322 



COMPARATIVE ZOOLOGY 



The "nerves" of an insect's wing consist 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 



FIG. 278. Ideal Section of a Bee: a, alimentary canal; //, dorsal vessel; t, trachea: 

 n, nervous cord. 



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, therefore, circulating 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 abdo- 

 men, it falls dead at once, being 

 suffocated. The largest spiracle is 

 usually found on the thorax, as 

 under the wing of a moth ; such 

 may be strangled by pinching the 

 thorax. 



In millepedes and centipedes, the 

 spiracles open into little sacs con- 

 nected together by tubes; in spiders and scorpions, the 



FIG. 279. Section of in- 

 jected Lung (highly mag- 

 nified) : a, a, free edges 

 of alveoli; c, c, partitions 

 between neighboring al- 

 veoli; b, small arterial 

 branch giving off capil- 

 laries to the alveoli. 



