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, hidcntata 

 appears to be as superior to hetularia, 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-broAvn twigs with Avhite spots, although 

 producing lighter larvoe than those upon unspotted but 

 otherwise similar twigs (compare figs. 4 and 3), did not 

 lead to the appearance of white marks upon the larvae 

 (fig. 4). 



Dr. Stacey Wilson's experience led me to try the same 

 experiments with an environment of lichen in the case of 

 A. hetularia. My friend Mr. Arthur Sidgwick kindly gave 

 me a small batch of eggs in the summer of 1893, and the 

 fourteen young larvse which hatched from them were sub- 

 jected, together with hidentata, to this form of environ- 

 ment, in Experiments XII to XV. It will be seen 

 however that eleven of the resulting larvie were yellowish- 

 green, two brownish-green, and one grey mottled with 

 brown. 



The same experiments produced the larvis of hidcntata 

 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, hetularia does not seem to be 

 nearly so sensitive or so specialized to this form of environ- 

 ment as hidentata. At the same time lichen must have 

 been the cause of the hetularia larvae, 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., 1802, pp. 331, 332). It will be 

 of interest to repeat these experiments upon a much 

 larger scale, and to introduce the larvae 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 — 820), and (2) the four small experiments, 

 conducted in 1893, do prove considerable sensitiveness to 

 lichen although they did not lead to the production of 

 lichen-like larvae. 



The fortunate discovery of a company of young larv;r 



