CHAPTER I.

_THE CRAB AND SCORPION GROUPS._

BY W. F. KIRBY, F.L.S.

This section of animals is often called a "sub-kingdom," and differs from back-boned animals in having the framework of the body outside. That is, instead of a skeleton, Crabs, Spiders, Centipedes, and Insects are protected either by a hard shelly casing, or by a tough skin, to which the muscles are attached; and this arrangement renders them much stronger and much less susceptible to injury, in proportion to their size, than vertebrate animals. They have cold blood, generally of a white colour; and their bodies and limbs are usually composed of a considerable number of separate joints.

The group is a very large one, and it is probable that there are at least 300,000 different kinds of insects already known, while the total number of species now existing is estimated by different entomologists as from two to ten millions. It is, therefore, no exaggeration to say that every word of our brief account of the Insects represents from ten to twenty known species at least. The other classes of the group are also very numerous. Our account must necessarily be very short; the characters of the principal classes of the Jointed Animals are referred to in their places.

CRABS, LOBSTERS, SHRIMPS, WOOD-LICE, BARNACLES, ETC.

Jointed animals are generally provided with one pair of long jointed organs, called "antennæ," a naturalised word derived from the Latin, in which language _antenna_ means a sail-yard. They are often called "feelers," and usually fulfil this function at least; but they are also frequently organs of smell, and sometimes probably of hearing and other senses. One peculiarity of the CRAB and LOBSTER group is that they are generally furnished with two pairs instead of one pair of these organs.

They are nearly all aquatic animals, by far the larger portion being marine, and they breathe with gills. They are provided with a hard calcareous or horny covering. The head is not separated from the trunk, as in insects; and they are provided with a number of jointed organs, usually classified as three pairs of jaws, three pairs of foot-jaws, and five pairs of legs in the more typical families; but in the smaller and more aberrant species the number is more variable. In their early stages they frequently pass through very extraordinary changes of form, but after assuming their adult shape they grow by casting their shells at intervals.

We will now notice a few typical examples of the different groups of these creatures.

The BRINE-SHRIMP is a little reddish creature about half an inch long, which prefers the concentrated solution of brine-pits to sea-water. It has eleven pairs of legs, and, notwithstanding its name, the front portion of its body is considerably broader and flatter in proportion than that of a real shrimp, the other half consisting of a jointed tail.

BARNACLES were formerly considered to be shell-fish, but are now usually classed with the crabs and lobsters, because, when they are young, they appear as freely swimming creatures, with one eye, two antennæ, and six pairs of jointed limbs. When they grow larger, they fix themselves to a rock or some other object by the head, and develop a shell, usually composed of several pieces. The commonest is the ACORN-BARNACLE, the white shell of which, measuring rather less than an inch across, swarms on rocks at the seaside. It is shaped like a limpet, but open at the top. The GOOSE-BARNACLES hang down by a stalk, and their jointed shells more resemble those of a mussel than that of a limpet, though they are composed of several pieces. Various species similar to both those mentioned are found on piers, rocks, the bottoms of ships, and even sometimes on the skin of whales.

In dark cellars in the country, under loose bark, or under pieces of wood which have been left in the fields, we often see creeping about brown creatures about half an inch long, with jointed bodies and antennæ, and short jointed legs. They are called WOOD-LICE, and several species roll themselves up into a ball when alarmed. These creatures feed chiefly on decaying vegetable substances; and there is a larger marine species much like them, which is common in holes and crannies in the rocks on the seashore.

There are other curious creatures, called WHALE-LICE and FISH-LICE, which are parasitic in their habits. Some of these look like spiders, and one or two have enormously long legs; but others are of strange and almost indescribable forms, and sometimes without legs at all. One species, found on the sprat, has two long appendages at the end of its body not unlike a pair of compasses.

SHRIMPS and PRAWNS are red when cooked, but when alive are very pretty semi-transparent objects, which may be seen swimming about through the glass of aquariums placed against the wall. Prawns are larger than shrimps, and have a strong serrated spine in front of the head. Shrimps and prawns, of which several kinds are found off the British coasts, generally prefer shallow water, with a sandy bottom. In most of these the first two pairs of feet are divided to form a pair of pincers at the extremity; but in such small creatures this is easily overlooked, unless special attention is directed to it. It is different with the LOBSTERS and CRAYFISH, which much resemble shrimps in form, but are very much larger, and armed with a pair of very large pincer-like claws, in addition to the other legs. Lobsters live in the sea, in holes in the rocks, into which they dart backwards, and there protect themselves with only the head and claws projecting at the opening, ready to face any intruder. Crayfish, which are smaller, live in holes in the banks of brooks and rivers. There are many species, some of large size and bright colours. The COMMON LOBSTER is black when living, but other species are red, blue, or variegated.

Sometimes, if we pick up a whelk-shell on the beach, we shall find it inhabited, not by a mollusc, but by a crab, with its legs and claws wedged together, so as to fill the aperture completely--one claw, much larger than the other, resting in front; and if we pull it out, we shall find that, though the front of its body and the legs and claws are hard, like those of an ordinary crab, it has a long, soft, fleshy tail, absolutely defenceless and unprotected. Crabs belonging to this curious section are called HERMIT-CRABS, and protect themselves by taking possession of shells which they have either found empty or appropriated by the easy and economical process of devouring the owners. The few species found on the British coasts are all small, and more frequently noticed in whelk-shells than in any others; but tropical species attain to a considerable size, and may sometimes be found in shells measuring 3 or 4 inches across at the opening.

Crabs are distinguished from the lobsters by their compact form, and by having the short tail turned in under the body. There are a great number of species, differing much in size, shape, and appearance. One of the best known in England is the large EDIBLE CRAB, which may often be seen in fishmongers' shops, and, unlike the lobster, does not change much in colour when boiled. Every visitor to the seaside must have seen numbers of the little greenish SHORE-CRABS, running about on the sand, or over seaweed-covered rocks, at low tide. These small crabs are harmless, but large kinds are able to give a very severe pinch. It is related that when the great chemist Sir Humphry Davy was a boy he used to maintain that pain was no evil, until a large crab gripped his toe one day when he was bathing, after which he changed his opinion.

_Photos by W. P. Dando. F.Z.S. Regent's Park._ _Printed at Lyons, France._

Some crabs are smooth and shining, but others are covered with bosses, excrescences, and spines, which give them a very formidable appearance, and must be a useful protection against any enemies to whose attacks they are exposed. In many species one of the two great claws is always much larger than the other. Some have round bodies, others are oval or nearly square; some have short legs, and others very long ones. The species differ much in their habits; and in tropical countries there are land-crabs which live entirely on shore, and others which are amphibious, and climb cocoanut-trees to get at the nuts. As a general rule, however, crabs are carnivorous and marine, and play the part of sea-scavengers.

The KING-CRABS differ very much from any now living in the British seas, but are generally considered to be allied to the Trilobites, an extinct family which appears to have been extremely numerous in very ancient seas. King-crabs are 2 or 3 feet long from the front of the body to the end of the tail. The front part of the body is entirely covered by a curved oval shield, while the hinder part of the body is much narrower, and armed at the sides with strong teeth directed backwards, and also with a long and strong spear, something like that of a sword-fish on a small scale, as long as the rest of the body. The few species known exhibit an instance of what is called "discontinuous distribution," since they are found only on the coasts of the Moluccas, East Indies, and the Southern United States and West Indies.

SCORPIONS, SPIDERS, AND MITES.

These creatures form a peculiar group in which there are only two principal divisions of the body, the head and thorax being fused into one mass, and the abdomen forming a separate division. In the Mites, however, the body forms a single round or oval mass, even the division between the thorax and the abdomen having disappeared. The members of the group have no antennæ, but two pairs of jaws and a pair of palpi, frequently very long, and armed with a pincer-like arrangement at the end, in which case they are called "foot-jaws." Except in some of the mites, which have only four or six, all the group have eight legs. They pass through no metamorphosis, but moult several times after quitting the egg before attaining their full growth. They have frequently several pairs of simple eyes, but no compound eyes like the large pair on the head of most insects.

In the SCORPIONS, of which there is a considerable variety in different parts of the world, the united head and thorax are comparatively short; but the abdomen is very long, and divided into a broad half, consisting of seven segments, and a narrow tail of five very movable segments, besides a sharp, curved sting at the extremity. There are from three to six pairs of eyes on the head and thorax, and in front of the body projects a pair of very large pincer-bearing foot-jaws. Scorpions are generally of a yellowish or black colour; and the largest black scorpions of Africa and India sometimes measure as much as 9 inches in length. They are nocturnal creatures, hiding under stones, or in holes in the ground, or in crevices in walls during the day. They kill the insects and other small animals on which they feed with their stings, the sting of one of the large black scorpions, like that of the large tropical centipedes, being as painful and dangerous as that of a snake. There are a few small and comparatively harmless species found on the shores of the Mediterranean, but most of the scorpions inhabit warmer countries.

The JOINTED SPIDERS are creatures 1 or 2 inches long, remarkable for having the head and the segments of the thorax separated from each other, so as to form distinct divisions of the body. They have rather long and very hairy legs, and only one pair of well-developed eyes, another pair being rudimentary. Some species are diurnal and others nocturnal in their habits. They feed on insects, and sometimes on small birds, etc., and can inflict a very painful bite. They are found in South-eastern Europe, Africa, Southern Asia, and from the Southern States of North America south to Chili and Argentina.

The FALSE SCORPIONS, or BOOK-SCORPIONS, are small animals resembling scorpions in shape, but with no sting, and the abdomen not narrowed into a tail. They are sometimes found in houses among dusty old books, as well as out of doors among moss, or under stones or bark. Sometimes they cling to the legs of flies; they are believed to feed on mites and other small creatures, but not to injure the flies, only employing them as a convenient method of being conveyed from one place to another.

The WHIP-SCORPIONS are not unlike scorpions, and have large claws, but the front legs are very long, slender, and whip-like, and there is either no tail, or else a long, slender, whip-like one without a sting. They are inhabitants of warm countries, and, rightly or wrongly, are reputed to be venomous. Different species measure from 1 inch to 4 or 5 inches in length.

The curious HARVEST-MEN have two eyes, a small, compact, oval body, large pincers, and very long, slender legs, longer and more slender in proportion to their size than those of crane-flies, and equally liable to be broken off, if the owner is roughly handled. They feed on plant-lice and other small insects.

We now come to the large and important group of SPIDERS, which more frequently attract attention in England than any others of the group. The abdomen is not usually divided into distinct segments, and is connected with the thorax by a short stalk. Spiders have strong, poisonous jaws, which make some of the larger species formidable even to man, and several pairs of eyes; while many possess an apparatus for spinning a strong silken web, in which they entangle their prey, consisting in England chiefly of flies and other winged insects.

The largest known spiders are usually placed first in the series. These are the great BIRD-CATCHING SPIDERS of South America, some of which have bodies 3 inches long, and strong, hairy legs. These large spiders have now been proved not only to feed on insects, but occasionally on humming-birds, and even sometimes on larger birds, such as finches.

The TRAP-DOOR SPIDERS are allied, but smaller, perhaps averaging about an inch in length. They construct a silken gallery in the ground, with a round door, which they shut behind them when they enter. There is only one species in England, which does not form a trap-door, but a silken tube. If any insect settles on it, the spider clutches it from within, tears a hole in the tube, drags its prey inside, and then repairs the rent.

Different spiders have many curious methods of capturing their insect-prey. Some catch insects by running after them, and others by leaping on them, while those which spin webs are also very dissimilar in their habits and in their abodes. The brown HOUSE-SPIDERS spin webs in any room left undisturbed long enough to allow them to construct them. On the other hand, the ORB-SPINNERS, or GARDEN-SPIDERS, construct elaborate webs out of doors. One of the most beautiful of these is the DIADEM-SPIDER, which is nearly an inch long, and of a green or reddish colour, with a white cross bordered with black on the back. The web is very regularly constructed, the principal threads radiating in all directions from a common centre, where the spider generally sits in fine weather, ready to rush out upon any insect which may become entangled in the web.

The GOSSAMER-SPIDERS spin light webs, which are easily carried up into the air, and upon which the spiders are borne from one place to another. Sometimes on an autumn morning the air may be seen to be full of these floating webs, which also cover the grass and bushes where they have settled. The WATER-SPIDERS, again, construct a habitation of water-tight silk under water, like a diving-bell, and inflate it by carrying down bubbles of air from the surface, entangled in the hairs of the body.

The nesting-habits of many spiders are very curious. The eggs are usually laid in a silken case, and the RUNNING-SPIDERS may often be seen with the egg-cases attached to the end of the body, as in the female cockroach.

The males of many spiders are much smaller than the females, and are very liable to be devoured by their partners.

Among the most curious of the group are the SPINY SPIDERS, strange, horny, semicircular creatures, studded with strong spines. They are allied to the Garden-spiders, but confined to the tropics.

The SPOTTED SPIDER is a very beautiful species, often seen among cases of mixed insects, etc., sent from India, It is black, with brown abdomen and numerous yellow spots, and about 1½ inch long; the body is much longer than broad, and the legs are about twice as long as the body.

Attempts have been made to turn spider-silk to commercial purposes, but the great difficulty is that spiders are so voracious and cannibalistic in their propensities that they cannot be kept in captivity, for they will kill and eat each other as long as there are any left, to the very last spider. The silk of some of the large tropical spiders is sometimes strong enough to cause a man much annoyance when riding through the woods, striking up against his face, and sometimes knocking off his hat.

The last section includes the MITES and TICKS, most of which are small or microscopic. The whole body forms one round or oval mass, with scattered hairs, and eight legs, though most mites have only six legs when young, while the PLANT-MITES have only four. The largest mites are those called TICKS. There are one or two small British species which are sometimes sufficiently troublesome; but in many warm countries they are a far more serious nuisance, lurking on the herbage, and fixing their proboscis in the skin of any passing man or animal, and retaining their hold till they are gorged with blood, and allow themselves to drop off by their own weight.

Among the smaller mites some species are parasitic on warm-blooded animals, causing itch, mange, and other diseases; while many infest insects, especially bumble-bees and dung-beetles. These are of considerable size for mites; and there are other bright scarlet species which are sometimes found on saw-flies, dragon-flies, etc. Many feed on decaying animal or vegetable matter, such as the CHEESE-MITE and the SUGAR-MITE, the former being a very familiar and interesting microscopic object; and others, again, are very destructive to plants, like the small scarlet mite known in greenhouses as the RED SPIDER.

Among the plant-feeding mites are the four-legged GALL-MITES, which produce galls or other excrescences on the plants which they infest.

Mites are probably almost as varied in their forms and habits and as interesting objects of study as insects or spiders; but the group is somewhat neglected by naturalists, owing to the small size of most of the species, and the consequent difficulty of collecting and preserving them.

CENTIPEDES AND MILLIPEDES.

These are creatures with long, worm-like bodies, composed of a number of rings or segments, each provided with one or two pairs of legs. They have one pair of antennæ, like insects, but they pass through no metamorphoses, nor do they moult. Instead of this, they begin their existence, on quitting the egg, without legs, or with only three pairs of legs, and continue to add to the number of their segments and legs until they have attained their full growth. They are called Centipedes, or Hundred-legs, and Millipedes, or Thousand-legs; but in the majority of species the number of legs is considerably below 100, though in some few it may exceed 300.

The CENTIPEDES have only one pair of legs attached to each segment of the body, and are carnivorous, being armed with a pair of strong mandibles, which are perforated poison-fangs. The British species are all small and harmless, but the bite of the large tropical centipedes is more painful and almost as dangerous as that of a snake. Centipedes are long, broad, flattened creatures, with about twenty-one pairs of legs, and sometimes measure more than a foot in length. A reddish centipede, belonging to an allied family, is common in England under stones and in loose mould. It has long antennæ and fifteen pairs of legs, and feeds chiefly on worms. It is about an inch long.

The ELECTRIC CENTIPEDES are much longer and more slender than the others in proportion to their length, with rather short antennæ, and short and very numerous legs. They are of a white or yellow colour, and 2 or 3 inches long. All are nocturnal in their habits, and feed on decaying animal or vegetable matter, and are fond of ripe fruit. They emit a pale phosphorescence, visible in the dark along the track over which they have crawled.

MILLIPEDES are not venomous, and feed chiefly on soft vegetable matter. Except the first three behind the head, which are provided with only one pair each, every segment bears two pairs instead of one pair of legs. The COMMON SNAKE-MILLIPEDE is about an inch and a half long, and is brown, with yellow rings and ninety-nine pairs of short white legs. It is nearly as destructive as the Wire-worms, which it resembles in its habits, and may often be seen clinging to a partly eaten potato. Millipedes are able to roll themselves up into a spiral. Many foreign kinds grow to a much larger size, measuring nearly a foot in length. They are more frequently sent to Europe from foreign countries than centipedes, probably because they are sluggish, harmless creatures which do not bite.

The members of one family of millipedes, called PILL-MILLIPEDES, are so similar to wood-lice in shape and appearance that they might easily be mistaken for them, and they exhibit the same habit of rolling themselves up into a ball. One species is not uncommon in England.

A curious genus, generally placed in a distinct class by itself, includes a few species which may be called SLIMY MILLIPEDES. The species are found in widely separated parts of the world, chiefly in the most southern regions, such as South America, South Africa, Australia, and New Zealand, though one or two are known from Ceylon and the West Indies. They resemble slimy caterpillars, with conspicuous antennæ, and from thirteen to forty pairs of legs. The body is not distinctly divided into segments, and it exudes a very viscous slime, which acts like bird-lime in capturing the small insects which form at least a part of the food of these creatures, but which will not adhere to their own bodies. These creatures are found among decaying vegetable matter.

INJURIOUS CREATURES.

We are sometimes inclined to complain of our English climate, but we have cause to congratulate ourselves that it is far less prolific of noxious creatures than many others. We have no venomous scorpions or centipedes, and are not obliged to shake such intruders out of our boots before we can venture to put them on. Since the country has been so well drained, we are very little troubled with gnats, which breed in standing water, and are equally troublesome in cold countries like Lapland, and warm countries like South America. Nevertheless, several very troublesome creatures, not native to this country, have taken up their abode with us permanently, and more care should perhaps be exercised in preventing the possible introduction of others. Among the most troublesome of our household insects are bugs, cockroaches, and house-ants, all of which have been introduced from abroad. Among field- and garden-pests, the American Blight (which destroys our apple- and pear-trees) and the Hessian Fly are probably invaders from abroad; but the latter does not seem to have committed great ravages in this country. Among pests which have not succeeded in establishing themselves here, but which we should be specially on our guard against, are the White Ants, which are found as far north as Bordeaux, and are terribly destructive to woodwork, wherever they are met with; the Gypsy-moth, very destructive on the Continent and in North America, but extinct as a British species, perhaps because there is something inimical to its constitution in our climate; and the Colorado Potato-beetle, which is only kept out of Europe by incessant vigilance. But apart from actually injurious insects, it is remarkable how many species which are common everywhere on the Continent are either absent from Britain, or are only met with in very restricted localities. Let us hope that we may long enjoy our comparative immunity from noxious insects in Britain.

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