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 becomo 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 * 
