolk has gradually been used up and the space which it occupied taken by the internal organs, which we have mentioned.

In the last stages of the development the appendages become larger still, the _heart_ makes its appearance, segmentation of the body is completed, and except that the seventh pair of walking legs are as yet rudimentary the woodlouse is completed. It is only after hatching that the pair of legs mentioned, attain to their normal length.

The process of segmentation of the egg and the formation of its layers lasts about a fortnight, while the completion of the development proceeds much more rapidly, for another three weeks bring it to an end.

After the first moult or change of skin the last pair of walking legs makes its appearance, and Mr. James B. Casserley [whose work one of us (75) has described elsewhere] found when keeping a number of the common pill-woodlouse (_Armadillidium vulgare_) in captivity that his specimens did not subsequently change their skins more than once in the six months during which he had them under observation. He also noted that the crustaceans go on growing after they are sexually mature. As his specimens grew older, Mr. Casserley noticed that their colour became darker, and a curious point recorded by him is that two examples of the same age may change their skins at the same time, and while one may have afterwards nearly twice as many markings, on the other very few at all will be seen. The time required for the growth of a woodlouse from the size of a pin's head to that of an adult example--say three-quarters-of-an-inch long--must be fairly considerable, taking into account the fact that any appreciable increase in size can only occur at a moult and Mr. Casserley's observations as to the infrequency of the process in _Armadillidium vulgare_. (See p. 14.)

=Habits and Economic Considerations.=--The construction of the breathing organs of woodlice, and the necessity which exists for these to be kept moist, restricts the habitats of the animals considerably. Woodlice are found under stones and logs, beneath the bark of dead and rotten trees, among decaying vegetable matter as well as living grass and moss in damp or wet situations. When looking for some of the common species under the bark of fallen trees it is surprising to notice that the crustaceans may be entirely absent from many trunks, while when another is examined which seems to differ very slightly, if at all, in condition or situation, they are found in swarms. There is no doubt but that the habits of woodlice would well repay the attention of naturalists, who are now recognizing that besides anatomy as such, and the classification which a knowledge of structure permits, there is the equally important consideration of the creatures as they live their own life and affect that of others. It is not our object to give a detailed account of the ecology of British woodlice, but rather to provide a basis from which it may be approached. Nevertheless a few general remarks may not come amiss. Many points in the life-history of woodlice may no doubt be learned by keeping them in captivity and there is just sufficient difficulty in doing this successfully to give an interest to the matter.

Apart from a supply of proper food, we take it that the chief object to be attained is the provision of the amount of moisture required by the particular species under examination, together with a sufficient supply of air.

A great many interesting observations can be thus carried out, such as those of Mr. Casserley, to which allusion has already been made. The process of moulting for instance is well worth watching, and although specimens with half their coat changed may be found in remote corners, yet the whole course of the moult can be seen much better in the case of captive woodlice. The following account is taken from Mr. Casserley's description (75) of what happens in the case of _Armadillidium vulgare_:--The approach of the moult is indicated by the appearance of a white border on each segment of the body, which becomes gradually more marked, while at the same time the animal is seen to be less active and often makes a small burrow in which to hide. Sometimes a sheltered corner against a stone is looked upon as affording sufficient protection, but in either case each woodlouse keeps to the place originally chosen. About ten days after the white lines have become visible the animal appears to be divided into two. Its skin is becoming loose and little movement can take place at the joints of its body with the exception of that between the fourth and fifth thoracic segments where the skin will ultimately break. The woodlouse spends a day or two in this condition and then, by suddenly walking forward, frees itself from the covering of the hinder portion of its body. The three last pairs of walking legs are carefully pulled out from the old skin, which now appears perfectly white, and at the same time the lining of the hind portion of the alimentary canal (hind gut) is also shed. After putting the tender half of his body well into his corner or burrow the woodlouse proceeds to eat the part of his skin that he has cast. The creature has now a very odd appearance. His front half with the exception of the white edges is as it was before, the rest of him instead of a light slaty blue, and is very soft as well as proportionately a little larger.

In three days or so the tail end becomes hard and attains the normal colour. Then the old skin from the front half is pushed off and the creature becomes practically defenceless, so much so in fact, that any of his species that happen to find him will attack him and eat all his front half, rejecting, however, his now hardened tail-end.

Provided that the moulting woodlouse has survived (and in captivity, to ensure this, he must be isolated), after three days his jaws will be sufficiently hardened to allow of his eating, and usually he first of all devours the second half of his cast skin. The operation of moulting does not occupy quite so long a time in the case of young examples. Specimens half-an-inch long do not moult more than once in six months and show but little increase in size after the process.

Woodlice do not appear to live on either animal or vegetable food alone, but adopt a mixed diet. It is, however, owing to their attacks upon cultivated plants that the creatures are looked upon as pests by the horticulturalist. The animals feed either in the night or in the very early morning, on seedlings, orchid tubers, mushrooms, or anything that comes to hand. Few of the accounts, however, of their ravages, mention that the crustaceans have been caught absolutely in the act of doing the damage ascribed to them. Some careful inquiries have nevertheless enabled us to discover several observers who have watched woodlice feeding. Mr. F. V. Theobald, of Wye College, and one of the students at Swanley Horticultural College are among the number. The former has also given us an account of the methods, out of many tried, which he has found most successful for getting rid of the crustaceans. Out of doors trapping with moss, sacking or horse-dung is best. In glass houses, fumigation with hydro-cyanic acid gas has cleared them out, and poison baits, especially potatoes cut and soaked in white arsenic, have done some good. Stable manure is especially favourable to these creatures, particularly when it is used "long": in this condition it should therefore be avoided.

It is interesting to note how the woodlice in winter simply remain where they happen to be so long as there is sufficient moisture, though they are ready to run about as rapidly, for a time, as in summer, should they happen to be disturbed.

No doubt many points of inter-relation between woodlice and other animals remain to be discovered. Mr. John W. Odell tells us that on Exmoor, in the open, he found no _Armadillidia_, though other forms occurred under nine out of every ten stones that he turned over, and here the smaller species of ants also abounded. Close to stone walls _Armadillidia_ were to be seen to the exclusion of all other genera, and this state of affairs was ascribed by Mr. Odell to the presence of swarms of the large wood-ants which he considers would make short work of any woodlice that could not protect themselves by rolling up.

We ought not to conclude this account without mentioning the fact that woodlice once played an important part in medicine.

Doctor Fernie (28) gives some interesting extracts with regard to the hog-louse and the woodlouse. The latter he seems to have identified quite correctly as _Oniscus asellus_. He calls the former, however, indiscriminately, "the common armadillo" (which is the old name for the pill-woodlice now known as _Armadillidium_), "the pill millipede" and "_Glomeris marginata_." The last two names are those of another creature, not a crustacean, which when it is rolled up can be very easily mistaken for an _Armadillidium_, though, when it uncurls, it will be seen to have many more than seven pairs of legs. The local appellations applied to the hog-louse by Doctor Fernie, and his remarks with regard to its commonness, tend to show that it is _Armadillidium vulgare_, to which he really refers, and the use of which in medicine was commonly general.

Hog-lice were prescribed for scrofulous diseases and obstructions of the liver and digestive organs, among other things, and the London College of Physicians directed that the creatures should be prepared by suspending them in a thin canvas bag placed within a covered vessel over the steam of hot spirit or wine, so that being killed by the spirit they might become friable. Hog-lice and Woodlice were also administered alive, while the former were also put down the throats of cows "to promote the restoration" of their cud, hence their name of "cud-worm." There seems to be considerable evidence that even in modern times Woodlice have had considerable remedial effect which depends upon "an alkalescent fluid" contained in them.

=Local Names.=--Among the local names by which these creatures are known are those of "sow bug," "lucre pig" (Berkshire), "carpenter" and "chiselhog" (Berkshire). Doctor Fernie (28) gives a number of others:--"thrush-louse," "tiggyhog," "cheslip," "kitchenball," "chiselbob," "lugdor," "palmer," and "cudworm." In the eastern counties the same writer notes that they are known as "old-sows" or "St. Anthony's hogs" while the Welsh call them "little grey-hogs," "the little old women of the wood" or "grammar-sows," grammar signifying a shrivelled up old dame. _Oniscus asellus_ was sometimes called "socchetre," "church louse," and "chinch."

=Methods of Collection and Preservation.=--Woodlice should be collected straightway into tubes or bottles half filled with 30 per cent. methylated spirit.[3] Woodlice dropped into this weak spirit become gradually narcotised and die, and they remain limp enough for purposes of examination or to allow, of their legs and antennæ being set out during the process of mounting. Specimens to be kept permanently should be placed in 70 per cent. alcohol. For storage purposes the specimens of each species from a given locality should be put together into a small flat bottomed tube such as is used for pillules by apothecaries or specially made for natural history purposes. A paper label on which the name, locality, date of capture and any other necessary particulars have been written with dark lead pencil, is not affected by the spirit. The tubes may be corked, though if not frequently examined all the spirit may evaporate, and cause the specimens to be spoilt. A safer method is to plug the tubes with cotton wool and keep all those containing a given species or specimens from a particular locality beneath the surface of spirit in a large wide-mouthed bottle, into which first of all some cotton wool has been put to prevent the tubes from coming into sudden contact with the glass at the bottom. For show purposes in museums, specimens taken direct from 30 per cent. spirit should be mounted on slips of opal glass by means of gum-tragacanth which has been powdered and shaken up in spirit before having water added to it. The slips can be exhibited in glass tubes, six inches high by one across, or in narrow stoppered museum jars. A variation of the method is to mount the animals on clear glass and to place behind them another strip of any colour that may be preferred.

=Classification.=--The various genera of woodlice are connected together so closely, by intermediate forms, that their division into families is, to a very great extent, arbitrary. Bate and Westwood described but a single family Oniscidæ (I), though they distinguished two sub-families:--Ligiinæ, which included the forms with many joints to the flagellum of the antenna, and Oniscinæ, which contained the rest.

Since then the pill-woodlice have been thought by some to be sufficiently different from the other genera to warrant their separation, and three families namely, Ligiidæ, Oniscidæ, and Armadillidæ have been recognized, as for instance by Dr. Scharff (63).

A fourth family--Trichoniscidæ--has been added by Professor G. O. Sars, who in his _Crustacea of Norway_ (59) alludes to the division of the tribe into the sections Ligiæ and Onisci and has adopted the following classification:--

_Order_--=ISOPODA.=

_Tribe_--ONISCOIDA.

_Family I._--LIGIIDAE.
_Ligia._
_Ligidium._

_Family II._--TRICHONISCIDÆ.
_Trichoniscus._
_Trichoniscoides._
_Haplophthalmus._

_Family III._--ONISCIDÆ.
_Oniscus._
_Philoscia._
_Platyarthrus._
_Porcellio._
_Metoponorthus._
_Cylisticus._

_Family IV._--ARMADILLIDIIÆ.
_Armadillidium._

All the genera described by Professor Sars are represented in the British Islands.

* * * * *

Below is a scheme of classification and synopsis of the characters of British genera of woodlice which we have compiled in order to render easy the determination of the genus to which any particular specimen may belong.

_SCHEME OF CLASSIFICATION AND SYNOPSIS OF GENERIC CHARACTERS._

_Order_--=ISOPODA.=

_Tribe_--ONISCOIDA.

_Section I._--=LIGIÆ.=

_The Two Divisions of the Tail Appendages alike in Shape._

(A.)--Flagellum with 10 or more joints; tail
appendages wholly visible; head without
lateral lobes LIGIIDAE.
(1.)--Abdomen broad; body large;
habitat, the sea-shore _Ligia._
(2.)--Abdomen narrow; habitat, wet
moss _Ligidium._

(B.)--Flagellum with less than 10 joints; head
with small lateral lobes, tail appendages
partly covered TRICHONISCIDÆ.
(3.)--Abdomen narrow; eyes compound;
flagellum usually with more
than 3 joints _Trichoniscus._
(4.)--Abdomen narrow; eyes simple or
wanting; flagellum with 4 joints _Trichoniscoides._
(5.)--Abdomen broad (comparatively);
eyes simple; back with
longitudinal ridges; flagellum
with 3 joints _Haplophthalmus._

_Section II._--=ONISCI.=

_The Outer Divisions of the Tail Appendages Broader than the Inner._

(A.)--Tail appendages projecting when the animal
is walking ONISCIDÆ.
(a.)--Unable to roll up into a complete ball.
(6.)--Flagellum with 3 joints; abdomen
broad; head, with lateral lobes _Oniscus._
(7.)--Flagellum with 3 joints; abdomen
narrow; head without lateral
lobes _Philoscia._
(8.)--Flagellum with 1 joint; eyes
wanting; abdomen broad; habitat,
ant's nests _Platyarthrus._
(9.)--Flagellum with 2 joints; abdomen
broad; frontal lobe projecting _Porcellio_.
(10.)--Flagellum with 2 joints; abdomen
narrow _Metoponorthus._
(b.)--Able to roll up into a complete ball.
(11.)--Flagellum with 2 joints; antennae
folded together over the thorax
when the animal is rolled up
into a ball _Cylisticus._

(B.)--Tail appendages not projecting when the
animal walking ARMADILLIDIIDÆ.
(12.)--Flagellum with 2 joints; antennae
hidden or carried at the sides
of the head when the animal is
rolled up into a ball _Armadillidium._

=British Species.=--Naturalists in this country paid little attention to the recognition or description of Woodlice, until the latter half of the nineteenth century.

In 1857 Kinahan read a paper before the British Association (32) in which he described fourteen species of woodlice from the British Islands, and eleven years later when Bate and Westwood published their book (1), the number had risen to seventeen. One of the species (_Oniscus fossor_), however, was doubtful, and although Dr. Scharff in 1894 (63) rejected it, his list contained also seventeen species, for in the meantime the Rev. T. R. R. Stebbing had found _Ligidium hypnorum_ in Surrey (70).

Since then the Rev. Canon Norman, Dr. Scharff, the Rev. T. R. R. Stebbing, and one of the present writers, have added other species, as will be seen from the following pages, in which all those found, up to the present time in the British Islands are described and figured.

We shall now consider in detail the British genera and species of woodlice and give their synonymy and distribution.

_Order_--=ISOPODA.=

_Tribe--ONISCOIDA._

_Section_--=LIGIÆ.=

THE TWO DIVISIONS OF THE TAIL APPENDAGES ALIKE IN SHAPE.

_Family_--LIGIIDÆ.

=Flagellum with ten or more joints; tail appendages wholly visible; head without lateral lobes.=

_Genus_--=LIGIA= Fabricius, 1798 (27), p. 301.

_Abdomen broad; body large; habitat, the sea-shore._

The genus _Ligia_ agrees with _Ligidium_ alone, in that the flagellum of the larger antennæ has more than ten joints. In both genera, there are no lateral lobes to the head, and the tail appendages are wholly visible from the upper surface of the body. The latter in _Ligia_ is, however, very many times bigger than in _Ligidium_ and shows no abrupt decrease in the width of its segments when the abdomen is reached.

=Ligia oceanica= Linné (The Quay-louse). PLATE I.

1767 _Oniscus oceanicus_ Linné (43), p. 1061.
1793 _Cymothoa oceanica_ Fabricius (26), p. 509.
1815 _Ligia scopulorum_ Leach (38), p. 374.
1868 _Ligia oceanica_ Bate and Westwood (1), p. 444.
1898 _Ligia oceanica_ Sars (59), 156, pl. LXX.

There is but one British species of _Ligia_, and this, the largest member of the whole tribe to be met with in these Islands, usually attains a length of two centimetres, while adult males may be nearly half as long again. It is the _Oniscus oceanicus_ of Linnæus and lives on the sea shore, where it may be found at low tide beneath stones and rubbish in the crevices of timber. _Ligia_ forms a connecting link between the woodlice proper and the many Isopods which actually live in the sea.

The colour of the animals is a greenish grey, and the compound eyes are almost black, so that they are very conspicuous; there are from eleven to fourteen joints to the flagellum of the outer antennae and this feature, taken in conjunction with the large size and habitat, is sufficient to identify the species in question.

On the coast of Essex the name "quay-lowders" is given to these crustaceans, "lowder" being apparently an old plural of louse.

It is worthy of mention that Mr. Webb, when in charge of the Marine Biological Station at Brightlingsea, examined a very large male specimen of _Ligia oceanica_, in which the maxillæ were duplicated and consisted of four pairs instead of two.

_BRITISH LOCALITIES:--_

_England_: Brightlingsea; (W.M.W.): Maldon; (W.M.W. from R.M.):
Southend; (J.A.M.): Whitstable; (W.M.W.): Herne Bay; Margate;
Dover; Folkestone; (J.A.M.)

_Scotland_: Shetland to Cornwall; (Norman, 49).

_Ireland_: East Coast; West Glengariff; Castletown; Berehaven;
Bundoran; (Scharff, 63).

_FOREIGN DISTRIBUTION:--_

_Europe_: France; (25): Spain; (12): Denmark; Prussia; Norway;
Faroe Islands; Belgium; (59).

_Africa_: Morocco; (16).

_Genus_--=LIGIDIUM= Brandt, 1833 (3), p. 173. Zia, Koch (34).

_Abdomen narrow; habitat, wet moss._

In _Ligidium_ there are numerous joints to the flagellum, lateral lobes are absent from the head, and the tail appendages are completely to be seen. All the segments of the abdomen are distinctly narrower than those of the thorax and in this it agrees with _Trichoniscus_, _Trichoniscoides_, _Philoscia_, and _Metoponorthus_. In these, however, the flagellum has never more than seven joints, the tail appendages (as in all genera but _Ligia_ and _Ligidium_) are partially hidden by the last segment, and in all the four but _Philoscia_ there are lobes to the head.

=Ligidium hypnorum=, Cuvier. PLATE II.

1792 _Oniscus hypnorum_ Cuvier (9), pl. XXVI., figs. 3-5.
1793 _Oniscus agilis_ Persoon, quoted by Koch in Panzer (51),
part 5, pl. XXIV.
1830 _Ligia hypnorum_ Bosc (2), p. 179.
1833 _Ligidium persoonii_ J. F. Brandt (3), p. 174, pl. IV.,
figs. 6-7.
1840 _Zia agilis_ Koch (34), part 34, pls. XXII. and XXIII.
1844 _Ligidium personii_ Zaddach (77), p. 17.
1853 _Ligidium personii_ Lereboullet (39), p. 14, pl. I., fig. 1,
pl. II., figs. 20-31.
1857 _Ligidium personii_ Kinahan (32), p. 275, pl. XXI., fig. 14,
pl. XXII., fig. 9.
1873 _Zia saundersii_ Stebbing (70), p. 286.
1873 _Ligidium agile_ Norman (48), p. 419.
1885 _Ligidium hypnorum_ Budde-Lund (8), p. 254.
1898 _Ligidium hypnorum_ G. O. Sars (59), p. 158, pl. LXXI.

This species, which like the last, is the only British representative of its genus, was added to our fauna in 1873 by the Rev. Thomas R. R. Stebbing (70) who found specimens in the neighbourhood of Copthorne Common, Surrey. Up to the present time, when we are pleased to announce that we discovered it in the spring of 1902 at Warley in Essex, _Ligidium hypnorum_ has not been recorded from any other place in the British Islands.

As the name of the species implies, it lives in wet situations and in its turn connects _Ligia_ with the forms which inhabit drier places. _Ligidium hypnorum_ might be mistaken for _Philoscia muscorum_, but as already pointed out in the generic description, the latter has but a few (three) joints to the flagellum, instead of from ten to thirteen. From _Ligia_, the species under consideration is distinguished by its small size, narrow abdomen, and habitat.

_BRITISH LOCALITIES:--_

_England_: Warley, Essex; (W.M.W.): Copthorne Common, Surrey;
(Stebbing, 70).

_FOREIGN DISTRIBUTION:--_

_Europe_: France; (25): Sweden; Denmark; Germany; (59): Turkey; (8).

_Family_--TRICHONISCIDÆ.

=Flagellum with less than ten joints; head with lateral lobes; tail appendages partly hidden.=

_Genus_--=TRICHONISCUS= Brandt, 1833 (3), p. 174.

_Abdomen narrow; eyes compound; flagellum, usually with more than three joints._

In _Trichoniscus_ the flagellum may have from seven to four (rarely three) joints. As in _Trichoniscoides_ and _Haplophthalmus_ there are lateral lobes to the head, though these are not very pronounced; the body is also of small size, the abdomen narrow with both divisions of the tail appendages equally so, and almost of the same length though slightly covered by the last segment. The compound eyes distinguish _Trichoniscus_ from the two genera named, and from _Platyarthrus_, while its small size and the character of its tail-parts mark it out from all others.

=Trichoniscus pusillus= Brandt. Plate III.

1833 _Trichoniscus pusillus_ Brandt (3), p. 174, pl. IV., fig. 9.
1838 _Itea riparia_ Koch (34), part 22, pl. XVII.
1844 _Itea lævis_ Zaddach (77), p. 16.
1857 _Philougria celer_ Kinahan (32), p. 281, pl. XXII., figs. 1-4.
1858 _Philougria riparia_ Kinahan (33), pp. 191 and 198, pl. XXIII.,
fig. 1.
1868 _Philougria riparia_ Bate and Westwood (1), p. 456.
1898 _Trichoniscus pusillus_ Sars (59), p. 161, pl. LXXII., fig. 1.

This tiny species is found commonly amongst the roots of the herbage in very moist places. It presents a horny translucent appearance and is of a reddish brown colour. It runs with considerable speed, and when it is moving, the white irregular lines with which it is beset are not evident. _Trichoniscus pusillus_ is very much like _Trichoniscus vividus_ in colour but the latter species is nearly twice as big and has from five to seven joints to the flagellum, while the former has never more than four. _Trichoniscus roseus_ is also much larger and its bright red colour (which it loses, however, when preserved in alcohol) is another means of distinguishing it from the species under consideration.

Professor Sars in his _Crustacea of Norway_ (p. 162) describes from Christiania, under the name of _Trichoniscus pygmæus_, a still smaller species. As this may possibly be discovered in this country a brief comparison between it and _Trichoniscus pusillus_ may be of value. The former reaches a length of but two millimetres; it is "whitish, semi-pellucid with a few light brown pigmentary ramifications across the segments and a double row of irregular opaque patches along the middle of its back" (p. 163). Its body is covered with minute tubercles and there are only three joints to the flagellum; its movements are by no means rapid.

The body of _Trichoniscus pusillus_ is smooth and polished. It has four joints to the flagellum--Dr. Scharff (63) says three or four--and it moves quickly.

_BRITISH LOCALITIES:--_

_England_: Brightlingsea; Warley; (W.M.W.): Epping Forest; (Bate
and Westwood, 1): Hanwell; Southall; Kew Gardens; Langley; Burnham
Beeches; Dropmore; Skirmett; Bluebell Hill, Maidstone; (W.M.W.):
Chislehurst; Plymouth; Polperro; Looe; (Bate and Westwood, 1):
Hertfordshire; Northumberland; Durham; (Norman, 49): Exeter;
(Parfitt, 53).

_Scotland_: Edinburgh; (Scott, 68): Cumbrae; (Robertson, 57).

_Ireland_: Connemara; (Norman, 49): Dublin; Wexford; Cork and
Kerry; (Percival Wright _teste_ Bate and Westwood, 1): Tyrone;
Waterford; Portlaw; Kilkenny; Wicklow; (Kinahan, 33).

_FOREIGN DISTRIBUTION:--_

_Europe_: France; (25): Spain; (15): Italy; (19): Norway; Sweden;
Denmark; Germany; (59).

_Africa_: Algeria; Tunis; Azores; (24).

_America_: Niagara; North America; (59).

=Trichoniscus vividus=, Koch. PLATE IV. (from a spirit specimen).

1840 _Itea vivida_ Koch (34), part 34, pl. IV.
1858 _Philougria vivida_ Kinahan (33), pp. 197 and 198, pl. XXIII.,
fig. 2.
1868 _Philougria vivida_ Bate and Westwood (1), Vol. II., pp. 458
and 459, figs.

This species is claret-brown in colour and under a lens it is seen to be marbled with white, indeed in appearance it is much like _Trichoniscus pusillus_ though twice the size. There are important differences between the two species as regards the number of joints to the flagellum. These vary from five to seven in _Trichoniscus vividus_ while in the other, as already pointed out, there are not more than four. The body is practically speaking smooth for it bears only very small tubercles, widely separated. In _Trichoniscus vividus_ the antennæ lack the bristles which characterise those of the other species in the genus. The species under consideration was discovered by Dr. Kinahan in March, 1858, at Portlaw, Co. Waterford and is active even amongst the snow.

_BRITISH LOCALITIES:--_

_Ireland_: Portlaw, Co. Waterford; (Kinahan, 33): Cappagh, Co.
Waterford; (Scharff, Irish Nat., Vol. IX., p. 158): Borris, Co.
Carlow; (Scharff, 64.)

_FOREIGN DISTRIBUTION:--_

_Europe_: Spain; (12).

=Trichoniscus roseus= Koch. PLATE V.

1838 _Itea rosea_ Koch (34), part 122, pl. XVI.
1858 _Philougria rosea_ Kinahan (33), pp. 197 and 199, pl. XXIII.,
fig. 3.
1858 _Philougria rosea_ Bate and Westwood (1), p. 460.
1898 _Trichoniscus roseus_ Sars (59), p. 163, pl. LXXIII., fig. 1.

The third British species of _Trichoniscus_ is of a deep pink colour and has a light yellow stripe down the back (in some habitats the animals are said to be quite white). Arranged in transverse rows upon the body are large tubercles, each of which under strong magnification will be found to end in a tiny hair. It is distinguished from _Trichoniscus pusillus_ by the larger size of its body, which is also comparatively broader, and from _Trichoniscus vividus_ by the four joints of the flagellum of its antennæ which latter have strong bristles upon them. In the former species there are five or more joints to the flagellum and the antennæ, though hairy, lack the bristles. _Trichoniscus roseus_ is to be looked for in old gardens.

_BRITISH LOCALITIES:--_

_England_: Warley; (W.M.W.): Maldon; (W.M.W. from R.M.): Stanmore;
Hanwell; Ealing; Wimbledon; (W.M.W.): Berkhamsted; Torquay;
(Norman, 49): Plymouth; (Bate and Westwood, 1 and B.M.,):
Grassendale, near Liverpool; (R.W.): Newtownards; (R.W., Irish
Nat, 1904, p. 260.)

_Scotland_: Tarbert; (Scot, 68).

_Ireland_: Dublin; Ballyfinder, Co. Down; (Scharff, 63):
Templeogue; Dundrum; Blackrock; Rathgar, Co. Dublin; Bray, Co.
Wicklow; (R.F.S.): Oakleigh; Kerry; (R.W.): Belfast; (Welch, Irish
Nat., 1896, p. 213.): At the grave of Josiah Welch (grandson of
John Knox), Castle Upton; Richhill, Co. Armagh; Castleconnell
Ferry; (R.W.): Glenade House, Co. Antrim; (R.W. from R. Ll.
Praeger).

_FOREIGN DISTRIBUTION:--_

_Europe_: France; (25): Spain; (12): Italy; (59): Denmark;
Germany; Holland; (39): Dalmatia; (18).

_Africa_: Algeria; Tunis; (24).

_Genus_--=TRICHONISCOIDES=, Sars, 1898 (59), p. 164.

_Abdomen narrow; eyes simple; (or wanting); flagellum, with four joints._

The members of this genus are very much like those of _Trichoniscus_. In the latter, however, the hinder legs are longer in proportion and the eyes are compound.

=Trichoniscoides albidus= Budde-Lund. PLATE VI.

1879 _Trichoniscus albidus_ Budde-Lund (7) p. 9.
1898 _Trichoniscoides albidus_ Sars (59), p. 165, pl. LXXIII.,
fig. 2.

We are able to include this species, as a specimen was found by Mr. Webb at Eton Wick in the summer of 1899. It is one of a number of species which the Rev. Canon Norman (49, p. 18) suggested as likely to be British. It is the only representative of its genus, which does not differ in any very important characters from the others in the family. The narrow elongated body will serve to separate it from _Trichoniscus vividus_ and _Trichoniscus roseus_, but on account of its size, which is much the same as that of _Trichoniscus pusillus_ and the two British species of _Haplophthalmus_, it will be advisable to give some further points of distinction. From the first its white colour will serve to differentiate it; the other two lack the narrow abdomen seen in _Trichoniscoides albidus_. Moreover, not one of the three shows the serrations on the side plates which characterise the species under consideration. _Platyarthrus hoffmannseggii_ is small and white and the edges of its side plates are toothed, but it is oval in shape, possesses no eyes, and its stout antennæ have but a single joint to the flagellum instead of four. On the Continent this species has been found in rich soil.

_BRITISH LOCALITIES:--_

_England_: Eton; (Stebbing, 71a): Sunderland; (Brady, 50a).

_FOREIGN DISTRIBUTION:--_

_Europe_: France; Wimereux and Lyons, Forêt (25): Norway;
Denmark; (59).

_Genus_--=HAPLOPHTHALMUS= Schöbl, 1850 (66), p. 449.

_Abdomen broad (comparatively); eyes simple; flagellum with three joints; back with longitudinal ridges._

The body of _Haplophthalmus_ is long in proportion to its width, but there is no abrupt decrease in the breadth of the abdomen as seen in _Trichoniscus_ and _Trichoniscoides_. The eyes are simple as in the latter genus and the lateral lobes of the head are rather large, while the side plates of the body are well separated.

=Haplophthalmus mengii= Zaddach. PLATE VII.

1844 _Itea mengii_ Zaddach (77), p. 16.
1860 _Haplophthalmus elegans_ Schöbl (66), p. 449.
1885 _Haplophthalmus mengii_ Budde-Lund (8), p. 250.
1898 _Haplophthalmus mengii_ Sars (59), p. 167, pl. LXXIV.,
fig. 1.

The Rev. Canon Norman discovered two specimens of this species in Ireland in June, 1900 (50); in the previous year one of us (Mr. Webb) found a single example at Eton Wick.

The main differences between the members of this genus and their allies are set forth in the generic description and incidentally elsewhere, so we shall content ourselves with giving the distinctive points of the two British species. _Haplophthalmus mengii_ has a number of raised longitudinal ribs on each segment of the thorax, the outer ridges being somewhat broken. There are also two prominent ribs upon the third segment of the abdomen.

_BRITISH LOCALITIES:--_

_England_: Eton; (Stebbing, 71a): Sunderland; (Brady, 50a).

_Ireland_: Corcumroe Abbey; Co. Clare (Norman, 50).

_FOREIGN DISTRIBUTION:--_

_Europe_: France; (25): Norway; Prussia; Germany; Bohemia; (59).

=Haplophthalmus danicus= Budde-Lund. PLATE VIII.

1870 _Haplophthalmus elegans_ Budde-Lund (6), p. 228
(not _Haplophthalmus elegans_ Schöbl).
1879 _Haplophthalmus danicus_ Budde-Lund (7), p. 9.
1881 _Haplophthalmus mengii_ Weber (76), p. 192, pl. V., figs. 7-9
(not _Itea mengii_ Zaddach).
1885 _Haplophthalmus danicus_ Budde-Lund (8), p. 250.
1898 _Haplophthalmus danicus_ Sars (59), p. 168, pl. LXXIV., fig. 2.

This species was added to the British list by the Rev. Canon Norman (49), who found a colony in his garden at Berkhamsted. It has rows of tubercles on its thorax instead of ridges, and there are no ribs at all upon the abdomen. The front of the head projects further comparatively and forms a more acute point than in _Haplophthalmus mengii_ and it is not so purely white in colour as the latter species.

_BRITISH LOCALITIES:--_

_England_: Warley Place; (W.M.W. from Miss Willmott): Queen's
Cottage, Kew Gardens; Stanmore; Hanwell, garden at Odstock,
Bennett's Nurseries; (W.M.W.): Berkhamsted; (Norman, 49):
Sunderland; (Brady, 50a).

_FOREIGN DISTRIBUTION:--_

_Europe_: France; (25): Denmark; Holland; Germany; (Dollfus, Feu
de Jeun, Nat., April, 1896): Norway; (Sars, 59).

_Section_--=ONISCI.=

THE OUTER DIVISIONS OF THE TAIL APPENDAGES BROADER THAN THE INNER ONES.

_Family_-ONISCIDÆ.

=Tail appendages projecting when the animal is walking.=

(1.) Unable to roll up into a complete ball.

_Genus_-=ONISCUS= Linné 1746 (41), p. 360.

_Flagellum, with three joints; abdomen broad; head with lateral lobes._

The characters given above taken in conjunction with the size of the animals will serve to distinguish the members of this genus.

=Oniscus asellus= Linné (The "Common Slater.") PLATE IX.

1761 _Oniscus asellus_ Linné (41), p. 500, No. 2058.
1792 _Oniscus murarius_ Cuvier (9), p. 22, pl. XXVI.
1838 _Oniscus fossor_ Koch (34), part 22, pl. XXII.
1868 _Oniscus asellus_ Bate and Westwood (1), p. 468.
1868 _Oniscus fossor_ Bate and Westwood (1), pp. 471-2.
1898 _Oniscus asellus_ Sars (59), p. 171, pl. LXXV.

_Oniscus asellus_ is one of the largest of our woodlice and it is also probably the commonest, though _Porcellio scaber_ is in many places quite as abundant. The body of _Oniscus_ is broad and expanded and the colour is usually a slate grey with yellowish markings more or less regularly arranged.

From the genus _Porcellio_ the species with which we are concerned is at once distinguished by the three jointed flagellum. _Porcellio_ has but two joints and has, besides, a prominent lobe projecting from the middle of the head, which is not seen in _Oniscus_. _Philoscia_, although it has three joints to the flagellum, has a narrow abdomen and lacks entirely the lateral lobes which are a feature of the other genera of Oniscidæ.

_Oniscus fossor_ of Koch (34) was recognized by Kinahan and by Bate and Westwood as a species. Dr. Scharff submitted specimens to Professor Budde-Lund who found no differences between them and _Oniscus asellus_. The former (63) mentions, however, that the characteristics of the supposed species are those of young examples of _Oniscus asellus_, and Professor Sars (59, p. 173) seems to be of the same opinion. Many young examples of _Oniscus asellus_ that we have examined have a curious whitish transverse band owing to the light colour of the dorsal plates of the first abdominal segments. The flagellum also does not seem to shew in young animals a distinct division into three joints.

_BRITISH LOCALITIES:--_

_England_: High Beach, Epping, including an albino; Maldon;
Brightlingsea; Iver; Hanwell; Eton; Kew; Pamber Forest;
Kingston-on-Soar; Bluebell Hill, Maidstone; (W.M.W.):
Lynmouth; (W.M.W. from J.T.C.).

_Scotland_: (Scharff, 63). Dinnet, Aberdeenshire; (W.M.W. from
Madame Christen).

_Ireland_: (Scharff, 63). Yellow form with black spots, Donegal (R.W.)

_FOREIGN DISTRIBUTION:--_

_Europe_: Almost throughout; (12): France; (25): Spain; (12):
Sweden Norway; Denmark; Germany; Holland; Italy; Iceland; (59):
Faroe Islands; Thorsharn; (R.F.S.)

_Africa_: Azores; (24).

_America_: Greenland; (59): North America; (Budde-Lund).

_Genus_--=PHILOSCIA= Latreille, 1804 (37), p. 43.

_Flagellum with three joints; abdomen narrow; head without lateral lobes._

If any further differences of an obvious kind be required to distinguish _Philoscia_ from _Oniscus_, one at least will be found in the much greater development of the hinder legs in the former genus.

=Philoscia muscorum= Scopoli. PLATE X.

[Not of Lereboullet, which is an _Oniscus_, see Bate and Westwood (1).]