of certain Larvce are due to Pigments derived from Food. 425 



Before discussing the conclusions, ib 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 larvae in (1) and (3), as they became mature. The 

 adult larvae of T. pronuba are sometimes bright green ; and some of 

 the larvae, 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 larvae 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 larvae 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 larvae 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, everyone who 

 breeds larvse knows that they are subject to diseases of various kinds, 

 and yet, so far as I am aware, the complete inability to form certain 

 classes of pigment has not been recognised as a symptom. Moreover, 

 the larvae fed on the thick succulent etiolated leaves (1) grew far more 

 rapidly then those fed on green leaves (3). 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 larvoe reared by 

 Miss Gould, and single larvae 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 larvae to get sufficient food from other parts, 



