January 2, 1893.] 



KNOWLEDGE 



suspension in the air if a sudden displacement of the 

 claspers should take place, and sometimes prevents a fall 

 to the earth, and saves the consequent labour of getting 

 over uneven ground to reach the tree trunk and then 

 climbing, perhaps, many feet of rough bark before the 

 former position can be regained, and at the same time it 

 forms a considerable security against the fatal results which 

 might follow from striking against intervening branches 

 while falling with gradually increasing velocity. 



The general resemblance to a small twig, caused by the 

 cylindrical shape and the brownish colour, is often heightened 

 by the presence on the backof excrescences of various kinds, 

 similar to what may be seen on the twigs of the food-plant. 

 A very abundant insect, the caterpillar of the brimstone 

 moth (Bw'iia cratmiata), a pretty sulphur-coloured moth 

 with a few reddish patches and streaks, and a common 

 inhabitant of gardens in summer-time, may be taken as an 

 excellent exemplification of this. It is tinted with almost 

 exactly the same mixture of purplish and reddish brown as 

 the twigs of its commonest food-plant, hawthorn ; its thick- 

 ness is about that of the young twigs amongst which it 

 lives, and halfway down its back it bears a wart-like 

 excrescence which very closely resembles the knotty little 

 protuberances seen in similar positions on short stumpy 

 twigs of hawthorn. This same insect exhibits also another 

 very beautiful arrangement whereby the resemblance to a 

 twig is rendered still more perfect. When the two pairs of 

 claspers are in position grasping a twig, it is evident that, 

 if that part of the body which lies between them were of 

 the usual shape, evenly cylindrical in outline, there would 

 be a space between its under surface and the twig, which 

 would be thrown into shadow, and the presence of this 

 clearly-defined dark line of shadow just where the pretended 

 twig ought to pass uninterruptedly into the parent stem 

 would be a flaw which would detract from the perfection 

 of the mimicry. Hence we find that the skin between 

 these two pairs of claspers runs out into numerous little 

 irregularities, forming flaps which reach to the twig itself, 

 and therefore break up the shadow and soften the contact 

 with the stem. If this irregularity of outline were found 

 all along the body, it would hardly be safe to lay much 

 stress upon its presence bet>veen the hind legs ; but this is 

 not the case. The fringe occurs only where the body comes 

 into contact with the twig, and its presence just where it 

 is needed, aud its absence elsewhere, lend probability to the 

 above explanation of its function. 



A thoughtful consideration of cases such as this will 

 serve to show what an endless field of iuvesiigation is 

 opened up, when even the knobs and humps aud ex- 

 crescences of caterpillars are found to be not the mere 

 meaningless freaks of some sportive and erratic force, as 

 they at first appear, but exquisite adaptations whereby the 

 organism has been brought more completely into harmony 

 with its environment ; adaptations, therefore, which have 

 some bearing either upon its preservation amidst its present 

 conditions of life, or upon its past history and the course 

 of events which have constituted its life as a species. 



In this latter connection we may refer to the caudal 

 horns of the caterpillars of the hawk moths (Sp/iiniiidir). 

 These insects have on the back of the last segment but one 

 a curved horn, which contributes in no shght degree to the 

 air of stateliness that characterizes them when they 

 assume the sphinx attitude. The hom usually points 

 more or less upward, and shows only a single curve, but 

 in some cases, as in the death's head moth (Aclurontia 

 Atropos), it is depressed, and doubly curved into an 

 S-shape. When a hawk moth caterpillar is first hatched, 

 the horn is much longer, proportionately to the body, than 

 when fully grown ; it is also straight, almost erect, and 



forked at the tip (Fig. 7), and its surfasd is bsjet with 

 prickles. As the insect grows, the prickly tubercles on the 

 horn become, in some species, less conspicuous, aud 

 finally entirely disappear, the horn becoming smooth and 

 polished ; in others they are retained throughout life. 



Fi&. 7. — Caudal lioru of CoiivoItuIus Moth Caterpillar, at 

 different stages. A, B, f, magnified 58, 3, and 2 diameters; D, natural 

 size. The magnification is not suffieient to slion- the tubercles. 

 (From Poulton.) 



The forked appearance of the tip disappears after the second 

 moult, and the horn curves more and more downwards. 

 Not only is the horn of the newly-hatched larva thorny, 

 but the whole surface of the body is covered with minute 

 tubercles, each of which emits a small hair^ In some 

 species, such as the poplar and eyed hawk moths, these 

 tubercles are retained throughout life, giving rise to that 

 roughness of skin which is described as shagreening. In 

 other cases, such as the privet hawk moth, they are lost 

 after the early stages, and the skin becomes quite smooth. 

 In this f.i.mily of moths, then — a family whicli consists of 

 insects whose general resemblance testifies to their real 

 relationship — we see caterpillars that commence life with 

 well-marked characters, which they afterwards lose to a 

 greater or less extent. Such characters do not seem to 

 bear any special relation to their present surroundings, and 

 may therefore be fairly regarded as common ancestral 

 relics, implying that the insects are descended from 

 progenitors with forked and tuberculated horns. Some of 

 the species, however, have advanced farther from this 

 ancestral condition than others. 



There is another family of large and handsome moths, 

 very different in shape, style, and habits from the hawk 

 moths, but nevertheless exhibiting some curious resem- 

 blances to them in the caterpillar condition. This is the 

 family Saturniilir, or emperor and silk-producing moths, 

 distinguished by the coloured or transparent, circular or 

 crescent-shaped eye-spots in the centre of the wings. The 

 family is very poorly represented in Great Britain, only 

 two species, the Kentish glory and the emperor moth, 

 being native with us. The former of these is in some 

 respects very unlike its associates, and the caterpillar is 

 quite distinct; but the emperor moth {Saturnia carpini) 

 may be regarded as a good illustrative type of the group. 

 Thfi caterpillars of this family are generally large and 

 fleshy creatures, adorned with rows of rounded tubercles 

 from which spinous hairs project. Our British species is 

 a bright green insect with a row of pink tubercles on 

 each segment. In some species there are not only the 

 usual tubercles, but various spines or hornlike projections 

 as well in diflerent parts of the body. These show a 

 tendency to be developed about the thoracic and caudal 

 regions. In some cases the spines are single-pointed, but 

 in others forked and tuberculated. Moreover, when the 

 spines are large, the tubercles on the rest of the body are 

 small, as though the spines were enormously-developed 

 tubercles, which, by reason of their superior protective 

 p iwers, could afford to dispense with their smaller and 

 weaker brethren. 



But the most remarkable facts of all are to be found in 

 connection with the caterpillar of the Tau emperor (Aglia 

 tau), a species about 2^ inches in expanse of wings, and so 



