The muscles of insects are usually whitish. Sometimes I have noticed quite a pinkish hue about the muscles of the thorax. They vary in form and position to accord with their use. The mechanism of contraction is the same as in higher animals. The ultimate fibers of the voluntary muscles, when highly magnified, show the striæ or cross-lines the same as do the voluntary muscles of vertebrates, and are very beautiful as microscopic objects. The separate muscles are not bound together by a membrane as in higher animals. In insects the muscles are widely distributed, though, as we should expect, they are concentrated in the thorax and head. In insects of swiftest flight, like the bee, the thorax (Fig. 7, _a, a, a_) is almost entirely composed of muscles; the œsophagus, which carries the food to the stomach, being very small. At the base of the jaws, too, the muscles are large and firm. The number of muscles is astounding. Lyonnet counted over 3,000 in a single caterpillar, nearly eight times as many as are found in the human body. The strength, too, of insects is prodigious. There must be quality in muscles, for muscles as large as those of the elephant, and as strong as those of the flea, would not need the fulcrum which the old philosopher demanded, in order to move the world. Fleas have been made to draw miniature cannon, chains, and even wagons many hundred times heavier than themselves. The nerves of insects are in no wise peculiar so far as known, except in position. As in our bodies, some are knotted or have ganglia, and some are not. The main nervous cord runs along the under or ventral side of the body (Fig. 8), separates near the head, and after passing around the œsophagus, enlarges to form the largest of the ganglia, which serves as a brain. The minute nerves extend everywhere, and in squeezing out the viscera of an insect are easily visible. The organs of circulation in insects are quite insignificant. The heart is a long tube situated along the back, and receives the blood at valvular openings along its sides which only permit the fluid to pass in, when by contraction it is forced towards the head and emptied into the general cavity. Thus the heart only serves to keep the blood in motion. According to the best authorities, there are no special vessels to carry the blood to various organs. Nor are they necessary, as this nutritive fluid everywhere bathes the alimentary canal, and thus easily receives nutriment, or gives waste by osmosis, everywhere surrounds the tracheæ or air-tubes--the insect's lungs--and thus receives that most needful of all food, oxygen, and gives the baneful carbonic acid, everywhere touches the various organs, and gives and takes as the vital operations of the animal require.