IX 



with it the whole corporal size of the Rhopalocera is strongly diminishing. 



Lastly there exists a fourth analogous process, namely the one which I have 

 treated of in detail in the essays mentioned before in the 2"'^, y^ and 4''' place 

 and which I have indicated with the name of evolution of colour. It is that 

 of the gradual destruction of the pigments to which the Lepidoptera owe 

 their pigmental colours and the disappearance of the scales which contain 

 them, beginning with a chemical process which causes the paling of the 

 original red pigment from blood-red into various shades of a paler red, after 

 that into orange, then into yellow and afterwards into white, while at last the 

 pigment entirely disappears and the scales become transparent ; finally they 

 fall off as useless remnants. This may be observed f. i. among the Sesiidae, 

 which in emerging from the pupa, have still such remnants of scales on the 

 wings, fastened so loosely, however, that they fall off as soon as the butterflies 

 begin to fly. The last period but one is certainly that in which the scales either 

 partly disappear or still exist, but colourless; which is probably the case with 

 the somewhat transparent spots which are fond on the wings of the 9 of the 

 European Aporia Crataegi l. In 1898 already I have mentioned this as a 

 supposition. Now it has been confirmed by the observations published by 

 B. Slevogt in n" 29 of the Entoniologisclies Wochenblatt, 1907 and in n° 14, 

 1908. We are informed that in 1907 in Courland and in the Caucasus many 

 specimens of Aporia Crataegi L. have been caught which had lost, in a 

 still much stronger degree, the scales either on the fore-wings or on all the 

 wings. It is just this irregularity which is of such a great importance, as 

 from it follows, that here the different stages of development exist, which 

 characterize every evolutional process, and that therefore an evolutional change 

 is taking place. 



During this process there often appears another similar phenomenon, the 

 cause and the course of which, though the fact itself is very distinctly visible, 

 are still very little known, viz., a strong- increase of the black colour, so that 

 scales which seem to contain a black pigment cover more or less the brighter 

 coloured ones, but only to eventually disappear again, showing once more the 

 brighter scales underneath, though in that case generally much paler, as their 

 evolution in the meantime has not stood still. The real nature of this phenomenon 

 of growing darker which is quite independent of the former, the getting paler, 

 though it often takes place at the same time on the same sides of the wings, 

 requires closer research. Is it that new scales are formed which contain a black 

 pigment or does the pigment in some scales turn into black, or does still some 

 other process take place ? This has not yet been investigated ; the fact, however, 

 that the black colour on the wings increases in the said manner and disappears 



II 



