INSECT STRUCTURES 



11 



secreted by the glandular walls of the stomach. Most 

 insects have several pouches opening into the stomach 

 near its union with the proventriculus and pouring into 

 it some of the digestive juices. After the stomach comes 

 the intestine. In some forms it is merely 

 a straight tube, but in others it is very 

 much curved and convoluted. Gene- 

 rally it is divided into more or less 

 distinct regions which are termed the 

 mid-intestine, the ileum, the colon, and 

 the rectum. Some absorption takes place 

 through the walls of the mid-intestine, 

 but this process .is started and largely 

 completed in the stomach. At the point 

 of union between the intestine and the 

 stomach arise a great number of very- 

 fine, convoluted tubes. These are the 

 kidney-tubules or Malpighian tubes. They 

 are supposed to function as kidneys and 

 to be excretory. They, with the intes- 

 tine, form the excretory system, such as 

 it is, of insects. 



12. The Circulatory System. The 

 circulatory system differs greatly from 

 that of vertebrates. There is a so-called 

 heart which consists of a long tube 

 lying just beneath the dorsal wall 

 or back of the insect. The front end of this tube 

 is called the aorta and is the only blood vessel in 

 the body. Blood is drawn into the heart from the 

 body cavity, where it fills all space not taken up by 

 the organs, through valves along the side of the heart. 

 A system of muscles causes an alternate contraction 



----- h.i 



FIG. 7. Digestive 

 System of an 

 Insect. 



ph., pharynx; oe., 

 oesophagus; s.g., sali- 

 vary gland; g.c., gastric 

 coeca; g., gizzard or 

 pro-ventriculus ; st., 

 stomach or ventric- 

 ulus; m.p., Malpighian 

 tubules; /.t. , fore- 

 intestine; m.i. , mid- 

 intestine; h.i., hind- 

 intestine; a., anus. 



