128 THE CANADIAN ENTOMOLOGIST. 



MELANISM AND HUMIDITY. 



BY J. W. TUTT, F. E. S., WESTCOMBE HILL, LONDON, ENGLAND.* 



At different times considerable attention has been paid to the general 

 darkening in colour of our British insects, compared with Central 

 European and with American forms. In Britain, many species become 

 much darker (some absolutely black) on various parts of the West Coasts 

 of Ireland and Great Britain, and, as a general rule, the more humid 

 districts produce the darker specimens, c. g., Acronycta ( Viminia) 

 euphorbice var. tnyricce, Xylophasia polyodoii vars. infiiscata and nigra, 

 Vimiiiia rumicis var. salicis, etc. We find, moreover, that certain 

 geological strata are more prone to produce dark varieties than others, 

 e. g. Guop/ios obscurata on peat, etc., becomes quite black ; and in manu- 

 facturing districts, where the surfaces of fences, trees, etc., get much 

 darker than is normally the case, insects which rest on them also become 

 darker, to wit., Amphidasys betularia var. doubledayaria, Hybernia 

 margitiaria (progemmaria) var. fuscata, Eupithecia rectangulata var. 

 nigrosericeata and Boarmia repandata var. fiigra, (a magnificent form 

 from Huddersfield, in which the whole area of the wings is intensely 

 black). In excessively moist districts, those insects which rest on the 

 ground, trees, rocks, etc., are those which are chiefly affected, because 

 here, the ground, trees, rocks, etc., becoming permanently darkened by 

 rain (vide "Entomologist's Record," Vol. I., pp. 123, 124), the darker 

 specimens are thus preserved by " natural selection." Where the 

 geological stratum is naturally dark in colour. " natural selection " acts 

 much in the same way. In manufacturing districts the atmosphere is 

 polluted with carbon particles, and when the rain falls the impurities are 

 brought down with it, but when the water evaporates the solid matters 

 are left behind, and thus surfaces of trees, etc., are artificially darkened. 

 There is no doubt that the great agent in effecting the darkening of 

 insects, which rest in such places, is " natural selection," aided, of course, 

 by the tendency that the surfaces of certain objects have to become darker 

 when continuously wet or damp. The intimate connection between 

 humidity and melanism is well illustrated by the fact that at high altitudes 

 (where the humidity becomes greater) melanism again shows itself, as in 



* Editor of tlie "Entomologist's Record and Journal of Variation." 



