INSECTS. 239 



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, the 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 im- 

 perfection in the blood-vessels is compensated for by the 

 peculiar character of the organs of breathing (respiration). 

 These consist of a number of tubes (tracliece) 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 exceed 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 enlargement of the abdomen, and 

 thus reaches all of the tissues of the body. Since breath- 

 ing is accomplished through the spiracles 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 

 nerve-cord passes on either side of the gullet, to unite 

 in a second enlargement (ganglion) behind. Thus, as 

 will readily be understood, the alimentary canal passes 



