OCR Text |
Show 1887.] VALUE OF COLOUR AND MARKINGS IN INSECTS. 215 Minchin has called my attention to some experiments made by M . de Niceville upon the imagos of Acrseinse &e. (Butterflies of India, Burmah, and Ceylon, vol. i. part ii. p. 318). " M . de Niceville has experimented with the carnivorous Mantis on many of the Butterflies believed to be offensive to birds, and he has found that A. viola is the ouly Butterfly which all the species of Mantis he has experimented with refuse to eat." One other consideration remains to be partially discussed before giving the list of experiments. The meaning of conspicuous coloration may be said to be clear and definite in the case of larvse, being only capable of the explanation that it is of value for protective purposes. But in the perfect forms the other explanation of colour is always possible, i. e. its use in courtship, and, as a consequence, its growth from small beginnings and its gradual perfection by sexual selection. By what criterion, then, is it possible to judge of the meaning of bright and conspicuous colours in any particular instance ? In the first place, the brilliant tints due to sexual selection alone rarely usurp the whole surface of an insect, and there are certain parts (exposed in the protective attitude of rest) in which we expect to find such brilliant tints replaced by the (usually) sober colours which harmonize with the environment. For instance, this is well known to be the case with the uppersides of the upper wings in most moths, and with the undersides of both wings in nearly all butterflies. If, therefore, such exposed parts are conspicuously coloured, strong presumptive evidence will be afforded for the explanation of the colour as belonging to the "warning class." Another test is found in the mode of flight, which may be expected to be such as will clearly display the colours no less than when the insect is at rest. The question is no doubt complicated by the two kinds of coloration running into each other. Thus Wallace has shown that the shapes and colours due to sexual selection may run riot in localities (certain islands) where enemies are largely excluded by barriers, and in the same way the brilliant colours of dangerous or nauseous insects may perhaps be sometimes explained by equal immunity, although due to other causes. But a conspicuous appearance gained in this way will be always liable to be used for an entirely different object as "warning coloration." But if such a change of use took place, we should expect some change of pattern or some new combination of colours, for this reason, that " warning colours" have one meaning which is not associated with sexual colours, i. e. the production of a conspicuous appearance and the more or less complete subordination of everything to this end. Nevertheless it is almost certain that the appearance of any species, however specialized for other ends, possesses sexual significance, and appeals as an adornment to the modified taste of the individuals concerned ; and we have a rough criterion of the extent of the modification in taste, when we compare such appearances with those which possess a sexual vulue alone, which are concealed except during flight and which are especially displayed during courtship. Prof. Meldola has recently drawn my attention to an observation of |