THE EXTERNAL STRUCTURE OF THE LEPIDOPTEROUS LARVA. 45 



(chlorophyll), by the effects produced by light and shade, by adaptation 

 to the form of the edge of the leaf (as in the serrated back of certain 

 Notodonts), by adaptation to the colours of different leaves and to the 

 stems, since shades of greens, yellows, reds, and browns, are almost as 

 common in the cuticle of caterpillars, as on the surface or cuticle of 

 the leaves and their stems, or in the bark of the twigs and branches. 

 He also adds that probably many have observed that the peculiar brown 

 spots and patches of certain Notodonts do not appear until late in larval 

 life, and also late in the summer, or early in the autumn, contem- 

 poraneously with the appearance of dead and sere blotches in the leaves 

 themselves. This phase of the subject will be dealt with at length in 

 a later chapter. 



Tactile hairs, defensive setae, locomotive setae, and spines of various 

 kinds, occur in worms ; these, too, often arise from fleshy warts or 

 tubercles. It is, therefore, not at all unlikely that the ancestral lepi- 

 dopterous larva was provided with piliferous warts, and that many of 

 the specialised spines, etc., now found in lepidopterous larvae, are 

 modifications of these ancestral simple structures. 



It may be safely assumed that spines, hair-tufts, etc., serve to pro- 

 tect the organism from external attack, probably also to strengthen 

 the shell or skin. That even the most complex spines are modifica- 

 tions of the tubercular structure is evident if one examines the cast 

 skin of a Vanessid larva when it has just been thrown off, and the 

 pupal state assumed. Packard, in a long argument. suggests that 

 " it is not improbable that tubercles, humps, or spines, may have in 

 the first place been developed in a few generations, as the result of 

 some change in the environment during the critical time attending or 

 following the close of the Palaeozoic, or the early part of the Mesozoic 

 age, the time when deciduous trees and flowers probably began to 

 appear." The same author refers to Darwin's significant remark -f- 

 that " organic beings, when subjected during several generations to 

 any change whatever in their conditions, tend to vary," further, that 

 " variations of all kinds and degrees are directly or indirectly caused 

 by the conditions of life to which each being and, more especially, its 

 ancestors have been exposed" (p. 241) and again, that " changes of 

 any kind in the conditions of life, even extremely slight changes, often 

 suffice to cause variability. Excess of nutriment is, perhaps, the most 

 efficient single exciting cause." 



Referring to the geological fact, that in the Cretaceous period, the 

 forests consisted of oaks, maples, willows, beech, poplar, etc., Packard 

 assumes that, in all probability, the low-feeding caterpillars of that 

 time began to desert the herbaceous plants to feed on trees, and that 

 they then experienced sufficient change to induce considerable variation, 

 and that, to a great extent, tree- feeding necessitated isolation. He 

 thinks, moreover, that the change from herbaceous to arboreal feeding, 

 not only affected the shape of the body, causing it to become thick and 

 fleshy, but also led to a hypertrophy of the piliferous warts, common 

 to all lepidopterous larvae. We deal with this at length, not because 

 we are inclined to agree with its assumptions, but because no other 

 explanation of the actual origin of the cause of the modification has 

 been offered. 



* Bombycine Moths of America, pp. 16 et seq. 

 f Variation of Animals and, Plants under Domestication, 2nd Edition, 1888, 



