G. T, PORRITT : PRESIDENTIAL ADDRESS. IO3 



maritima, a pale green plant, with stripes of a whitish powder- 

 ing-, and bearing- pink flowers. In Aug-ust, i88g, I beat out a 

 considerable number of these larvae, and the colour and marking- 

 were so exactly similar to the leaves and stems of the Arie?nisia 

 maritima that it would probably have been almost impossible to 

 have detected them by searching-. But I found further, that a 

 g-ood number of the full grown larvae, besides being-of the colour 

 of the plant, were very prettily marked with pink, precisely 

 assimilating- to the flowers of the Artemisia, which was also just 

 g-etting- well into bloom. A still more curious fact is to be 

 related. From the moths reared from these larvse I obtained 

 two further g-enerations, consisting- of many hundreds of the 

 larvae, but of course in Huddersfield was unable to feed them on 

 their natural food, except by sending- to the coast for it, which 

 I had done with the original lot of larvae collected. Experiment- 

 ally, Artemisia vulgaris and A. abrotamim, the latter the 

 familiar ' Southernwood ' of our g-ardens were tried, and 

 althoug-h the former was almost refused, they took to the 

 southernwood with relish, feeding- up in every way satisfactorily. 

 But what did I find ? That the larvae all assimilated at once in 

 colour to the difl^erent tint of g-reen which A. abrotamim 

 possesses as compared with A. vtaritima ; and still more 

 extraordinary, that not the slig-htest trace of the pink ever 

 appeared on a single larvae, in complete harmony with the fact 

 that southernwood does not flower in West Yorkshire. This 

 experiment has been absolutely verified bj^ everyone who has 

 since tested it. 



But perhaps it is the tree-feeding- species which afford our 

 finest illustrations. Who is there among- us who has not at 

 some time attempted to break off an apparently dead twig- from 

 a garden rose bush, and on grasping it with the fingers has as 

 suddenly ' let go ' on finding it the soft wriggling caterpillar of 

 Odontopera hidentata? The resemblance was perfect, and so it 

 is with scores of species. Professor Poulton and others have 

 made numerous experiments with the larvae of this group, and 

 the more one experiments, the more fascinating the study 

 becomes. One of Professor Poulton's first subjects consisted of 

 larvce of the common brimstone moth, Rumia cratcegata, a 

 familiar object, probably with everyone in this room, who has 

 taken evening walks along hawthorn hedges in early summer. 

 The caterpillars of this month precisely resemble in appearance 

 the short gnarled twigs of the hawthorn, but they are of two 

 distinct varieties in colour, one very dark, almost black, the 

 other green. In common with nearly all the species which 

 affect mimicry, these larvae feed chiefly at night, when colovir 

 apparently is of no advantage, and Professor Poulton wished to 

 find out why some should be green and others black. By 

 constant and close observation he found that the dark larvae 

 almost invariably rested on the dark stems of the hawthorn, 

 whilst the green larvae were almost always to be found among 



