ANATOMY OF INSECTS- -INTERNAL 



29 



cord, which form the nervous system. On either side of each seg- 

 ment is a small opening through the body wall, called a spiracle, 

 through which air is admitted to the breathing tubes, which branch 

 to all parts of the body and form the respiratory system. The re- 

 productive organs are found in the posterior segments of the abdo- 

 men and have a separate opening just below the anus. 



The digestive system. The digestive tract, or alimentary canal, 

 consists of a more or less straight tube, occupying the larger part of 

 the center of the body and divided into parts with special functions, 

 whose development depends upon the food habits of the insect. 



Pharynx. The food, after being torn to pieces and ground up 

 by the mouth-parts, is received into the pharynx (often called the 



m 



FIG. 33. Digestive and excretory system of a grasshopper 



r, crop; g, gizzard, or proventriculus concealed by caeca; g.c, gastric caeca; /.*', large intes- 

 tine ; m, mouth ; tti.t, Malpighian tubes ; o, esophagus ; r, rectum ; s, stomach ; s.g, salivary 



glands ; s.i, small intestine 



mouth), lying within the head, and in which it is acted upon by the 

 saliva. In sucking insects the pharynx acts as a pumping organ, as 

 already described. The saliva is secreted by the salivary glands, 

 which lie along the esophagus in the thorax, whose ducts open at 

 the base of the tongue (hypopharynx). The saliva acts on starch, 

 changing it into glucose as in the vertebrates ; in some carnivorous 

 insects it acts on the proteids and is sometimes used to poison the 

 prey ; in mosquitoes the poisonous saliva prevents the coagu- 

 lation of the blood of animals, though its original function may 



