138 INSECT TRANSFORMATIONS. 



breathe by the mouth, and are not furnished with 

 lungs; for though good air is no less essential to their 

 nutrition and existence, it is brought to act on the di- 

 gested food in a different manner. In caterpillars, and 

 in most perfect insects, the air is respired by breath- 

 ing-tubes — usually eighteen in number — placed along 

 the sides,* the mouths of which may be seen moving, 

 as the air passes in and out, from ten to thirty times 

 in a minute. When these are covered with oil, or any 

 other matter preventing the entrance of the air, the 

 insect, being unable to breathe, is suffocated and dies, 

 as was observed two thousand years ago by Aristotle. f 

 The breathing-tubes all run into what may be called a 

 wind-pipe, one of which lies along each side of the 

 insect; and these two wind-pipes send off innumerable 

 small branches swith air to the vessels containing tlie 

 digested food, supplying it with oxygen for the pur- 

 poses of nourishment. A fluid is thus prepared ana- 

 logous to the blood of the larger animals, and stored 

 up in a large dorsal vessel; but this is not at all like 

 a heart, for though it has been observed to beat, its 

 motions do not seem to be constant or regular, and 

 no blood-vessels go oft' from it. The fluid analogous 

 to blood may perhaps pass through this singular reser- 

 voir, as water does through blotting-paper; but as yet 

 this process has not been accurately investigated. A 

 more distinct notion, however, of the process of insect 

 breathing may be obtained from Swammerdam's sketch 

 of the interior of the water-grub of a May-fly [Ephe- 

 mera). 



It is further conjectured that the portion of the blood 

 not immediately wanted for nourishing tlie organs al- 

 ready formed, goes to form a mass of thickish muci- 

 lage, contained in floating membranes of a white, yel- 



* Insect Archi., p. 303. > Aristotle, Hist. Animal., viii, 27. 



