and extends from one end to the other. The heart (h) is a tube open at both ends, and lying between the alimentary canal and the muscles of the back.
The central part of the nervous system (n) is a series of small masses of nervous matter, connected by two longitudinal chords; one of these masses, the brain, lies above the alimentary canal; the others are situated, one in each segment, between the alimentary canal and the ventral wall of the body; the two chords connecting these masses, or ganglia, pass one on each side of the oesophagus to the brain. The reproductive organs (r) lie in the cavity of the abdomen and open near the hind end of the body. The respiratory organs are omitted from this diagram for the sake of simplicity.
The respiratory system.—The most striking peculiarity in the structure of insects is the form of their organs for breathing, for they do not breathe through the mouth as we do. If an insect be carefully examined, there can be found along the sides of the body, a series of openings; these are the openings through which the air passes into the respiratory system, and are termed spiracles. The spiracles of the honey-bee are small, and are not easily found by one not trained to look for such things; but if the reader will examine the sides of one of our larger caterpillars, he will have no trouble in seeing them. Typically, there is a pair of spiracles, one on each side of the body, in each of the body-segments, but they are lacking in the head and in some of the other segments. The spiracles lead into a sys-
