HOW ANIMALS BREATHE. 



115 



the blood wherever it circulates. To keep the pipes ever 

 open, and at the same time leave them flexible, they are 

 provided inside with an elastic spiral thread, 

 like the rubber tube of a drop-light. Res- 

 piration is performed by tlie movements of 

 the abdomen, as may be seen in the Bee 

 when at rest. This "air-pipe system," as 

 it may be termed, is best developed in In- 

 sects. 



The "nerves" of an Insect's wing con- „ ^ 



° Fig. 80. —Tracheal 



sist of a tube within a tube: the inner one Tube of an insect, 



1 . . 1 , highly magnified, 



IS a trachea carrying air, and the outer one, showing elastic 

 sheathing it, is a blood-vessel. So perfect 'p'™' ""'""'• 

 is the aeration of the wiiole body, from brain to feet, 

 the blood is oxygenated at the moment when, and on the 

 spot where, it is carbonized; only one kind of fluid is,. 



Fio. 81.— Ideal Section of a Bee: a, alimentary canal; h, dorsal vessel; «, tracheal 

 n, nervous cord. 



therefore, circulating^ — arterial. It is difficult to drown 

 an Insect, as the water cannot enter the pores ; but if a 

 drop of oil be applied to the abdomen, it falls dead at 

 once, being suffocated. The lai-gest spiracle is usually 



