138 INSECT TRANSFORMATIONS. 



do not 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 acton 

 the digested food in a different manner. In caterpil- 

 lars, and in most perf»ct insects, the air is respired by 

 breathing-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 

 ar.d dies, as was observed two thousand years ago by 

 Aristotle t- 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 with air to the ves- 

 sels containing the digested food, supplying it with 

 oxygen for the purposes of nourishment. A fluid is thus 

 prepared analogous 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 off from it. The fluid analo- 

 gous to blood may perhaps pass through this singu- 

 lar'reservoir, as water does through blotting-paper ; 

 but as yet this process has not been accurately inves- 

 tigated. 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 (Ephemera). 



It is further conjectured that the portion of the 

 blood not immediately wanted for nourishing the or- 

 gans already formed, goes to form a mass of thickish 

 mucilage, contained in floating membranes of a white, 



* Insect Archi. p. 308. t Aristotle, Hist. Animal., viii. 27. 



