THE EXTERNAL STRUCTURE OF THE LEPIDOPTEROUS LARVA. 41 



directly below the vent. In appearance, it is somewhat like the egg- 

 guide of the Acrydii, though the latter is thin and flat." Its use is, 

 evidently, to aid in tossing the pellets of excrement away, so that they 

 may not come in contact with the body. 



Packard, in an article describing the larva3 of certain species of Centra, 

 gave it as his opinion, that the "stemapoda" or filamentous anal pro- 

 cesses of these caterpillars Avere homologous with the anal prolegs of 

 other Notodonts, and, to show this, figures the anal prolegs of Dasy- 

 lophia anguina in its first larval stage. He points out, in his comparison, 

 that it is intermediate in form between the normal anal proleg and 

 the stemapod, and remarks that it " has no crochets, but the planta, 

 of which the flagellum of Centra seems to be the homologue, is re- 

 tracted, and the retractor muscles, one of which is divided, are much 

 as in the filamental legs of Centra. It, however, is not the general 

 opinion of British entomologists that the stemapoda are modified anal 

 prolegs. Hellins regarded them as " dorsal appendages, somewhat 

 after the fashion of the anal spines of the larvae of the Satyridae." 

 Packard discusses this view, and concludes: — "After repeated com- 

 parisons of the filamental anal legs of Centra with those of Macruro- 

 campa marthesia, and comparing these with the greatly elongated anal 

 legs of young Heterocampa unicolor, as figured by Popenoe, and taking 

 into account the structures and homologies of the supra-anal and 

 paranal flaps, one can scarcely doubt that those of Centra are modified 

 anal legs." There appears to be no doubt whatever that Packard is 

 quite right, and that the view hitherto held by British entomologists, 

 is a wrong one. 



The ancestral lepidopterous larvae probably lived, at first, on 

 grasses and low growing plants, and the arboreal habit was possibly 

 assumed at a comparatively late period of larval evolution. This 

 view is fully borne out by the geological evidence, for it is generally 

 considered that flowering plants and trees were probably developed in 

 the Cretaceous or Tertiary periods, and that our present race of lepi- 

 dopterous insects became evolved side by side with the great changes 

 that then took place in the flora of the world. 



Many of the most highly developed groups of Lepidoptera — most 

 of the Noctuids, Arctiids, Pierids, Satyrids, etc. — feed, even now, 

 almost exclusively upon low plants, and we find that, amongst larvse 

 with this particular habit, the caterpillar is usually devoid of spines, 

 and smooth or covered with a short, dense, velvety pile, whilst the 

 markings consist chiefly of longitudinal lines of various shades of 

 green, grey, etc., running from the head to the anus, dor sally, 

 laterally, and ventrally. There are, of course, many very hairy and 

 spiny larvse that feed on low plants, but these live usually a more or 

 less exposed life — neither hiding under leaves (like the Satyrids) nor 

 stones (Noctuids and Crambids) by day — and the great development of 

 hairs, pencils, spines and bristles, appears to be due often to the cater- 

 pillars having changed their mode of life from a concealed to an 

 exposed condition, the change having frequently been accompanied by 

 a move from a herbaceous to an arboreal feeding ground. 



Just as the caterpillars of grass-feeding larvaa are green or grey in 

 colour, and are chiefly ornamented with longitudinal lines of various 

 shades, so the larvae of arboreal caterpillars — Catocalids, Geometrids, 

 etc. — have their bodies usually of a grey or ash colour, ornamented 



