Yet another point may be noticed. In each wing there is a space, the discoidal cell, j [Fig. 3], at the apex of which several nervures join, forming knots. These are points at which obstacles exist to the flow of the contents, and they are almost always marked by a distinct pattern. We thus have a discoidal spot in very many butterflies, in nearly all moths; and in the other orders of winged insects the decoration is even more pronounced, as any one may see who looks at our dragon-flies, wasps, bees, or even beetles.
In some insects the decoration of the body is very marked, as in our small dragon-flies, the Agrions. In one species, for example, A. Puella, the male is pale blue banded with black, and the female bronze black, with a blue band on the segment, bearing the sexual organ; the ovipositors are also separately decorated. The male generative organs are peculiar, in that the fertilizing fluid is conveyed from one segment to a reservoir at the other end of the abdomen. Both the segments bearing these organs are marked by special decoration. The peculiar arrangement of the sexual organs in dragon-flies is very variable, and certain segments are modified or suppressed in some forms, as was shown by J. W. Fuller.[32] In every case the decoration follows the modification. In the thorax of dragon-flies, too, the principal muscular bands are marked out in black lines. This distinct representation of the internal structure is beautifully shown in Æschna and Gomphina, and in the thorax of Cicada, as shown by Dr. Haagen in the paper quoted in the last chapter.
We may, then, safely pronounce that the decoration of insects is eminently structural.
Simple Variation. Cases of simple variation have been already cited in our description of spots and stripes, and it only remains to show that in this, as in all other cases, the variation is due to a modification of original structural decoration.
To take familiar examples. Newman, in his British Butterflies, figures the varieties of the very common Small Tortoiseshell (Vanessa urticæ). In the normal form there is a conspicuous white spot on the disc of the fore-wings, which is absent in the first variety, owing to the spreading of the red-brown ground colour. This variety is permanent on the Mediterranean shores. In variety two, the second black band, running from the costa across the cell, is continued across the wing. The third variety, Mr. Newman remarks, is "altogether abnormal, the form and colouring being entirely altered." Still, when we examine the insect closely, we find it is only a modification of the original form. The first striking difference is in the margin of the wings, which in the normal form is scalloped with scallop-markings, whereas, in the variety the margins are much simpler, and the border pattern closely corresponds with it, having lost its scalloping. In the fore-wing some of the black bands and spots are suppressed or extended, and the extensions end rigidly at nervules. The dark colouring of the hind-wings has spread over the whole wing. We thus see that the decoration, even in varieties called abnormal, still holds to structural lines, and is a development of pre-existing patterns.
No one can have examined large series of any species without being impressed with the modification of patterns in almost every possible way. For instance, we have reared quantities of Papilio Machaon, and find great differences, not only in the pattern, but in the colour itself. A number of pupæ from Wicken Fen, Cambridgeshire, were placed in cages, into which only coloured light could fall, and though these experiments are not sufficiently extended to allow us to form any sound conclusions as to the effect of the coloured light, we got more varieties than could be expected from a batch of pupæ from the same locality. The tone of the yellow, the quantity of red, the proportion of the yellow to the blue scales in the clouds, varied considerably, but always along the known and established lines.
The variations in the colour of Lepidoptera has been most admirably treated by Mr. J. Jenner Weir in a paper, only too short, read before the West Kent Natural History Society.[33] He divides variations into two sections, Aberrations or Heteromorphism, and constant variations or Orthopæcilism, and subdivides each into six classes, as under:—
In some cases, he remarks, variations are met with which may with equal propriety be classed in either section.
Albinism he finds to be very rare in British species, the only locality known to him being the Outer Hebrides. This reminds us of Wallace's remark upon the tendency to albinism in islands. Xanthism, he finds to be more plentiful, and quotes the common Small Heath (Cænonympha pamphilus) as an illustration. In these varieties we have simply a bleaching of the colouring matter of the wings, and therefore no departure from structural lines. Melanism arises from the spreading of large black spots or bars, or, as in Biston betularia, a white moth peppered with black, dots by the confluence of small spots; for this insect in the north is sometimes entirely black. It is singular that insects have a tendency to become melanic in northern and alpine places, and this is especially the case with white or light coloured species. (See [Plate IV., Fig. 17]) It has recently been suggested that this darkening of these delicate membranous beings in cold regions is for the purpose of absorbing heat, and this seems very probable. [34]