and what might have been expected, but we are confronted with the difficulty that if the most highly organised insects pass through the most profound transformations, then insects present a remarkable and puzzling exception to the general rules of development among animals, as has already been pointed out in the first chapter of this volume (p. 7).  A few students of insect transformation have indeed supposed that the crawling caterpillar or maggot must be regarded as a larval stage which recalls the worm-like nature of the supposed far-off ancestors of insects generally.  Even in Poulton’s classical memoir (1891, p. 190), this view finds some support, and it may be hard to give up the seductive idea that the worm-like insect-larva has some phylogenetic meaning.  But the weight of evidence, when we take a comprehensive survey of the life-story of insects, must be pronounced to be strongly in favour of the view put forward by Brauer (1869), and since supported by the great majority of naturalists who have discussed the subject, that the caterpillar or the maggot is itself a specialised product of the evolutionary process, adapted to its own particular mode of larval life.

The explanation of insect transformation is, in brief, to be found in an increasing amount of divergence between larva and imago.  The most profound metamorphosis is but a special type of growth, accompanied by successive castings and renewings of the chitinous cuticle, which envelopes all arthropods.  In the simplest type of insect life-story, there is no marked difference in form between the newly-hatched young and the adult, and in such cases we find that the young insect lives in the same way as the adult, has the same surroundings, eats the same food.  This is the rule (see Chapters II and III) with the Apterygota, the Orthoptera, and most of the Hemiptera.  In the last-named order, however, we find in certain families marked divergence between larva and imago, for example in the cicads, whose larvae live underground, while in the coccids, whose males are highly specialised and females degraded, there succeeds to the larva—­very like the young stage in allied families—­a resting instar, which in the case of the male, suggests comparison with the pupa of a moth or beetle.

Turning to the stone-flies, dragon-flies and may-flies, whose life-stories have been sketched in Chapter IV, we find that the early stages are passed in water, whence before the final moult, the insects emerge to the upper air.  Except for the possession of tufted gills, adapting them to an aquatic life, the stone-fly nymphs differ but slightly from the adults; the grubs of the dragon-flies and may-flies, however, are markedly different from their parents.  In connection with these comparisons, it is to be noted that the dragon-flies and may-flies are more highly specialised insects than stone-flies, divergent specialisation of the adult and larva is therefore well illustrated in these groups, which nevertheless have, like the Hemiptera and Orthoptera, visible external wing-rudiments.