140 HOW INSECTS BREATHE. 



lected into one mass called the lungs, but are scattered 

 throughout the whole body. 



The cause for this dispersion we may probably find in the 

 imperfect circulation of the blood ; in the Mammalia, care 

 has been taken that all the blood should be subjected at very 

 short intervals to the action of the air, and though in the 

 reptiles this is not the case, yet even here a part of the blood 

 thus aerated is carefully mixed with the remainder, which 

 has not been so purified. 



But in insects the organs for the circulation of the blood 

 appear much less developed, and though very likely future 

 anatomists may discover far more than we are at present aware 

 of, yet we may perhaps conclude, that, if the blood was aerated 

 in one part only of its course, a large portion would remain 

 without this necessary purification. 



As insects never breathe through their mouth, they have 

 of course no voice properly so called, and when they make a 

 noise it is in some different manner, and is usually caused, 

 like the so-called chirping of the cricket and grasshopper, by 

 the rubbing of one part of the body against another. The 

 humming of the bumble bees, blue bottles, &c, forms how- 

 ever, to a certain extent, an exception to this rule, although 

 for this sound three causes have been suggested. 



Chabrier (Essai sur le Vol des Insectes, Ann. du Musee, 

 1820, vol. vi. p. 456), speaking of the blue bottles, observes, 

 that the metathoracic spiracles are covered by several little 

 scales, which are of the colour of the skin, and are fit to give 

 extension and continuity to the sound by their vibrations or 

 their resonance ; and if they are removed with care, the insect 

 can still fly, but can scarcely be heard. Burmeister, how- 

 ever, Manual of Entomology, p. 468, says, " If the wings 

 be cut off, the fly produces its former sound, although some- 

 what weaker .... If the scales be removed, the sound is 



