230 Genetical Studies in Moths 



from Poecilopsis pomonaria. Both, likewise, when scale development is 

 taking place are exposed to low temperatures, the former to the frosts 

 of a Murmansk and North Finland spring, and the latter to autumnal 

 frosts at an elevation of 800 — 1000 metres in the Alps of the Tyrol. 



Now laboratory experiments on lines imitating these climatic con- 

 ditions have been undertaken by Standfuss, Pictet, Federley, Merrifield, 

 Fischer and others. Pupae just at the critical point in scale develop- 

 ment were exposed to various low temperatures and as a result insects, 

 sometimes, but not always, melanic, were produced. Most of these 

 authors contented themselves merely with securing the aberrative forms, 

 but Federley^ tried to correlate the melanism with scale condition. He 

 discovered that the temperature employed had not only lessened the 

 actual number of scales, but, in addition, it had diminished the volume 

 of the individual scales, and this is precisely the condition seen in D. vinula 

 var. phavtoma and Poecilopsis isahellae. Let us consider the effects 

 of this ; during histolysis and histogenesis in the pupae notable changes 

 are taking place ; tissues are breaking down and reforming themselves 

 with a necessary elaboration of waste products which must be eliminated. 

 One of the media of this removal is the vestiture of scales and hairs on 

 the insect's body. Consequently, when the secretion of the haemolymph, 

 which is the mechanism of the passage, flows or is injected into the 

 newly formed scales, instead of supplying a large number of scales with 

 a normal capacity it supplies few scales with small capacity. Therefore, 

 if the secretion responds as usual to enzyme action, each scale present 

 will receive a greater quantity of the pigment so generated than usual, 

 which means that the insect would appear melanic. Melanism is not a 

 case of the genesis of new colours but is quantitative. For example, it 

 is quite well known that straw-coloured — nay almost white — Melitaea 

 athalia, M. parthenia and Brenthis euphrosyne, as well as those almost 

 black, can be captured ; yet these almost diametrically opposed colours 

 are precisely the same, only in the latter case the great quantity of the 

 brown pigment produces an outward semblance to black. 



This then serves to explain not only the natural forms of D. vinula 

 var. phantoma and P. isahellae but also some of the artificial results of 

 temperature experiments on Lymantria dispar and other species. 



From this line of argument it would seem likely that, unless 

 registered in the germ plasm, the melanism of the two natural instances 

 would yield to simple experiment in the laboratory as does that in the 



1 Federley, " Lepidopterologische Temperatur. Experimente mit besonderer Beriick- 

 sichtigung der Fliigelscbuppen," Festschrift f. Palmen, Helsingfors (1905). 



