THE TEMPERATURE OF INSECTS. 291 



equalized by prolonged hunger. This points to one of those 

 curious relations between sex and alimentation, which are so 

 striking and yet so difficult to fix exactly. 



In this short review of Prof. Bachmetjew's results, it is hoped 

 that enough has been said to show that a considerable foundation 

 has been laid down for further researches ; but reference should 

 be made to the book itself, which is full of carefully tabulated 

 experiments, and most clearly expressed deductions from them. 



It is clear that in dealing with the temperature of insects we 

 have to do with a complex phenomenon dependent on a variety 

 of interacting factors, some of which we have already touched 

 upon. In the remainder of this paper I intend to consider one 

 factor which I believe will have to be taken into account if we 

 wish to gain a complete idea of the temperature relations of the 

 so-called poikilothermic animals, i. e. animals whose tempera- 

 tures vary with the surrounding medium. This factor is colour. 

 The radiant powers of differently coloured surfaces are notably 

 different ; those surfaces which absorb the long-waved colours 

 are better radiators than those which absorb chiefly those colours 

 which lie at the other end of the spectrum. The emissive and 

 absorptive radiant powers of a substance are directly propor- 

 tional — a good radiator is a good absorber ; it must also be 

 remembered that a good reflector of radiant heat is a bad 

 absorber and radiator, and vice versa. It has for long been 

 pointed out that a dark coloured animal would be able to take 

 advantage of sunshine more readily than a light coloured one, 

 and Lord Walsingham used this fact in explaining certain phe- 

 nomena of melanism in Lepidoptera. In the controversy which 

 arose on this head nothing conclusive was reached, but a certain 

 amount of evidence was brought forward to show that on the 

 whole, in regions where the sunshine was intermittent, a melanic 

 tendency in the Lepidoptera became the rule rather than the 

 exception. My chief collecting-ground for Lepidoptera abroad 

 has been Haute Savoie, in the neighbourhood of Mont Blanc, 

 and I have been struck there with the fact that the two kinds of 

 butterflies which frequented the highest mountain regions were, 

 on the one hand, the dark brown Erebias, and, on the other, the 

 white Pierids and pale Coliads. This contrast struck me for some 

 time as inexplicable on the theory that the colouration bore any 



