ORGANS OF BREATHING. 71 



Fresh air, however, is as essential to insects as to 

 other animals, though it enters their bodies in a dif- 

 ferent manner ; since, instead of one, two, or three 

 opening's for breathing, there are usually eighteen, 

 generally very obvious in the larger caterpillars ; their 

 function being demonstrable by stopping them up 

 with oil or grease, when the animal is soon suffocated 

 and dies. 



These openings are termed spiracles ( ! ), and are of 

 two sorts, simple and composite. 



A spiracle magnified to show the lips open to inhale the air in 

 breathing. 



The simple spiracles ( 2 ) are usually situated on the 

 sides of the abdomen, a pair (one on the right and 

 one on the left) to each ring, at the junction between 

 the back arch and the belly arch. Like the nostrils 

 in man and other animals, these spiracles are often 

 beset with hairs, crossing and meeting, for the purpose 

 of preventing the entrance of what might prove inju- 

 rious, while the air may pass pure. 



The position assigned them, as well as their number, 

 varies considerably, to suit the habits of the insect. 

 In the maggots of some flies, for instance, which feed 

 on greasy substances, or live in water, there is only 

 one spiracle at the end of the last ring which can be 

 stretched out like a telescope into the air, while the 

 body is enveloped. 



The second and third rings of the body of caterpil- 

 lars answering to the mid and hind corselet, having 

 no spiracles in caterpillars, and the exact parts where 



(1) In Latin, Spiracula, improperly Stigmata. 

 (2) In Latin, Spiracula simplicia. 



