114 COMPARATIVE ZOOLOGY. 
tube of a drop-light. Respiration 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 Insects. 
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, 
rio. 80, Tracnen SUCAathing it, is a blood-vessel. So perfect 
TubeofanInsect, ig the aération of the whole body, from 
highly magnified, : - me 
showing elastic brain to feet, the blood is oxygenized at 
Spiral thread. the moment when, and on the spot where, 
it is carbonized; only one kind of fluid is, therefore, cir- 
culating —arterial. It is difficult to drown an Insect, as 
the water can not enter the pores; but if a drop of oil be 
applied to the abdomen, it falls dead at once, being suf- 
Fra. 81.—Ideal Section of a Bee: a, alimentary canal; h, dorsal vessel; ¢, trachea ; 
nm, nervous cord, 
focated. The largest spiracle is usually fonnd on the 
thorax, as under the wing of a Moth: such may be stran- 
gled by pinching the thorax. 
In Millipedes and Centipedes, the spiracles open into 
