Chapter VI., and need not be repeated.

The hind margin of both wings is very commonly emphasized by a border, of which _V. Antiopa_, Pl. III. Fig. 3, is a very perfect example.

The border pattern may consist of one or more rows of spots, lines, bands, or scallops;[31] and there is frequently a fine fringe, which in many cases is white, with black marks, and to which the term bead-pattern may be applied.

A definite relation subsists in most cases between the shape of the hind margin and the character of the border-pattern. The plain or simple bordered wings have plain border patterns, and the scalloped wings have scalloped borders; or rather scalloped borders are almost exclusively confined to scalloped wings. In our English butterflies, for instance, out of the 62 species:--

33 have plain margins to the wings. In all the border is plain, or wanting.

20 have the fore-wings plain, and the hind-wings scalloped, and in all the hind-wings are scalloped and the fore-wings plain, or with slightly scalloped border-patterns.

9 have scalloped margins and scalloped border-patterns.

Another relation between structure and pattern is found in those insects which have tailed hind-wings, for the tail is very frequently emphasized by a spot, often of a different colour from the rest of the wing as in the Swallow-Tails, Plates IV. and V.

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:--

_Heteromorphism._

Albinism ... ... white varieties.

Melanism ... ... black do.

Xanthism ... ... pallid do.

Sports ... ... or occasional variations not included in the above.

Gynandrochomism ... females coloured as males.

Hermaphroditism ... sexes united.

_Orthopæcilism._

Polymorphism ... variable species.

Topomorphism ... local varieties.

Atavism ... ... reversion to older forms.

Dimorphism ... ... two constant forms.

Trimorphism ... ... three do. do.

Horeomorphism ... seasonal variation.

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]

Of ordinary spots it is merely necessary to remark, that they are all cases in our favour. Thus, in _Satyrus hyperanthus_ we have "the ordinary round spots ... changed into lanceolate markings"; this takes place also in _C. davus_. The other cases of aberration do not concern us.

When, however, we come to the cases in which a species has two or more permanent forms, it is necessary to show that they are in all cases founded on structure lines. The patterns, as shown in Plate V., Figs. 1-13, are always arranged structurally, and the fact that not only are intermediate forms known, as in _Araschnia porima_, Plate V., Fig. 6, but that the various forms are convertible into one another, would in itself be sufficient to show that in these cases there is no departure from the general law. In _Grapta interrogationis_, Plate V., Figs. 8-10, we see in the central figure one large spot above the median nervure, in the left-hand form this is surmounted by another spot above the lowest sub-costal branch, and in the right-hand figure this latter spot is very indistinct. We have here a perfect gradation, and the same may be said of the colouration of the lower wings. Take again the three forms of _Papilio Ajax_ in the same plate, Figs. 11-13, and we have again only modifications of the same type.

In local varieties, as in seasonal forms, we have again nothing more than developments of a given type, as is well shown in Plates IV. & V., Figs. 13-18 & 1-13.

When, however, we come to mimetic forms, whether they mimic plants, as in Plate I., or other species, as in Plates II. & III., a difficulty does seem to arise.

The leaf butterfly (_Kallima inachus_), Plate I., offers no trouble when we view the upper surface only with its orange bands, but its under surface, so marvellously like a dead leaf that even holes and microscopic fungi are suggested, does seem very like a case in which structure lines are ignored. Take, for instance, the mark which corresponds to the mid-ribs, running from the tail to the apex of the upper wing; it does not correspond to any structure line of the insect. But if we take allied and even very different species and genera of Indian and Malayan butterflies, we shall find every possible intermediate form between this perfect mimicry and a total lack of such characters. To cite the most recent authority, the various species of the Genera Discophora, Amathusia, Zeuxidia, Thaumantis, Precis, &c., figured so accurately in Distant's Rhopalocera Malayana, will give all the steps.

In the cases of true mimicry, as in Figs. 1-3, Plates II. & III., where insects as different as sheep from cats copy one another, we find that of course structure lines are followed, though the pattern is vastly changed. The _Papilio merope_, Fig. 1, Plate II., which mimics _Danais niavius_, Fig. 3, does so by suppressing the tail appendage, changing the creamy yellow to white--a very easy change, constantly seen in our own Pieridæ--and diffusing the black. A similar case is seen in Figs. 4-5, Plate III., where a normally white butterfly (_Panopoea hirta_) mimics a normally dark one of quite a different section. Here again the change is not beyond our power of explanation. Where a Papilio like _merope_ mimics a brown species like _Danais niavius_, we have a still greater change in colour, but not in structural pattern.

If we ascribe to these insects the small dose of intelligence we believe them to possess, we can readily see how the sense of need has developed such forms.

Local varieties present no difficulty under such explanation. The paramount necessity for protection has given the Hebridran species the grey colour of the rocks, and the desert species their sandy hue.

Finally, to take the case of caterpillars, Weismann has admirably worked out the life history of many forms, and shows how the complex markings have arisen by development. Broadly, a caterpillar consists of 13 segments, the head being one. The head is often marked with darker colour, and the last segment with its clasping feet is also very frequently emphasized, as in Figs. 1 & 3, Plate VII. The spiracles are generally marked by a series of spots, and often connected by a line. Here the tendency to repetition shows itself strongly, for not only the spiracles themselves, but the corresponding points in the segments without spiracles are frequently spotted, and, moreover, these spots are frequently repeated in rows above the spiracular line. Of this, _Deilephila galii_ and _D. Euphorbiæ_, Figs. 1-5, Plate VII., are good examples.

The segmentation is also generally emphasized, as shown in all the examples on the plate, but in its simplicity in Fig. 10.

Running down the centre of the back a more or less distinct line is often seen, as shown in the figures. This corresponds with the great dorsal alimentary canal lying just below the skin, and Weismann has shown that in young larvæ this line is transparent, and the green food can be seen through the skin. We have here, perhaps, a relic of the direct colouration noticed in the transparent coelenterata.

Where larvæ possess horns either upon the head, as in _Apatura iris_ and _Papilio machaon_, or on the tail, as in many of the sphyngidæ, like Figs. 1-5, Plate VII., these appendages are always emphasized in colour. As they are frequently oblique, we often find that this obliquity is continued as a slanting spot, as in _D. galii_ and _euphorbiæ_, and sometimes repeated as a series of oblique stripes, as in Fig. 4.

It must be admitted that in insects we have strong evidence of structural decoration.

[31] In the true scallop pattern the convexity is turned towards the body of the insect. [32] J. W. Fuller on the Breathing Apparatus of Aquatic Larvæ. Proc. Bristol Nat. Soc. [33] Entomologist, vol. xvi., p. 169, 1883. [34] Nature. R. Meldola on Melanism, 1885.