XII 



this manner the different degrees of paHng which characterize the said process, 

 often, however, they are either dark or hght brown, sometimes even paling 

 into a slightly brownish white, sometimes also entirely a deep black. Finally 

 there are also caterpillars of this family whose heads are concolorous with 

 the rest of the light green body. Besides the entirely black heads, black 

 appears more or less on all the others in the form of spots, streaks or lines, 

 in whose number, size and placing is a very great difference, yet in the same 

 species mostly so constant, that it gives rise to a marked characteristic of the 

 species. Not always, however ; and in such cases a mistake between the species 

 is easily made. The black spots on the large heads give to many of these 

 larvae a peculiar appearance. 



The cause of this difference in the marking of the heads, and of the 



circumstance that this arises principally from the black that appears on them, 



cannot be ver}' doubtful to one, who regards this in the light of the said 



evolutional phenomenon. Besides the paling of the original red colour into 



white, the black colour has evidently followed the usual process of colour-evolution 



as far as the black pigment is concerned, by extending itself in different ways 



until the head had become entirely black, in which stadium, in fact, some 



species are still, and then again by decreasing; this decreasing process, however, 



progresses very unequally in the several species, so that there remain various 



streaks and spots as relics of it. While in such species in which meanwhile 



also the paling had already grown from red to white, the decrease of the black 



pigment sometimes weakened the colour of the whole head regularly into brown 



of various shades, until at last there only remained a very little dark pigment 



mixed with white. In every one of these stages some species are to be found. 



The question then arises with each species whether the black colour on the 



head is still in a state of increase or already in a state of decrase. To a 



certain degree this question can be answered from the study of the first larval 



stadia, which show the earliest phylogenetic state. If in them, black is more 



extended than is the case with the larva in its present grown up form, the 



evolutional change has evidently caused a decrease of it, in the other case, it 



has brought an increase. The phenomenon of colour-evolution, however, was 



not yet known to me when I made my observations concerning the larvae in 



question ; I did not understand the phenomenon until later days, hence I have 



made no purposed researches in this respect; entomologists coming after me, 



may devote their attention to this. Nevertheless I have here and here, because the 



phenomenon, though as yet not understood by me, drew my attention, noted 



down some observations connected with it, and I can make some use of them here. 



In the young larva of Tagiades Japetus Cram. I found the head red, 



