422 BRITISH LEPIDOPTERA. 



varieties of Smerinthus ocellatus larvae) as the demarcation 

 between a lighter dorsal and a darker lateral tint, the junction 

 being itself slightly lighter than either. Traces of the line were 

 also visible on the thoracic segments, and the whole marking 

 was especially well seen on looking at the larva from a little 

 distance so as to obtain a general impression. Again, the traces 

 of the shagreen-dots on the dorsal surface are far less modified 

 than in the other variety, and are accompanied by less altered 

 dots, which are similar to those upon the lateral and ventral 

 surfaces. Those which are modified are larger than the others 

 and have small circular reddish patches round their centres, 

 which are marked by the scar of the bristle which formerly 

 existed in this position. Below the oblique borders traces of shagreen- 

 dots are very abundant and distinct, being especially crowded 

 upon the dark ventral surface. Each is a light - coloured 

 circular patch, which is not raised above the larval surface. 

 Within each patch is the dark scar, with a minute central white 

 point, which appears to be sometimes developed into the rudiment 

 of a bristle. The prominence of these lower patches affords a 

 strong contrast to their condition in the yellow larvae. 

 Hence this strong contrast between the markings of the two 

 varieties in a dimorphic species (quite apart from their difference 

 in colour) seems to suggest that they may have originally 

 belonged to different stages in the ontogeny. Whatever be the 

 interpretation, it is quite clear that the differences are of a much 

 more strongly marked kind than those usually expressed by the term 

 ' dimorphism ' as applied to larvae." Powell writes {in litt.) : " My ex- 

 perience of the larvae, at Hyeres, is that the brown variety of the larva 

 of M. atropos only develops its peculiarities at its penultimate moult. 

 Before that it is similar to the ordinary yellow-green and purple form after 

 this moult. To begin with, it is a hybrid-looking thing, intermediate 

 between the type and the variety. The last moult gives it its very dis- 

 tinctive dark appearance with the white marks on the first few segments." 

 Weismann asserts {Studies in Theory of Descent, transl. pp. 323-324) 

 that the larvae in the fourth stadium are greener than the more purely 

 yellow ones of the fifth stadium (which, however, are also frequently 

 green), whilst some adults are of a dark brownish-grey without any 

 striking colours. He considers that, in this dimorphism, he sees the 

 commencement of a new process of adaptation, viz., that the larva is 

 becoming adapted to the soil in* and on which it conceals itself by 

 day. He explains the blue-coloured stripes as being imitations of 

 the oblique shadows of the leaf-ribs, and he says that it is "quite in 

 accordance with this explanation that we see caterpillars with the 

 brightest-coloured stripes concealing themselves in the earth by day, 

 and betaking themselves to their food plants only in the dusk of the 

 evening or dawn of morning and even during the night, i.e., in a light 

 so faint that feeble colours would produce scarcely any effect. The bright 

 blue of the larva of M. atropos, for example, would give the impression 

 of oblique shadows without any distinctive colour." Weismann further 

 adds : "The geographical distribution of the dark form indicates that, 



* There is no evidence whatever that the larva of this species conceals itself 

 either in or on the soil during the dav. 



