Camouflage and selective predation in caterpillars of the poplar and eyed hawkmoths (Laothoe populi and Smerinthus ocellata)

Final instar caterpillars of the poplar hawkmoth ( Laothoe populi ) are usually yellow-green when they have been feeding on Salix fragilis but white when on Populus alba. Similarly final instar caterpillars of the eyed hawkmoth (Smerinthus ocellata) vary in colour from yellow-green when fed on Salix cinerea or S. fragilis to grey-green when fed on Salix ripens or S. alba. Most caterpillars of these two moths are thus very cryptic to the human eye. It has already been shown that colour matching is brought about by young caterpillars changing colour depending on the colour of the substrate perceived by their eyes. Evidence is presented here for selective predation, probably by birds. Yellow-green poplar hawk caterpillars disappeared from P. alba more rapidly than did white caterpillars, but there was no significant difference in the survival of the two colour forms on S. fragilis. Yellow-green caterpillars survived better on S. fragilis than on P. alba , but there was no significant difference in the survival of white caterpillars on the two plants. Reasons why selective predation did not occur in all of the experiments are discussed.