| OCR Text |
Show 188/.] VALUE OF COLOUR AND MARKINGS IN INSECTS. 265 Just as it was considered to be probable that warning-colours possess a sexual value for the species concerned, so it is probable that the most extreme cases of protective resemblance also have a similar significance. And in fact, when the most specialized instances of the latter kind are detected and are looked at in themselves, they are often seen to possess great beauty, which is absent from the objects they protectively resemble. To take an extreme case, the imago of Melanthia albicillata sits upon the upper-side of a leaf in the usual attitude of the Geometers, with its wings extended as if " set," and in such a position its creamy-white groundcolour and dark lines and blotches are very conspicuous, but most forcibly suggest the appearance of bird's excrement which has fallen on to a leaf from a great height, and has therefore been spread out into a large wide patch. But when the insect is detected and examined, it is seen to possess the greatest beauty. Thus Mr. Beauchamp says of it:-"The perfect insect, when bred, seems to me almost without a rival for purity and exquisite delicacy of design. I should doubt whether in the range of natural objects a more beautiful line is to be found than that exquisite cool grey streak upon the rich creamy ground of the fore wing " (Newman's ' British Moths,' p. 156). While entirely agreeing with this description, we should all maintain that it is very far from applying to the object suggested by the Moth, and which it nevertheless resembles very faithfully. And it is probable that in all cases the appearance of a sexually mature insect possesses this among its other meanings. Thus I believe that the brightly-coloured underwings of the genus Tryphana have the same significance as those of Catocala and of Sphinx and Smerinthus, and the same significance as the bright colours of the uppersides of both wing in most Butterflies, which are also concealed during rest. But in Tryphana alone among these the bright sexually selected adornment has another meaning as well, and has also come under the independent action of natural selection. For the black and yellow colours of these wings, together with the colours of the undersides of both wings, seen during their rapid vibration in flight, greatly aid the protective resemblance to a dead leaf whirled along by the wind. And yet the very similar arrangement of red and black on the upper and undersides of the underwings in Catocala are comparatively non-protective and seem to have almost purely sexual significance. If, therefore, these brilliant colours of Catocala were modified by natural selection as a response to some unusual activity on the part of its foes, if they became yellow and black instead of red and black, and the habits were correspondingly modified, we should have no reason to conclude that they had in consequence lost their sexual significance, and there is no reason for forming such a conclusion in the case of the genus Tryphana. Jenner Weir has suggested that these brightly-coloured underwings have another protective meaning-that they are conspicuous, and hence form the mark of an enemy, and yet when seized they readily give way without doing harm to the insect. Again, he |
