322 THE WORLD OF LIFE CHAP. 



or spots of blue or green on the upper pair, while the lower 

 have a band of spots near the posterior margin of a brilliant 

 crimson. Among perhaps a hundred species with this 

 general style of coloration, there are a few (perhaps a 

 dozen) in which the red of the hind wings, when viewed very 

 obliquely from behind, changes into opalescent and then 

 into a curious bluish phosphorescence of intense brilliancy. 



I am informed by Dr. K. Jordan (of the Tring Zoological 

 Museum) that in these insects the black ground of the wing 

 changes also into metallic blue, which seems to spread over 

 the red and to aid in the production of the phosphorescent 

 effect. This is so marked that Mr. Bates gave to one of the 

 new species he described, the name of Papilio phosphorus. 

 One of the small Erycinidae (Euselasia prczclara) found in the 

 Upper Amazon valley, is of a yellow buff colour, with a 

 wonderful opalescent reflection which is said to be the most 

 intense and brilliant in the whole order of Lepidoptera and 

 probably the most brilliant colour known. 



All metallic reflections in the animal world are what are 

 called interference-colours, and are produced by excessively 

 fine lines or rugosities on polished surfaces, or by equally 

 thin transparent laminae. It is probable that in the remark- 

 able changing glows now described, both these causes may 

 come into play, producing, when viewed at certain angles, 

 an intensity of hue resembling those of the finest opals, 

 or sometimes imitating the most brilliant glow - worms 

 or fire-flies by means of reflected light. It seems probable 

 that these rare hues may be of a protective nature, since a 

 pursuing bird might be startled by the sudden flashing out of 

 so brilliant a light and thus allow the insect to escape ; but 

 that does not render it more likely that the infinitely 

 complex arrangements by which such structures are produced 

 and transmitted unfailingly to offspring, should have been 

 brought about for this purpose alone, when thousands of 

 other species arrive at the same end by simpler means. 



Now if there was a difficulty in the view that all the 

 wealth of colour and beauty in birds has been developed 

 solely on account of its utility to themselves, that difficulty 

 becomes greatly increased in the case of these insects. The 

 described butterflies alone are already far more numerous 



