Page:The Zoologist, 4th series, vol 6 (1902).djvu/346

292 relation to the temperature, but a little consideration showed that the white wings of the Pierids might act as reflectors of heat, glancing off the sun's rays on to the black body of the insect, which would thus absorb a greater quantity of heat. I have since tried many experiments in order to test this hypothesis; the bulb of a sensitive thermometer is tied round with black cloth, and hung up in bright sunshine. This morning a thermometer so prepared registered a temperature which varied between 30° and 31° C. I then backed the thermometer with a sheet of white paper folded so as to imitate the position of a butterfly's wings when expanded upon a flower. In three minutes the temperature had risen to 35° C, and was still rising when I removed the paper; the temperature immediately dropped. I repeated the experiment, substituting the cups of variously coloured flowers—such as poppies, Canterbury-bells, and so forth—to take the place of the white paper, and I obtained rises of temperature through two or three degrees, according to the reflecting powers of the various colours; the worst reflector being a dark purple larkspur, and the best a bright red poppy, which increased the temperature from 30·3° to 33·5° C. in a few minutes.

I surmise therefore that the influence of colour on the temperature of Lepidoptera is not so simple as it is usually assumed to be; on the one hand, the wings may absorb heat directly; on the other, they may be used as reflectors. It has been urged that the absorption of heat into the wings is a useless proceeding, since they are largely composed of dead structures; but it must be remembered that hæmolymph is present between the lamellæ of the wings, and I conceive that a circulation of this hæmolymph occurs from the body to the wings, and vice versâ, owing to the movements of the wings and body.

Butterflies and flowering plants afford us, both in variety and brilliance, the greater part of the great boon of colour in animated Nature; both of these orders of beings are in general dependent for the fulfilment of their vital functions on warmth and sunshine. The dark centre of the poppy, where the sexual products are matured, is encircled by a broad open tent of crimson, which flashes from its walls the most potent of the sun's rays. If we are to compute the circumstances favourable to the certain and