314 Professor E. B. Poulton on colour-relation 
unlikely, however, that under entirely normal conditions 
special detailed adjustments of this kind may be brought 
about. With regard to the sensitiveness to lichen, bidentata 
appears to be as superior to betwlaria, as it is inferior to the 
latter larva in sensitiveness to green leaves, so that the 
two species may be considered about equal in the power 
of colour adjustment. It is interesting to observe that 
dark purplish-brown twigs with white spots, although 
producing lighter larvae than those upon unspotted but 
otherwise similar twigs (compare figs. 4 and 3), did not 
lead to the appearance of white marks upon the larvee 
(fig. 4). 
Dr. Stacey Wilson’s experience led me to try the same 
experiments with an environment of lichen in the case of 
A. betularva. My friend Mr. Arthur Sidgwick kindly gave 
me a small batch of eggs in the summer of 1893, and the 
fourteen young larve which hatched from them were sub- 
jected, together with bidentata, to this form of environ- 
ment, in Experiments XII to XV. It will be seen 
however that eleven of the resulting larva were yellowish- 
green, two brownish-green, and one grey mottled with 
brown. 
The same experiments produced the larvee of bidentata 
of which typical examples are represented in Plate XVI, 
figs. 6—11. So far as any conclusion can be drawn from 
these four small experiments, ledwlaria does not seem to be 
nearly so sensitive or so specialized to this form of environ- 
ment as bidentata. At the same time lichen must have 
been the cause of the betularia larvee, with one exception, 
becoming green; for ordinary bark tends strongly to 
the production of dark forms of this species, even in 
the presence of a great preponderance of green leaves 
(Trans. Ent. Soc. Lond., 1892, pp. 331, 332). It will be 
of interest to repeat these experiments upon a much 
larger scale, and to introduce the larve immediately after 
hatching ; but it does not appear to be probable that this 
species often exhibits the kind of susceptibility to lichen 
observed by Dr. Wilson; for (1) it is remarkably sensitive 
to other surroundings almost throughout its life-history 
(see pp. 318—320), and (2) the four small experiments, 
conducted in 1893, do prove considerable sensitiveness to 
lichen although they did not lead to the production of 
hchen-like larvee. 
The fortunate discovery of a company of young larve 
