The feet of insects would make an interesting chapter alone, ranging from the curious, sucking, padded foot of the fly (Fig. 158) to the claws of others. The anatomy of insects is more or less complicated. The method of breathing is particularly interesting. It is very natural to imagine all animals breathing by the mouth or nostrils, but insects breathe by a singular system of air tubes (Fig 159), or tracheæ, some having lungs as well. The air tubes are wound with threads upon the inside. This preserves their shape and keeps them open. If we examine a grasshopper (Fig. 160), we shall see along the sides openings, which under a powerful microscope resemble eyelike organs (Fig. 161). These are air holes, windows, or spiracles, which lead to the air tubes, and by minute thread-lined tubes reach all over the body. To obtain air, or to breathe, the bee keeps its abdomen continually in motion, forcing air through the body, carrying oxygen to the blood tissues. [Illustration: FIG. 158.--Foot of a fly.] [Illustration: FIG. 159.--Breathing tubes of an insect.] [Illustration: FIG. 160.--Grasshopper, showing spiracles, _s_.] The insects, with some exceptions, deposit eggs, and the young pass through many strange changes, or metamorphoses, before the full-grown form is attained. The eggs of beetles hatch into larvæ (Fig. 162), which may live weeks or months or even years in the ground. The eggs of other insects, as moths, may become caterpillars, which finally spin a cocoon, as in the case of the silkworm, and from this cell-like room appears the perfect moth. These changes, so infinite in their variety, are among the most interesting features of insect life, and are never failing sources of wonder and amazement on the part of those who devote time to the study. [Illustration: FIG. 161.--Breathing hole or spiracle, highly magnified.] [Illustration: FIG. 162.--Larvæ of insects.] XIX. LOWER FORMS OF INSECTS In the previous chapter we have by the aid of illustrations glanced at the structure of insects, and noted some of the features which distinguished them from the crabs. Now we may take up some of the more important and interesting groups and observe how Nature has adapted them to their peculiar surroundings, and for the various offices they fill in the world. [Illustration: FIG. 163.--The Peripatus.]