200 SUMMARY OF CUERENT RESEARCHES RELATING TO 



by the blcacliing, could not be tlie producers, at least not alone, of 

 iridescence, as in all uot-colour-changing scales the striae are exactly 

 of the same arrangement and distance, just as fine and approximate 

 as in the iridescent ones. Therefore it may be presumed that the 

 lamellee of the iridescent scales are more distant one from the other, 

 less firmly glued together, and therefore easier affected by the bleach- 

 ing fluid, and the coloured substance between the lamellae easier 

 bleached. But why are the corresponding not-iridescent scales of 

 the under side of the wing also affected, and at the same time with 

 those of the upper side ? It can only be supposed that the quick 

 effect upon the scales on one side of the wing gives easier access to the 

 scales on the other side. The author confesses that he is not entirely 

 satisfied with this explanation, but he does not know of a more satis- 

 factory one. For the first experiment the wings were cut through 

 the middle of the colour-changing part, and were therefore perhaps 

 more quickly affected. In subsequent experiments with entire wings 

 of Eti.j)loea superha the iridescence was gone in three-quarters of an 

 hour, but the wing was only less dark even in the colour-changing 

 part. In the same space of time wings of Apaiura iris and ilia, and 

 of Thecla querciis were entirely bleached, those of Lyccena Damon only 

 partly. The question whether the strise of scales with more distant 

 lamell£e will help to produce iridescence which the same kind of striae 

 of scales with not-distant lamellsB does not do, he is unable to answer. 



The colours of butterflies change mostly from purple to blue, 

 sometimes to yellow. Probably a calculation based upon the appear- 

 ance of these colours might help to solve the question. 



Interference colours are also produced by very small impressions 

 in juxtaposition. Such an arrangement is found on the feathers of 

 birds ; for instance, on the neck of jjigeons and elsewhere. In the 

 hairs of Aphrodite and Eunice this arrangement may be compared 

 with striae. Perhaps this kind of interference colours is found more 

 frequently among insects than is commonly known. At least there 

 are often parts of insects, and theii* limbs in appearance yellowish, 

 but in a certain direction changing to brown or blackish. Dr. Hagen 

 knows of no other explanation of this not uncommon fact on the legs 

 of Diptera, of Hymenoptera, and of Phryganidae. 



Natural colours are of two different kinds. (1) the pigment is 

 deposited in the form of very small nuclei in the cells, or in the 

 product of cells, in the cuticula ; (2) the pigment is a homogeneous 

 fatty substance, a kind of dye somewhat condensed. 



The first kind belongs to the cuticula, and may be called dermal 

 colours. Dr. Hagen considers them to be produced mostly by oxida- 

 tion or carbonization, in consequence of a chemical process originating 

 and accompanying the development and the transformation of insects. 

 To a certain extent the dermal colours may have been derived from 

 hypodermal colours, as the cuticula is secreted by the hypodermis, 

 and the colours may have been changed by oxidation and air-tight 

 seclusion. The cuticula is in certain cases entirely colourless. The 

 dermal colours are persistent, never becoming obliterated or changed 

 after death. 



