WAYS AND MEANS OF LIVING
reason, the head of an insect may be cut off and the rest of the creature may still be able to walk and to do various other things until it dies of starvation. Similarly, with some species, the abdomen may be severed and the insect will still eat, though the food runs out of the cut end of the alimentary canal. The detached abdomen may lay eggs, if properly stimulated. Though the insect thus appears to be largely a creature of automatic regulations, acts are not initiated without the brain, and full coordination of the functions is possible only when the entire nervous system is intact.
The active elements of the nerve centers are nerve cells; the nerve fibers are merely conducting threads extended from the cells. If the nerve force that stimulates the other kinds of cells into activity comes from nerve cells, the question then arises as to whence comes the primary stimulus that activates the nerve cells. We must discard the old idea that nerve cells act automatically; being matter, they are subject to the laws of matter—they are inert until compelled to act. The stimulus of the nerve cells comes from something outside of them, either from the environmental forces of the external world or from substances formed by other cells within the body.
Nothing is known definitely of the internal stimuli of insects, but there can be no doubt that substances are formed by the physiological activities of the insect tissues, similar to the hormones, or secretions of the ductless glands of other animals, that control action in other organs either directly or through the nervous system. Thus, some internal condition must prompt the insect to feed when its stomach is empty, and the entrance of food into its pharynx must stimulate the alimentary glands to prepare the digestive juices. Probably a secretion from the reproductive organs of the female, when the eggs are ripe in the ovaries, gives the stimulus for mating, and later sets into motion the reflexes that govern the laying of the eggs. The caterpillar spins its cocoon at the proper time for doing so; the
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