INSECTS. 215 



mouth-cavity open the salivary glands. In those forms 

 which eat solid food a "chewing-stomach" with hard 

 horny teeth occurs. Behind this comes the true stomach, 

 and following this the intestine, to which are attached a 

 varying number of Malpighian tubes (2-100 or more) 

 which, like the kidneys of higher forms, serve to carry away 

 nitrogenous waste from the body. 



The circulatory organs are poorly developed. A dorsal 

 tube, or heart, is present, lying above the alimentary canal, 

 and this pumps the blood forward, into an aorta of varying 

 length. Soon, however, the blood leaves this tube and 

 flows between the muscles and viscera and finds its way to 

 the hinder part of the body, where it again enters the heart 

 through openings in its sides. This imperfection in the 

 blood-vessels is compensated for by the peculiar character 

 of the organs of breathing (respiration). These consist of 

 a number of tubes (tracheae) which open to the outside by 

 paired openings (spiracles) in the sides of the body. These 

 spiracles occur in the thorax and. abdomen, and never ex- 

 ceed a pair to a somite, and from three to ten pairs may 

 occur. Internally the tracheae branch again and again, 

 until the finest twigs penetrate to every part of the body. 

 Frequently the various tracheae are connected on either side 

 of the body, and in the strong-fliers these connecting tubes 

 are enlarged into air-sacs, which thus render the body 

 lighter. Air is drawn into the tracheae by the enlarge- 

 ment of the abdomen, and thus reaches all the of tissues of the 

 body. Since breathing is accomplished through the spira- 

 cles in the sides of the body, one can see that one cannot 

 readily kill an insect by putting chloroform on its head. 



The nervous system consists of an enlargement or 

 "brain" in the head, in front of the mouth, and from 

 this nerves go to the eyes and antennae, while a stronger 



