How Insects Breathe. 



139 



that they purify their blood bv means of gills, 

 which will be described later. 



As already pointed out in earlier pages, 

 the blood flows freely into all available spaces 

 of the body without being confined in arteries 

 and veins ; therefore the ordinary apparatus 

 of lungs, where alone the blood can be brought 

 into contact with the air to absorb its oxygen, 

 would not be useful. Under such an arrange- 

 ment every particle of blood would be brought 

 in turn to the air ; in the Insect it is the aii 

 that is brought to the blood, no matter in 

 what remote part of the body it may be. 

 This is effected by means of a wonderful 

 system of elastic pipes into which the spiracles 

 open. These pipes, known as trachea, con- 

 tinually branch from the mains and subdivide 

 into most minute ramifications like the veins 

 of the higher animals. In their case the tubes 

 are kept distended by the contained blood 

 being continuously pumped through them with 

 force, owing to the unceasing and rhythmical 

 activitv of the heart. The trachea of Insects 

 being open to the exterior would collapse under 

 the pressure of the internal organs but for the 

 fact that they are strengthened by a spiral 

 wire of chitin running right through them, which keeps them distended whilst 

 allowing bends and curves, and the greatest freedom of movement of the body. 



There is no part of the Insect that is not reached by these air-tubes. They 

 ■extend through the wings, to the tips of the 

 antenna, to the delicate extremities of the 

 legs, and into the muscles. Wlicrcxcr they go 

 they are bathed by the blood, and lluir texture 

 is so finc^ that the oxvgcn ol the air ]iass(>s 

 through and i> taken up h\' tlic blood. In this 

 wa\' the necessity for an arterial system for 

 circulating the blood is ob\iated. Th(^ circula- 

 tion and renewal of the air in i\\v>v lubes is 

 maintained h\- a r]i\-thmical distension and 

 contraction ol the upper and lower walls of 

 the hind-bodw wliieh is often ])ereeptible to 

 the e\(,', and can al\\'a\'s be detected b\' holding pimtoby] [w. west. 



an\- largv Insect between finger and thumb. Breathing Mouth ok W.xter-Beetle. 



f>.r , .111 i 11 .1 One of the spiracles seen in the prcct'diiig photogr.iph is 



Ul course, such a niellUXl does not rt'UeW all llie here magnified to twenty-s^-ven times the natural size, to 



, . , . , show the fringe of branching hairs by which the air is 



air at once, as happi'iis m the expan>ion and liuered before entry. 



Pholo by] 



How Insects Breathe. 



[W. WUst 



.\ few of the breathing months, or spiracles, of a water- 

 bcetl •. They are arranged along the margins of the back 

 and covered by the homy wing-covers, which hold a store 

 of air for their supply. They are here shown magnified 

 seven times larger than their actual size. 



