44 
Piends which have become mimics of the orange Nymphalids, have 
really changed their colour from white to orange. The chemical 
change, Dr. Hopkins has shown, is easy. How has the change been 
biought about ? It is in this direction, among others, that we are 
waiting for information. 
Since my early papers on these subjects, I have supplemented my 
previous crude ideas by the publication of other papers_“Pupal 
development and the colour of the resulting Imago” (Ent. Rec., vol. 
iv., pp. 311—315), “Variation considered biologically” {Ent.’Iiec., 
\ol. vi., pp. 181 et set].'), etc. In these I have attempted to show how 
the physical condition of the pupa, its vitality (in excess or defect), 
and external influences affecting the pupa, may act on the colour in 
process of formation, and Prof. Weismann appears in part to have 
adopted this view, for I read ( Seasonal dimorphism of Lepidoptera 
p. 10 : Translation by W. E.Nicholson)“The cause of the seasonal 
dimorphism of Chrysophanusphlaeas is not a question of the suppression 
of one of two schemes of development, but of a modification of the 
chemical processes in the colour formation of the scale.” 
But our view of the nature of insect colours has been largely 
helped by the discovery of the course of development of a lepidopterous 
scale and its contained “ pigment-factor ” as defined by Dr. Kidim*. 
The observers, who have given special attention to scale development' 
have laid down the course of evolution as follows :—(1) Transnarent’ 
(2) White. (3) Yellow. (4) Matured colour of the scale. The white 
stage would appear to be that in which the scale is distended with 
air, and before the pigment-factor enters the scale. Dimmock (Psyche 
1883, p. 66) stated that the white scales of Pieris rapae contain air’ 
and no appreciable colouring matter. In cases like this, it would 
appear that the scale never gets beyond the second stage.. The yellow 
stage is that in which a deposit of “pigment-factor” takes place 
witlun the scale. Here, evidently, we have an explanation of the 
xanthic patches, pallid wings, etc., of Satyrids, Argynnids, Lycienids 
and numberless other lepidoptera. The arrest of the scale in the first 
stage would produce transparency, a phenomenon not at all uncommon 
m aberrations belonging to certain families. The application of these 
principles, too, coupled with the changes which we now know can be 
accomplished by rapid metabolism plus heat, must provide a broader 
groundwork for the explanation of certain of Mr. Merrifield’s tem¬ 
perature phenomena. 
The part that the wing membrane itself plays in ornament must 
not be overlooked. . Until I insisted (1892) on this point (British 
A octuae, ii., pp. iii.-iv.) it was generallyassumed that the wing membrane 
was not pigmented, but that the whole of the colour was due to the 
scales. When, therefore, some two years later, Dr. F. Gowland Hopkins 
succeeded in isolating certain pigments from the membrane, I was much 
gratified, as it was a direct proof of my previous contention. 
ihe paper of Dr. Hopkins, which 1 have here (and once previously) 
referred to, was entitled, “ The pigments of the Pieridas: a contribu¬ 
tion to the study of excretory substances which function in ornament ” 
(Loyal Society,. 1894). In this ho demonstrated that the pigment 
lactor in the wings of the Pieridae was of an excretory nature, con¬ 
taining uric acid, and that the same substance (or a derivative of it) 
