Page:Jardine Naturalist's library Entomology.djvu/165

Rh 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 branchiæ, which are nothing else than tracheæ 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 tracheæ, thus become fit for the support of life, acts in the same manner as in aerial insects."

Branchiæ are not known to exist in any insect