R l 2 T Q krirrird I \oiemhrr 1988, cii:rplrd lor puhlt:n/iori I7 brhruary 1989 (htcrpillsrs of the poplar and eyed hawkmoths (Laothoe populi and Smerinlhus ocellala respc-rtively) were reared under dilleretit conditions in order ti) detcrrniric why final instar caterpillars vary in ( d o u r . Poplar Iiawkmotli caterpillars iiormally rest o n the undersides of leaves. Dull grrrn and rcdspotted caterpillars arc genetically dctrrrninrd polyrnorphisms. Caterpillars that are riot dull grren, however. can 1)ccorric white whcii lid on f':ipulus d b a or yrllow-green when fed on Salix /ra,~ilis.Experinients showed that i t is the rcflcctivr qualities of the leaves that determines which colour the caterpillar develops: if ihc young larva sccs white then i t hecomrs white, but if i t sees green, grey o r I h c k the11 i t becomes yrllow-grern.Yourig cyrd hawknioth larvar always devr1opc.d into grey-green final instar caterpillars under our rather ~)oorly-iIIunii~iat~~d rearing conditions, but when reared on wild plants in white muslin sleeves ihcy 1)cc;inic whitish-green. In this species also i t appears that colour of the final instar is determined hy the rcllcctancr of the ~ut)stratc prrc civrd by the young caterpillar.

No abstract

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 caterpillan of the eyed hawkmoth (smcrinthus ocellata) vary in colour from yellow-green when fed on Salix cinerea or S. fragilis to grey-green when fed on Salix repms 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. aha, 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.

Carotenoids and chlorophylls a and b were extracted from final instar caterpillars of the poplar hawkmoth (Laothoe populi) and the eyed hawkmoth (Smm'nihus occllafu), as well as from their food plants. Both species of caterpillar absorb the two chlorophylls and the carotenoids lutein, cis-lutein and B-carotene in the gut and deposit lutein and cis-lutein in the integument. It is the lutein, together with pterobilin, that is largely responsible for the colour of the insect: yellow-green poplar hawkmoth caterpillars have more lutein in the integument than dull green ones which in turn have more than white ones. Yellow-green and dull green caterpillars both sequester lutein and cis-lutein in the gut wall, but the yellow-greens translocate more of these pigments to the integument than the dull greens. The white caterpillars absorb very little lutein and cis-lutein into the gut, and so they have much less also in the integument. The mechanism by which the reflected light perceived by the caterpillar is translated into differential absorption of pigment by the gut and deposition in the integument is not known.