of certain Larvee are due to Pigments derived from Food. 425 
Before discussing the conclusions, it will be best to consider the 
possible effect of certain conditions incidental to the experiments. 
Darkness.—The almost continuous darkness probably affected the 
colours of the larve in (1) and (3), as they became mature. The 
adult larve of 7’. pronuba are sometimes bright green; and some of 
the larve, hatched from the batch of eggs which supplied these ex- 
periments, reared by Miss L. J. Gould in the light and with green 
surroundings, remained a distinct, although dark, green until 
maturity, quite unlike any of those referred to above. 
Although it is thus probable that the larve are sensitive, like so 
many others, to the colour and degree of illumination of their sur- 
roundings, the results of these experiments are not affected in any 
essential respect ; for there was abundant opportunity for comparison 
before the changes referred to had taken place (e.g., October 10), 
and when the majority of the larve in (1) and (3) were green (see 
Plate 3, fig. 1). Furthermore it is also evident that the comparison 
was equally valid after the change had taken place, inasmuch as the 
brown ground colour, no less than the green, is shown to be due to a 
modified plant pigment. 
Nutritive Value of Pigmentless Food.—The extremely slow growth 
of the larvee fed upon the white mid-ribs, and the death of all except 
one of them in an early stage, may be looked upon as an argument 
that they were in a pathological condition, one result being the in- 
ability to form a certain kind of pigment. Such an interpretation 
would, of course, upset the conclusions I have arrived at. 
On the other hand, it may be urged that the single larva in (2) 
which survived until it was advanced in the last stage was certainly 
not pathological, and yet was unable to form the pigments in question. 
Although it grew very slowly in early youth, it began to be con- 
spicuous by its size on November 2, and from this date it grew 
rapidly and fed largely (see Plate 4, figs. 1 and 2); judged by all 
standards, it was perfectly healthy. Furthermore, everyoue who 
breeds larvee knows that they are subject to diseases of various kinds, 
and yet, so far as | am aware, the complete inability to form certain 
classes of pigment has not been recognised as a symptom. Moreover, 
the larvee fed on the thick succulent etiolated leaves (1) grew far more 
rapidly then those fed on green leaves (38). This species hybernates 
in the larval state, and, as is usually the case in such species, the 
rate of growth is extremely irregular. Many of the larvee reared by 
Miss Gould, and single larvee fed upon green leaves by me, lagged far 
behind the others and yet remained healthy. 
I believe that the retarding effect of the mid-ribs was not due to the 
absence of plant pigments, but to the rapid drying and oxidation of 
the cut surfaces (left by the removal of the rest of the leaf) and the 
inability of the young larve to get sufficient food from other parts, 
