THE CANADIAN ENTOMOLOGIST. 161 



NOTES ON THE CAUSE OF THE BLUE COLORATION 

 OF THE BLUE LYC^NIDS. 



BY H. M. SIMMS, MONTREAL. 



The question has recently been raised by certain entoftiologists 

 whether the blue tint of the blue species of the genus Lycaena and 

 its allied genera is actually due to blue pigment or dye in the scales 

 of the upper surface of the wings, or whether it is due to a kind of 

 "construction" similar perhaps to the apparent construction of a 

 green tint on the under surface of the wings of certain species of 

 the genus Euchloe. The latter, as is well known, on examination 

 by a microscope, is found to be caused not by a field of green scales, 

 but by one of black and yellowish scales intimately mixed in 

 approximately equal quantities. I do not think that it has ever 

 been suggested that the blue colour of the Lycaenids is produced 

 by an exactly similar mixing of scales of two distinct colours, but 

 it has been suggested that the blue tint is due to the superposition 

 of a layer of white, practically transparent scales over a layer of 

 dark-brown or blackish scales, and that the blue effect is in reality 

 either an illusion as in the case of the Euchloid green tints, or else 

 a phenomenon of interference of light, analogous, perhaps, to the 

 production of the rainbow tints between two reflecting surfaces 

 very close together and separated by a transparent medium, these 

 being known as "Newton's Rings." They are familiar to all in 

 the bright colours visible on the surface of soap bubbles as well 

 as the bright tints seen on the surface of stagnant water, or on the 

 fine film of oil covering the surface of oily water and at times in 

 puddles of automobile oil on roads. Now it can, I think, be clearly 

 shown that this latter suggestion is not the correct explanation of 

 the present case. The colours of "Newton's Rings" depend upon 

 the angle from which they are viewed, as well as upon the distance 

 between the surfaces. If one looks at stagnant water from different 

 angles, it will be seen that the colour at any one j-point varies 

 according to the angle, and if the blue colour were due -.^ ^ny such 

 cause as those which produce "Newton's Rings," the colour should 

 pass right through the spectrum from red to violet or vice versa, as 

 the angle of sight was shifted. But not only is this not the case, 

 but if the wing of a blue butterfly is placed between two strips of 

 glass and subjected to pressure, thus changing the distance between 

 the two surfaces of any one scale and also between the two layers 



May, 1915 



