68 THE ENTOMOLOGIST. 



ON MELANISM. 

 By T. D. a. Cockerell. 



Mr. Dobree's paper on this subject (Entom. 25) is certainly 

 most interesting and suggestive, and tlie facts he brings forward 

 seem to me extremely significant, though the deductions he 

 draws from them are perhaps open to question. Fi'om the 

 presence of melanic forms in mountain regions, and in the west 

 of Ireland and Scotland, it seems only natural to suppose that 

 the peculiar features of these regions are responsible for the 

 variation; and of all causes that seem probable from this point 

 of view nothing comes more prominently before us than the 

 extreme mistiness and dampness of the atmosphere. The view 

 that melanism is due to cold has been prominently set forth on 

 many occasions, and, indeed, has in its favour no small share of 

 evidence ; yet, as Mr. Dobree has shown, melanism does not 

 occur in the cold and dry districts of Russia, Siberia and 

 Canada, and does, on the contrary, occur in the much warmer and 

 damper region of Western Ireland. 



To further illustrate this argument, I will take a few examples 

 which, though not of insect species, have no small bearing upon 

 the question. It has been demonstrated in the case of Limax 

 arhorum, a species of slug, Scottish specimens are much darker 

 in colour than English,* and that examples from elevated 

 situations both in Italy and in Ireland are entirely suffused and 

 black in colour, while those from the lowlands are pale gre.y, 

 spotted or striped. f Another variable slug, Avion ate?', occurs in 

 dry situations of a brick-red or brown colour, while specimens 

 from damp and marshy spots are almost invariably pitchy black. 

 In both these cases I believe the darkening of colour to be 

 caused, perhaps partly by cold, but certainly in great measure by 

 the humidity of the atmosphere. 



Mountain-regions are often enveloped in mist, and here it is 

 that a darkening of colour occurs, as in well-known alpine forms 

 of many Lepidoptera, found constantly in similar situations, as 

 well as many more isolated cases, such as the dark variety of 

 Acidalia contiguaria found near Bettws-y-coed.+ It seems un- 



* Eoebuck, ' Journ. Conch.' 1885, p. 276. + ' Zoologist,' 1886, p. 341. 



+ ' Entom.' 1879. p. 67. 



