Among the best-known and attractive members of the lower animal kingdom are the insects, represented by the gorgeous butterflies, the iridescent beetles, the fierce spiders, and many others. The crustaceans may almost be called the insects of the ocean, as in general appearance they closely resemble these animals; but the real insects are higher forms. [Illustration: FIG. 154.--Parts of a typical insect.] The skeleton (Fig. 154) of an insect is divided into three distinct parts instead of two. The head is distinct from the body, as in the crabs, and the skeleton, like that of the crabs, is external, and formed of a horny substance called chitin. As in the crabs, the body is made up of rings, or segments: four in the head generally, three in the thorax, and ten or eleven in the abdomen. It is interesting to glance at the various parts of insects, and later on, in reading about the many species, to note the many different purposes to which each is adapted. The mouth is a very complicated organ in the crabs, and equally so in the insects. It is generally separated into four distinct parts: the upper lip, labrum, the jaws, or mandibles, a second pair of jaws smaller than the above, and the lower lip or jaws, labium. These are formed into sucking organs in the mosquito, biting organs in the ant, and tremendous graspers in the centipede, all displaying the most remarkable variety. [Illustration: FIG. 155.--Head of the grasshopper.] [Illustration: FIG. 156.--Eyes of a fly.] The eyes of insects are wonderful organs, being both simple and compound. In the grasshopper (Fig. 155) the two are easily seen, the compound eye being the larger. The fly has a remarkable compound eye (Fig. 156), and in the center of the two eyes are three simple ones. The compound eye in the fly is made up of vast numbers of six-sided eyes crowded together, appearing under a glass like a honeycomb; yet each of these facets is a complete eye. In a sectional view of the eye of a beetle (Fig. 157) we can see the nerve that reaches every one. In using the eyes hundreds of images of the same object must reach the brain of the insect, yet the image of but one is seen. Attached to the head of insects are various sense organs, feelers, or antennæ, which are very ornamental, as in the beetles. The central portion of the skeleton bears the wings. In the beetle the wing covers are formed of hard chitin. When its wings are not in use this insect stores them away in covers provided for the purpose. [Illustration: FIG. 157.--Sectional view of the eye of a beetle.] The third or last part of the skeleton, the abdomen, often bears a weapon of defense, as a sting or a drill for boring holes in trees, or machines for making silk, as in the spiders. Here also we find a remarkable variety of tails, ranging from that of the dragon fly to the long tail of the scorpion with its dangerous sting or dagger.