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 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 wdiole 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. 
Fig. 80.—Tracheal 
Tube of an Insect, 
highly magnified, 
showing elastic 
spiral thread. 
n 
Fig. 81.—Ideal Section of a Bee: a, alimentary canal; h , dorsal vessel; t, 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 
