Crabs are closely allied to lobsters, and belong to the highest orders of the crustaceans. The lengthened, loose-jointed abdomen of the typical crayfish is wanting, and there is a general concentration of the parts; all the most important viscera being included in the thorax, and covered by a single, closely compacted shield. There are many species of crabs, differing in other respects as well as in the form of the shell or back, which in some is nearly orbicular, in others it is oblong, longer than it is broad, or broader than it is long. They differ in the smoothness of their shells, and in the length of their legs, which they stretch out from under their horny covering. Their first pair of limbs is not fitted for locomotion, but shows a vigorous development of the strong claws and pincers of other decapod crustaceans. Though found in almost all seas, they are poor swimmers, their legs being formed for walking or creeping, rather than as oars to propel them through the water. They are found in pools, among seaweeds, and particularly in marshy places left by the receding tides. Most species live in water, some in moist places on land. Many kinds of crabs are used for food. Its black claws and broad carapace readily distinguish it from other species. From activity in seizing, tearing, and devouring their food, and from their pugnacity, crabs are interesting inmates of the aquarium. They also moult, or cast off their shells; not at regular seasons, but when the demand for more room requires it.
Class II.—Arachnida are closely related to the crustaceans, having, like them, the body divided into two sections—cephalo-thorax and abdomen. To the former are attached four pairs of legs, but the abdomen has no appendages for locomotion. There are about 5,000 species, produced from eggs, and undergoing no metamorphoses in their development.
The lowest forms, under the common name of Acarina,[7] have the anterior part in a mass with the abdomen, and short legs near the head, terminated in little claws suitable for taking hold of hairs and feathers. They are mostly parasitic, and all birds and animals, even parasites themselves, are liable to suffer from acarina peculiar to their own species. Pedipalpi[8] (scorpions), and Araneina[9] (spiders), though much larger, belong to this class. The body of the scorpion is divided into segments, though the anterior of the abdominal part seems but a continuance of the thorax, and is as large. It, however, soon tapers off into a long, jointed, tail-like process, in the terminus of which is its hooked sting, perforated and connected with the poison sac. In striking, the tail is raised over the back and struck down. Its other weapons are the crab-like claws on the strong forearms. The Araneina, at least some classes of them, are well known. The soft, unjointed body is separated from the thorax by a narrow constriction or tie, and at the posterior end there are little appendages called spinnerets, through which the silken lines issue that form the web. The hinder feet are skillfully employed in arranging the gossamer threads after patterns that are instinctively followed.
Class III.—Myriapoda, Centipedes,[10] have the thorax merged with the elongated abdomen, while the head is free. They resemble worms in form, but the skin is stiffened with chitine,[11] and the many legs are articulated. There are two orders: the Chilognatha,[12] which move slowly, and are harmless, the “thousand legged worm” is a representative, and the Chilopoda,[13] more active, and having a flattened body of about twenty segments, each carrying one pair of legs. Their mouths are armed with formidable fangs connected with poison glands. They are carnivorous, and may be distinguished by their general appearance, quicker movements, and by having longer antennæ than the innocent vegetarians.
THE HEAD OF AN INSECT.
Ex.—A, gula, or throat; b, ligula, or tongue; c mandibles; d, maxillæ, or inner jaws.
Class IV.—Insecta. The distinguishing characteristics of this class are that the head, thorax, and abdomen are distinct; that they possess three pairs of jointed legs, one pair of antennæ, and, generally, two pairs of wings. The skin is hardened, and to it the muscles are attached. The eyes are usually composed of a number of facets, from fifty in the ant to many thousands in the winged insects. As the eyes are not movable, these facets enable them to see in many directions.
The several parts of the head and its appendages are shown in our illustration. The sensitive palpi, or feelers, with the delicate hair-brush tips at the ends, may also be noticed. The mouth differs in different species, and is fitted for biting and masticating, or puncturing and sucking. The adaptation seems perfect. Of all animals belonging to the articulate type, the Insecta possess the highest instincts. To this class belong the following orders: I. Neuroptera,[14] or lace-winged insects, of which the Dragon-fly, or Devil’s Darning Needle, is a good representative. II. The Orthoptera[15] (straight-winged). They have four wings, the front pair thick and narrow, overlapping along the back; the hind pair broad, net-veined and folded upon the abdomen. The representative forms are Crickets, Grasshoppers, Locusts, and Cockroaches. III. Hemiptera[16] (half winged). To this order belong the wingless Bed-bug, the Squash-bug, the Seventeen-year Locust, and the Cochineal.
Coccus-cacti[17]. The Mexican cochineal insect is of great value as a dye, and from it the most beautiful scarlet and crimson colors are obtained. The female is wingless, and, as an uncomely parasite, lives and feeds on cactus plants, especially those of the nopal[18] species, native in Mexico and Peru. The male only is represented in the diagram, and magnified somewhat larger than life. They are comparatively few in number, and of no commercial value. The plants are cultivated for the purpose, and the care of the insects, which increase very rapidly, is an industry giving employment to thousands of laborers, and in some parts of Mexico the product of the cochineal farms is among the most valuable of their exports. Cortes[19] in the sixteenth century received instructions from the Spanish court to obtain cochineal in as large quantities as possible. The export became very large, both Spaniards and others becoming skilled in the use of the beautiful dye stuff, nearly a hundred years before its real nature was known. The dried insect being very small, and crushed in preparing it for market, it was supposed to be the seed of some plant; and it was not until in 1703 that its true nature was discovered by microscopic observations. The industry still flourishes in Mexico, but both plant and the insect have been taken to other countries, and do well. The annual export of cochineal from the Canary islands has, in the present century, amounted to over 6,000 tons, valued at more than $4,000,000. The manner of collecting the insects is very simple. When of sufficient age, some already dead, and others yet alive, they are brushed off into bags, and the living killed by holding them either in boiling water or heated ovens, and then exposed in the sun till quite dry. The dried insects have the form and appearance of irregular fluted and concave grains, of which it is estimated there are 70,000 in a pound.