OF INSECTS. 159 



of a different nature or density, two contrary currents 

 are established through the sides of the bladder, 

 the one conveying the liquid from without into the 

 latter, and the other having an opposite effect. Gases, 

 besides, have this peculiarity, that if we enclose a 

 mixture, in certain proportions, of oxygen, carbonic 

 acid, and azote, and plunge the bladder containing 

 it in water having air in solution, the two currents 

 established in the manner mentioned, continue till 

 there remain in the bladder only oxygen and azote 

 in the proportions which constitute atmospheric air. 



"This double phenomenon takes place equally 

 well through a living organic tissue and an apparatus 

 employed for experiment. Accordingly, it is easy 

 to perceive, that if some of the air-tubes of an insect 

 were to become external to its body, and floating in 

 the water, the carbonic acid which they contain 

 after the blood has been decarbonised, will escape 

 through their walls, and be replaced by the oxygen 

 of the air which is mingled with the water. This, 

 in fact, is exactly what takes place with branchiae, 

 which are nothing else than tracheae closed at the 

 extremity, and contained in a membrane remarkably 

 permeable. These tubes extract the oxygen from 

 the water, rejecting at the same time the carbonic acid 

 they contain; and the air enclosed in the interior 

 tracheae, thus become fit for the support of life, acts 

 in the same manner as in aerial insects."* 



Branchiae are not known to exist in any insect 



* Lacord. Intro, a 1' Ent. II. 91. 



