From these necessary details let us turn to make the difficult, but important inquiry--How are the wings expanded? by what means do they, from being thick, soft, and moist, become thin, hard, and dry? It is to be regretted that this inquiry cannot be answered with that satisfactory certainty which it deserves. It is very difficult to catch the insects in which the expansion of the wings is best observed at the right moment; and even then it is extremely difficult to say whether one cause more than another may contribute to this phenomenon. The following explanation of it is given by Messrs. Kirby and Spence, whose general accuracy on such subjects is well known. "As soon as the insect is disclosed, a fluid enters the tubes or nervures, and being impelled into their minutest ramifications, necessarily expands their folds; for the nervures themselves are folded, and as they gradually extend in length with them, the moist membranes attached to them are also unfolded and extended. In proportion as this takes place, the expanding membranes approach each other, and at last, being dried by the action of the atmosphere, become one. To promote this motion of the fluid seems the object of the agitations which, in many instances, the animal from time to time gives to its unexpanded wings. That a kind of circulation, or rather an injection of an aqueous fluid into these organs, actually takes place, may be ascertained by a very simple experiment. If you clip the wings of a butterfly during the process of expansion, you will see that the nervures are not only hollow, but that, however dry and empty they may subsequently be found, they at that time actually contain such a fluid. Swammerdam, who appears to have been the first physiologist that paid attention to this subject, was of opinion, that an aëriform as well as an aquiform fluid contributes to produce the effect we are considering. He had observed that if a small portion be cut off from the wing of a bee, a fluid of the latter kind exuded from its vessels in the form of pellucid globules, becoming insensibly drops, which he concluded proved the action of the latter; and he noticed also that the wings were furnished with _tracheæ_, which were at that time distended by the injected air; whence he justly surmised, that the action of the _air_ was also of great importance to produce the expansion of the wing." But Swammerdam appears only twice to have seen the expansion of the dragon-fly, in which this phenomenon is best observed. Herold, an eminent naturalist, also attributes the expansion of the wing to the flow of an aqueous fluid into the nervures. M. Chabrier, a French entomologist, having observed a fluid in the interior of the nervures of the wings of insects, thinks it probable that they can introduce it into them and withdraw it at their pleasure, so as to facilitate their unfolding. When we call to mind the force with which the blow-fly, or flesh-fly, and the dragon-fly, are able to expand their heads by forcing air into them, we need scarcely ask for any other explanation than simply that the tracheæ are distended with air, and by that means the soft and yielding wings are made to assume their distended state.