the colours of certain Lepidoptcra. 



33S 



is far keener than to green, and this corresponds with 

 the fact that the dark hirvse are much more perfectly 

 concealed than the green. Although the proportion of 

 hrown to green in the surroundings may be very small, 

 it is still to the advantage of the average larva of this 

 species to resemble the former, and the average larva 

 does so. 



The green larvae are of a yellower shade than that 

 which appears when they are fed upon Popnlus nujra. 

 This corresponds to the difference between the leaves 

 themselves. 



D. Dark Surroundings near the larva, but not 



ACTUALLY IN CONTACT, AND COMPxVRISON EXPERIMENT 

 WITH GREEN ALONE. 



The details of the experiment are described below : — 



VI. 



On green leaves alone, with dark 

 twigs outside cylinder. 



July 28. — 21 larva\ previously 

 surrounded by green leaves alone, 

 were carefully divided into two lots 

 as much alike as possible ; when 

 any diii'erence was unavoidable, the 

 darker larvre were put in VI., the 

 greener here, in a small cylinder 

 containing green leaves alone, but 

 surrounded by a large cylinder with 

 dark twigs packed between the two. 



Aug. 2. — 1 is certainly brown, 

 though not a very dark one; the 

 rest green. 



Aug. 13. — The dark twigs were 

 absent Aug. — 12. The first be- 

 came mature. The single larva still 

 remained brown ; all others green. 



►Sept. 2. — The dark larva was 

 nearly mature (quite so Hept. (i), 

 and was a brownish intermediate 

 larva. All the others remained 

 bright green, and matured at a 

 rather earlier date. 



VII. 



On green leaves alone, for 

 comparison with VI. 



July 23.— The 21 larvte divided at 

 this date between VI. and VII. were 

 small and nearly all greenish, ex- 

 cept one, which was large and green. 

 The latter was placed here with 10 

 of the small ones, on the wliole 

 sUgltili/ darker than those in VI. 



Aug. 2. — All green, or evidently 

 rapidly becoming so. 



Aug. 4.-6 are still in last stage 

 but one, 4 in last stage, 1 changing 

 last skin ; all bright green, except 

 tlie smallest, which is changing in 

 that direction. 



Aug. 13. — The first became ma- 

 ture. From this date onwards the 

 larvn3 gradually sought pupation, 

 all being bright green. 



