Page:Popular Science Monthly Volume 39.djvu/235

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NSECTS, arachnids or spiders, myriapods and crustaceans, are all included in the sub-branch of the arthropods or joint-limbs. Of the characteristics by which they are distinguished we mention here only the most salient. Insects have six legs, arachnids eight, and myriapods a more considerable number, but always short of ten thousand. The legs of crustaceans are variable in number; they have a carapace consisting of the external skeleton, which, being impregnated with carbonate and phosphate of lime, is always consistent and sometimes very hard.

The body of insects is divided into three distinct parts: the head, the thorax, and the abdomen. We may therefore define insects as articulated animals with six legs, having head, thorax, and abdomen distinct.

The life of the insect comprises four successive stages: those of the egg, the larva, the pupa, and the perfect insect. The rule, however, is subject to some interesting exceptions. The reproduction of insects usually takes place through sexual connection. The eggs are fertilized while passing through the oviduct of the female; but the females of certain bees, some of the butterflies, and several aphides, lay fertile eggs without the assistance of the male; this phenomenon is known as parthenogenesis. With some other species the females are viviparous.

The egg is composed of a firm shell, containing a limpid liquid that includes the germ of the embryo and the vitelline globules which are destined to nourish it. The eggs of insects are of the most diversified forms. The shells of many are adorned with remarkable figures. At the moment of hatching, the shell breaks, or rather opens like a hinged box-cover. A large number of the eggs look like seeds. Some are round, others cylindrical, conical, or hemispherical. Others represent solid forms, either flattened or terminating in a point. A curious study is furnished by the art with which the females deposit their eggs. Here we find single eggs; there we see them collected in considerable numbers within a parchment or silken protective envelope, which floats on the water, or is inserted in bark or attached to a stone. At other times we find a chaplet of symmetrical beads, arranged in closed rings around the branch of a tree. To deposit her eggs, the female bores with her ovipositor into the stems of plants, the tissues of animals, or the timbers of our houses. There are no old tree-trunks or cracked walls that may not serve her for a