INSECTS
not been determined. In any case, the tracheal method of respiration must be a very efficient one; for, considering the activity of insects, especially the rate at which the wing muscles act during flight, the consumption of oxygen must at times be pretty high.
The activity of insects depends very much, as every one knows, upon the temperature. We have all observed how the house flies disappear upon the first cold snap in the fall and then surprise us by showing up again when the weather turns warm, just after we have taken down the screens. All insects depend largely upon external warmth for the heat necessary to maintain cellular activity. While their movements produce heat, they have no means of conserving this heat in their bodies, as have "warm-blooded" animals. That insects radiate heat, however, is very evident from the high temperature that bees can maintain in their hives during winter by motion of the wings. All insects exhale much water vapor from their spiracles, another evidence of the production of heat in their bodies.
The solid matter thrown off from the cells in activity is discharged into the blood. These waste materials, which are mostly compounds of nitrogen in the form of salts, must then be removed from the blood, for their accumulation in the body would be injurious to the tissues. In vertebrate animals, the nitrogenous wastes are eliminated by the kidneys. Insects have a set of tubes, comparable with the kidneys in function, which open into the intestine at the junction of the latter with the stomach (Fig. 68, Mal), and which are named, after their discoverer, the Malpighian tubules. These tubes extend through the principal spaces of the body cavity, where they are looped and tangled like threads about the other organs and are continually bathed in the blood. The cells of the tube walls pick out the nitrogenous wastes from the blood and discharge them into the intestine, whence they are passed to the exterior with the undigested food refuse.
We thus see that the inside of an insect is not an unor-
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