HOW ANIMALS BREATHE. 



459 



the annelids a complex system of tubes conveys water to the tissues, 

 so in the insects all parts of the body, and the nutritive fluid, are 

 bathed by air admitted through a complex tube-system. So complete 

 is the aeration of the "whole structure, that practically the blood of an 



Fig. 17. A, lava ; B, pupa of a mosquito, d represents the water-line. (After Morse.) 



insect is wholly arterial. The necessity for this lies in the slow and 

 imperfect circulation, coupled with an unparalleled activity. 



The air-tubes, " tracheae," traverse every part of an insect's body, 

 even the brain and eye, and form the nervures or frame-work of the 

 wings, where they are sheathed by another tube conveying blood. 

 They are prevented from collapsing by 

 a mechanism, one of the most admirable 

 and exquisite found in nature. A deli- 

 cate, elastic thread is wound in a close 

 spiral between the two membranes of 

 the walls of the tubes a contrivance 

 which we have imitated in the flexible 

 gas-tube of a drop-light. 



The openings to the trachea?, termed 

 " spiracles " or " stigmata," are gener- 

 ally located on the sides, of the abdo- 

 men and thorax, a pair for each seg- 

 ment, and exhibit great variety and 

 adaptation to their purpose. So con- 

 structed as to admit air, they exclude 

 water and dust, and can be opened and 

 closed at the will of the creature. Some- 

 times they are a mere slit, like a button- 

 hole. In the soft-skinned larvae they 

 are kept open by a horny ring. The 

 aperture is sometimes protected by in- 

 terlacing hairs ; sometimes it is grated, 

 or in other species closed by a thin mem- 

 brane pierced by innumerable small pores a real sieve. A drop of oil 

 placed upon the abdomen immediately fills the spiracles and smothers 

 the creature. 



Fig. 18 Grub of Chameleon-Fly. 



