J 5 THE INSECT WORLD. 



1-6 transverse branches ; in short, that the body of this creature is 

 traversedTn all directions by i,57^ aeriferous tubes which are visib e 

 to Ae eye by the aid of a magnifying glass, without taking into 

 account those which may be imperceptible. 



The complicated system of the breathing apparatus ^v^^'^J^ J^ ^re 

 describing is sometimes composed of an assemblage of tubes and 

 meS^rZus pouches, besides the elastic tubes which we have ^ready 

 mentioned These pouches vary in size, and are veiy elastic, 

 " panLgwhen the air enters, and contracting when it leaves them, 

 as rtiey are altogether without the species of framework formed by 

 AespU thread of the tubular trachece, of which they are only 

 enlargements. ^_^^^^^^ ^^ ^^^^^ ^^ of aspiration. 



The resDiratorv mechanism of an insect is easily understood. 

 "TlTeaSfnaP cavity," says M. Milne-Edwards, ','in which is 

 placed the greater part of the respiratory apparatus is »sceptible 

 of being contracted and dilated alternately by the play of the 

 dfferent segments of which the skeleton is composed, and which 

 a e placed fn such a manner that they can be drawn into each other 

 to a geater or less extent. When the insect contracts its body, 

 the trachea: are compressed and the air driven out. But when, 

 on the other hand, the visceral cavity as.sumes its normal size 

 or dUates" these chmmels become larger, and the air with which 

 tlev are filled being rarefied by this expansion, is no longer in 

 equUibrkim with the outer air with which it is m communication 

 Zo gl he medium of the spiracles. The exterior air is then 

 Imp^d into the interior of the respiratory tubes, and the inspiration 



'' Thefespiratory movemems can be accelerated or diminished, 

 according o the wants of the animal; in general, there are from 

 thirty o fifty to the minute. In a state of repose the spiracles 

 Ire ope° and all the trachea are free to receive air whenever 

 the yFsceral cavity is dilated, but those orifices may be closed, 

 and Te insect thus possesses the faculty of stopping all commu- 

 nkation between the respiratory apparatus and the surrounding 



''"si'e'fnsects live in the water; they "e . therefore obliged to 

 come to the surface to take the air they are in need of, or else to 

 Possess themselves of the small amount contained in the w-ater. 

 K theseTetliods of respiration exist under different forms m 



■^^"to inhalfatmospheric air, which is necessary for respiration, 



