xvrir 



be more satisfactorily explained in this way. On the upperside of the cT there 

 is still darkening, as shown by the light brownish structural colour, which in 

 the 9, and on the underside of both sexes, more even in the c/ than in the $, 

 has already to a great extent become white. 



In this species also, therefore, the darkening would be diminishing, and 

 allowing the original colour, faded to white, to reappear in many places. The 

 great difference in shade of the structural colour in the various species would 

 then be explained; they would have to be considered as dependent upon the 

 condition of the pigmental colours over which they extend. 



My former opinion, expressed for instance upon p. xvi of my third 

 introduction, that the structural colours were entirely dependent upon the form 

 of the wing scales, would thus need revision. As a matter of fact, in itself 

 it is incomplete ; not only the form of the wing scales is of importance here, 

 but also the way in which they are placed on the surface of the wing. But 

 considering what a specialist as Dr. B. Walter says upon this subject in his 

 work " Oberjidchen — oder Schiller — Farben" (1895) — I do not consider my 

 present view to be impossible. I cannot however discuss this, or the general 

 question of the nature of structural colours further here. I must also, for the 

 present, leave the question of whether the influence of light plays a part in 

 this, which seems to me by no means improbable, in consideration of the 

 experiments by Poulton on the colour of the pupae of Rhopalocera, discussed 

 in the introduction to my third monograph. To be able to form a judgment 

 upon this matter, it would be necessary first to undertake a special physico — 

 entomological research, which, as far as I known, has not yet been done, and 

 which I am not able to undertake. 



In the same way an explanation can be given of the phenomenon that is 

 so striking in the Lycaena Strabo F. (PI. XXII, b^a.b). There are two 

 colour-forms in the males of this species. While there are no differences in the 

 underside of the cf, nor in the 9 , nor in the larvae or pupae, yet in the same 

 places and at the same time males are found that have a bright blue structural 

 colour on their upperside, and, simultaneously, others which are distinguished 

 under the name of Lithargyria Moore (PI. XXII, 64^) in which the blue is 

 very whitish; both shades being consistently as different as between many 

 different species. Our theory would explain this otherwise enigmatical pheno- 

 menon ; in this species also, the fading process is evident, the underside even 

 of the cf has become very light in colour ; we may assume that on the upperside 

 too of the cf this process has reached an advanced stage, in some individuals 

 having gone so far that the structural colour has also assumed a much lighter 

 shade. In the male of Arhopala Eumolphus Cram (PI. XXIV, \o2)a.b.c) the 



