74 The Irish NatitraUst. [ March, 



OBSERVATIONS ON THE DEVEEOPMENT OF 

 MELANISM IN CAMPTOGRAMMA BIEINEATA. 



BY WIIyl^IAM F. DE V. KANE, M.A., F.E.S. 

 [Report to the R.I. A. Flora and Fauna Committee]. 

 Many species of Eepidoptera are polymorphic, and exhibit 

 an invStability of character in the imaginal stage which appears 

 to arise from constitutional tendencies rather than immediate 

 environmental influence. Their varieties are not restricted 

 to locality, but occur in the same brood with the type, and in 

 wide distribution. Liiperina testacea and Apamea didyma may 

 be cited as typical examples of this heterogeneous polymor- 

 phism. 



There are other species, however, which, while showing 

 considerable instability of coloration and pattern in most 

 localities, apparently respond more or less directly to external 

 influences, and produce topomorphic varieties. These last 

 offer peculiar opportunities for studying the influence of 

 environment and natural selection in stereotyping aberrations 

 into local races, or eliciting new forms. The Geometer Camp- 

 togramiiia bilineata is a notable example in point. It is one 

 of the most widely spread and numerous of our common 

 species. Feeding on low plants and grasses, it is in no way 

 restricted to locality by the necessities of food supply, and 

 its constitution apparently enables it to acclimatise itself over 

 a wide distributional range, being found in North Scandinavia, 

 as well as in Syria and Siberia. In almost every British 

 locality the yellow ground-colour is variable in strength of 

 tint in different specimens, and the pattern of dark waved lines 

 is sometimes distinctly marked, btit often almost obsolete, 

 prodttcing a rather unicolorous form. Similarly the white 

 waved lines are sometimes strongly represented, and often quite 

 absent. The median band often present, especially in the 

 females, is also very variable in strength, and a well known 

 aberration occurs in which its exterior edge is darkly 

 shaded, and defined sharply externally but suffused internally. 

 The inner margin of the band in some examples is also 

 similarly shaded. This form with its various phases I shall 

 call the '* banded aberration." It occurs very widely, btit is 

 usually somewhat scarce. 



