THE VARIATION OP THE IMAGINES OF THE LEPIDOPTERA. 65 



is black, so also is it on the dark rocks of Perthshire ; in Sussex, 

 on the chalk, it is white, and the response of this moth, in ground 

 colour, to the colour of the rocks on which it rests, is very remarkable. 

 The black specimens found on peat in the New Forest, and on the 

 dark rocks of Perthshire, have a similar melanic appearance, the 

 colour evidently having been induced under such entirely different 

 environments, by a similar process of selection. But it is in the wet, 

 mountainous, and western districts of the British Islands, where the 

 rocks are blackened with moisture, and, even in summer, do not lose 

 one lot of wet until they have received another, that we find the most 

 striking cases of melanism. Thus, on the coasts of Scotland, the 

 Isle of Man and Ireland, we find black races of Agrotis lucemea, an 

 insect that is quite pale on the chalk rocks of the Isle of Wight. In 

 the Isle of Man the dark ab. mancrni of Dianthoecia caesia, quite unlike 

 the mottled Continental type, occurs. The aberrations nigra and in- 

 fuscata of Xylophasia monoglypha, an insect which rests upon the ground, 

 are found in all districts where the rocks are naturally dark, or where 

 there is a heavy rainfall. On the west coast of Ireland, melanic 

 forms of Camptogramma bilineata are found resting on the rocks, and 

 contrasting greatly with the beautiful golden specimens that hide on the 

 undersurfaces of leaves in our gardens, whilst the aberrations suffusa, 

 intermedia, oehrea and obliterae of Dianthoecia conspersa are found on 

 our northern and western coasts, and respond so perfectly to the rocks 

 upon which they rest, that the professional collectors can tell almost 

 the exact localities in various parts of the Shetlands and Hebrides, 

 from which individual specimens have come. In Shetland, again, 

 the little whitish Emmelesia albulata of our southern pastures and 

 meadows, becomes of a deep unicolorous leaden colour. 



In all these cases, moisture plays an important, if indirect, part. 

 In the first case, it brings down, in manufacturing districts, the soot 

 in the air, which, when evaporation takes place, is left behind and 

 forms a coating on the tree-trunks, fences, or rocks on which the 

 insects hide. In the second, it permanently darkens the rocks in 

 mountainous districts, and more or less so in the western areas, where 

 there is a heavy rainfall. It makes, therefore, the work of natural 

 selection in the direction of producing melanic aberrations exceedingly 

 easy. This aspect of melanism has been already worked out at con- 

 siderable length.* 



There have been occasionally general statements made to the effect 

 that insects from high latitudes are usually melanic. This is so, if only 

 the coast districts and areas with a heavy rainfall be taken into account ; 

 but if the open areas of high latitudes be considered, we find that, 

 although there is a general suffusion of markings and a tendency to ill- 

 developed pigment, due probably to the extreme conditions under which 

 development takes place, yet, as a rule, melanism is rare. Mr. Merrifield 

 has, however, shown us two cases in which temperature tends to pro- 

 duce melanic forms. These are remarkable from the fact that the 

 exposure of the pupa to a low temperature in one case, Eugonia poly- 

 chloros, produces a melanic form ; in the other, Chrysophanus phlaeas, 

 exposure of the pupa to a high temperature produces a somewhat similar 

 result. These, and parallel cases, are not difficult of explanation. 



* Tutt, Mela7iism and Melanochroism in British Lepidoptera, 1891. 



* E 



