Polymorphism in Hawkmoth caterpillars - an ecological and biochemical study of crypsis in Smerinthus Ocellata (L.) and Laothoe Populi (L.)

Abstract

Extensive field surveys in Lancashire and Merseyside have shown that for both Smerinthus ocellata (the eyed hawkmoth) and Laothoe populi (the poplar hawkmoth) there is a correlation between larval and foodplant coloration. For L. populi, experiments have shown that foodplant determines the colour of full grown caterpillars. Siblings reared on Salix fragilis L. (which has green leaves) became yellow-green, intermediate-green or dull-green whereas those reared on Populus alba L. became either white or green. Further experiments have shown that reflected light intensity may be the vital cue which determines whether a caterpillar becomes white or green. Thus for this species. the polymorphism is environmentally rather than genetically determined. However, the genetic background appears to have some effect on the proportion of dull-green and intermediate-green morphs in broods. For S. ocellata, both laboratory and field experiments have shown that the coloration of caterpillars is also determined by some environmental factors related to light. There is no simple genetic or nutritional control of larval colour in this species.
Pigment extraction and analysis have shown that the proportions of different carotenoids in the main foodplants are very similar, as are the chlorophyll a to b ratios and carotenoid to chlorophyll ratios. The principal carotenoid in the food of both species of caterpillar, lutein, is sequestered by the insects in the integument and contributes to the animal's coloration. Cis-lutein is also present in small quantities in the integuments of both L. populi and S. ocellata caterpillars. Yellow-green L. populi larvae contain more lutein in the integument than dull-green morphs and in white caterpillars this carotenoid is barely
detectable.
Field predation experiments indicated that white caterpillars of L. populi are at a selective advantage compared with yellow-green morphs on the white undersurface of P. alba leaves. However, both white and green morphs suffered similar predation on S. fragilis bushes.