628 SCIENCE PROGRESS 



one as merely a larger one than the other. Yet the difference 

 between their larvae is simply enormous. The small one is 

 bright soft green, with only the faintest of markings and a 

 nearly smooth body, while the large species has a caterpillar 

 bluish green in colour, and dotted over with black warts 

 studded with hairs. 



In varying families, each different method is more or less 

 developed, though perhaps green colouring, corresponding 

 more or less perfectly with the environment, is the most general. 

 Shape varies immensely, not only in different families, but in 

 individuals of the same family, and forms commonly known 

 as " woodlouse " shaped, from their extremely contracted 

 appearance, are met with in several groups. Hairy, spiny, and 

 wart-bearing species are common, and the protective devices 

 of the Puss Moth are possessed by a whole group of butterfly 

 caterpillars. In the Swallowtails, the forked appendage is 

 found as a regular feature, but, instead of being placed on the 

 last segment, it is on that immediately behind the head. In 

 this case it is the seat of a powerful and not pleasant odour, 

 which by implication must be regarded as a protection against 

 enemies. The acid ejection occurs among several Vanessa 

 larvse, but, in this case, is not thrown to a distance, but merely 

 discharged in a large drop when the insect is interfered with. 

 Silk is utilised in only a few cases, generally to form a common 

 web under which the whole brood feed during the earlier part 

 of their lives. 



Evolution of form must, of course, be allied to or corre- 

 lated with evolution of habit, and this fact should never be lost 

 sight of, if a true view of the complex causes which have led 

 to the immense variety of form found amongst lepidopterous 

 caterpillars is to be obtained. As we have seen, there is 

 scarcely any marked deviation from the simple grub type, 

 until habits of roaming abroad for food are acquired. Con- 

 versely, the habit of roaming could scarcely become profitable 

 unless qualities were acquired which counteract the risks 

 occasioned by it. How delicately this correlation is adjusted 

 may perhaps be best understood by the consideration of one 

 or two individual cases. Take that of the Privet Hawk 

 Moth, for example. Large and conspicuous though this insect 

 is when studied as a solitary object, I know from experience 

 that, when feeding on its native bushes, its green colour, broken 

 by the oblique white stripes, conceals it so effectively that it 

 is not an easy matter to find one or two out of several dozens 

 which may be feeding in quite a limited area. Place one on 

 the ground and it instantly stands out as a most startlingly 

 definite object. The Privet caterpillar goes down to the 

 ground at the end of its term, for its pupa is invariably placed 



