Because they are merely elongations of the skin. It is otherwise with scales. Some of these are inserted into the skin at one end and left free at the other, and in some insects are so feebly connected, as to fall off by touching them with the finger. These scales, in the butterfly, bear a remote resemblance to feathers in their form.
Because the thighs of their hind legs are of uncommon size, to give room to the requisite number of muscles.
Because the muscles which move their wings take their rise in the breast, and are capable of executing their functions with great celerity.
The flying insects do not possess rapidity of flight proportional to the number and size of their wings. In the coleoptera, the body hangs down during flight, while in other classes it preserves nearly a horizontal position.
The wings are composed of two membranes, an upper one, in which nervures or cords are traced; a lower one, separable from the upper. These nervures or cords contain a spiral vessel, " whence they appear," says Kirby, " to be air vessels communicating with the trachea in the trunk. The expansion of the wing at the will of the insect is a problem that can only be solved, by supposing that a subtle fluid is introduced into these vessels, which seems perfectly analogous to those in the wings of birds; and that thus an impulse is communicated to every part of the organ, sufficient to keep it in proper tension: we see by this, that a wing is supported in its flight like a sail by its cordage."
Because they, in a great measure, furnish the characters employed in classification. Thus, the presence or absence of wings - their number and appendices - their texture and consistence, together with their size, position, and manner of folding up, yield marks which are easy of detection, and which experience has found to be perfect. - Fleming.
Because they serve as a covering to the inferior ones. Strictly speaking, these elytra are not wings, since they perform no other motion than elevation and depression, and serve merely to protect the wings when at rest, not to assist them when flying.
Because the skin is probably smeared with some unctuous matter; comparative anatomy hitherto having failed in detecting any glands subservient to the functions of the skin. In some instances, indeed, the skin resists being wet, even after the death of the animal has taken place for some time, but previous to becoming dry. - Fleming.
Dr. Arnott, physically attributes it to the weight of the insects not being sufficient to overcome the cohesion of the particles of water among themselves.
In the tribes which swim, the legs are either flattened like the blade of an oar, or produced and ciliated (fringed) on the edges. Some swim upon their back, others upon their belly. Some keep always floating upon the surface, others dive and perform their movements at various depths, regulated by the condition of the organs of respiration. - Fleming.