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 mat- 

 ter, 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 sub- 

 stances 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 in- 

 ternal 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 diges- 

 tive 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 cater- 

 pillar spins its cocoon at the proper time for doing so; the 



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