42 THE SIX-FOOTED INSECTS 



The abdomen or tliird region of the body is composed of 

 about nine or ten visible segments that do not bear any 

 segmented appendages, but the terminal segments are 

 modified to form the external reproductive organs. The first 

 seven or eight segments have on either side small openings 

 into the respiratory system, the spiracles, and there are also 

 usually two spiracles located on the thorax. They are so 

 small as to be scarcely visible without magnification. They 

 are connected internally with the delicate respiratory tubes, 

 the trachea, which extend throughout the body, so that the 

 air is distributed to all of the tissues and the respiratory 

 process is consequently carried on in all parts of the body. 

 The minuteness of the pores, as has been already mentioned, 

 makes it possible for the insect to be suffocated by a very 

 small amount of oily material spread over the pores, closing 

 them. 



Some further details of structure will be given in connec- 

 tion with the general characters for each order. 



The internal structure of insects may seem at first sight 

 to be of little relation to economic problems, but if it is 

 recognized that their modes of feeding and the character 

 of food depends upon the digestive organs, and their mode 

 of respiration is very directly connected with certain impor- 

 tant modes of treatment, and that all of their special senses 

 associated with the attraction to certain kinds of plants, 

 the attraction or repulsion to light or to odors, and in fact 

 that the activities of the insect, as a whole, are dependent 

 upon the organization, it can be realized that these structures 

 have a very direct relation to their injuries and to methods 

 of control. 



The fligestive system of the insects is in general like that 

 of all arthropods, the mouth opening being connected with 

 a pharynx, the esophagus merging with the crop, commonly 

 the first part of the stomach (proventriculus), in which the 

 food is received and undergoes some digestive changes; 

 following this the true stomach around which are a number 

 of gastric ceca that secrete the gastric fluids. Following 

 the stomach is the intestine, divided into the ileum, colon, 



