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 with an elastic spiral thread, like 



the rubber tube of a drop-light. Respira- 



tion is performed by the 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- 

 sist of a tube within a tube: the inner one 

 is a trachea carrying air, and the outer one, 

 sheathing it, is a blood-vessel. So perfect 

 is the aeration of the whole 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, 

 h 



w 8ft _ Tracheal 

 Tube <>fau insect, 



highly magnified, 



showing elastic 

 8piral threa<L 



FIG. 81. Ideal Section of a Bee: a, alimentary canal; h, dorsal vessel; f, trachea; 

 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 largest spiracle is usually 



