MELANISM AND MELANOCHROISM, 33 



exhibit a decidedly lighter deviation of colours from the normal 

 types. 



" By way of adding a few facts in support of these remarks, 

 I may state that even in the most favourable seasons, lepidop- 

 tera collected on the plains here are invariably paler than 

 individuals of the same species occurring on the lower hills or 

 in wooded districts near the Ranges. The rainfall is in all 

 seasons much greater on and around the base of the Ran^-es 

 than out on the open plains. The higher we ascend the Alps 

 the more humidity we meet with, and the greater the darken- 

 ing of the lepidoptera, until we reach the summit, when they 

 become perfectly black. Percnodaiuion pinto, Erebiola butleri 

 and occasionally Ltathmonyma heciori are cases in point, and 

 doubtless many more species that yet remain to be discovered. 

 Most entomologists will naturally suppose that all mountain 

 forms of lepidoptera are endowed with a more hardy constitu- 

 tion than other or allied species occurring at lower altitudes ; 

 but the cause of melanism at high altitudes is not so much a 

 question of cold or heat as its great advantage over other 

 colours in a wet or humid region. Doubtless Lord Walsing- 

 ham's theory of the greater absorption of heat by dark- 

 coloured lepidoptera would occasionally be advantageous to 

 certain melanic forms as well as in a protective sense, but as 

 Mr. Tutt has shown {Ent. Record, vol. i., 232), it leaves the 

 origin of melanism wholly unexplained. Again, we know that 

 melanism is prevalent in some seasons that are at the same 

 time both mild and moist. A certain degree of moisture is 

 necessary in the egg, larval, and pupal stages to perpetuate the 

 typical colours of a species, and we know it is only in exceptional 

 seasons or under exceptional conditions that lepidoptera vary 

 most. Still, we must allow that in hot, dry seasons, the colours 

 of many species of lepidoptera are rendered paler, or obsolete, by 

 the sun's absorption, or bleaching of the colours affected ; but we 

 are not so much interested in this question at present as we are 

 in the origin and causes of melanism. One remarkable instance 

 which would appear to oppose Lord Walsingham's theory 

 occurs in the habits of Nyctemera aiinidata, a jet-black diurnal 

 moth with two white marks on each forewing and one on the 

 hindwing. The moth in a natural state, or when bred, almost 

 invariably emerges from the cocoon and flies about on dull and 

 drizzly days. It may sometimes be seen soaring in the cool 

 early morning, but at all times it shuns the hot sunshine. It 

 would therefore appear that the theory of absorption of heat 



c 



