The nervures have two functions, they support and strengthen the wing, and being hollow serve to convey nutritive fluid and afterwards air to the wing.
The wings are moved by powerful muscles attached to the base of the wings close to the body and to the inside of the thorax, all the muscles being necessarily internal. "There are two sets which depress the wings; firstly a double dorsal muscle, running longitudinally upwards in the meso-thorax;[28] and, secondly, the dorso-ventral muscles of the meso- and meta-thorax,[29] which are attached to the articulations of the wings above, and to the inside of the thorax beneath. Between these lie the muscles which raise the wings and which run from the inner side of the back of the thorax to the legs."[30] When we consider the immense extent of wing as compared with the rest of the body, the small area of attachment, and the great leverage that has to be worked in moving the wings, it is clear that the area of articulation of the wing to the body is one in which the most violent movement takes place. It is here that the waste and repair of tissue must go on with greatest vigour, and we should, on our theory, expect it to be the seat of strong emphasis. Accordingly we commonly find it adorned with hairs, and in a vast number of cases the general hue is darker than that of the rest of the wing, and so far as we have been able to observe, never lighter than the body of the wing. Even in the so-called whites (Pieris) this part of the wing is dusky, and instances are numerous on [Plate IV].
The scales, which give the colour to the wings, deserve more than a passing notice. They are inserted by means of little stalks into corresponding pits in the wing-membrane, and overlap like tiles on a roof; occasionally the attachment is a ball and socket (Morphinæ), in which case it is possible the insect has the power of erecting and moving its scales. The shapes are very numerous, but as a rule they are short. To this there is a remarkable exception on the wings of the males of certain butterflies, consisting of elongated tufted prominences which appear to be connected with sense-organs. They are probably scent-glands, and thus we find, even in such minute parts as scales, a difference of function emphasized by difference of ornamentation, here showing itself in variety of forms; but, as we have said, ornamentation in form is often closely allied to ornamentation in colours. In some butterflies, indeed, these scales are aggregated into spots, as in Danais, and have a different hue from the surrounding area.
The scales are not simple structures, but consist of two or more plates, which are finely striated. The colouring matter consists of granules, placed in rows between the striæ, and may exist upon the upper surface of the upper membrane (epidermal), or the upper surface of the under or middle plate (hypodermal), or the colour may be simple diffraction colour, arising from the interference of the lightwaves by fine striæ.
Dr. Haagen, in the admirable paper before mentioned, has examined this question thoroughly, and gives the results set forth in the following table:—
The hypodermal colours are usually lighter than the epidermal, and are sometimes changed by a voluntary act. Hypodermal and epidermal colours are, of course, not peculiar to insects; and, as regards the former, it is owing to their presence that the changing hues of fishes, like the sole and plaice, and of the chameleon are due.
The great order Lepidoptera, including butterflies and moths, seems to the non-scientific mind to be composed of members which are pretty much alike, the differences being of slight importance; but this is not in reality the case, for the lepidoptera might, with some accuracy, be compared to the mammalia, with its two divisions of the placental and non-placental animals. Comparing the butterflies (Rhopalocera) to the placental mammals, we may look upon the different families as similar to the orders of the mammalia. Were we as accustomed to notice the differences of butterflies as we are to remark the various forms of familiar animals, we should no longer consider them as slight, but accord to them their true value. When in the mammalia we find animals whose toes differ in number, like the three-toed rhinoceros and the four-toed tapir, we admit the distinction to be great, even apart from other outward forms. So, too, the seal and lion, though both belonging to the carnivora, are readily recognized as distinct, but the seals may easily be confounded by the casual observer with the manatees, which belong to quite a different order.
Thus it is with the Lepidoptera, for from six-legged insects, whose pupæ lie buried beneath the soil, like most moths, we pass to the highest butterflies, whose fore-legs are atrophied, and whose pupæ hang suspended in the open air; and this by easy intermediate stages. Surely, if six-legged mammals were the rule, we should look upon four-legged ones as very distinct; and this is the case with the butterflies. It is necessary to make this clear at starting, in order that we may appreciate to its full value the changes that have taken place in the insects under study.