i84 THE BIOLOGY OF INSECTS 



young insects, which clearly illustrates increase in divergence 

 of larva from imago. It also helps to explain that feature 

 of insect metamorphosis according to which the trans- 

 formation becomes most profound in the most highly 

 specialised groups. It is well known that among animals 

 generally, marked transformation in the course of the life- 

 history characterises creatures of comparatively primitive 

 organisation, w^hich live in the sea and as a rule produce eggs 

 of small size. This combination is illustrated by the pro- 

 found transformations undergone by starfishes and other 

 echinoderms, or by the marked change of form in growth 

 after hatching to be observed in most fishes, compared with 

 the young of terrestrial reptiles and birds hatched from 

 large-yolked eggs in a condition already well-developed. 

 Insects form a class of creatures essentially terrestrial and 

 aerial, whose eggs are of relatively large size. Yet the 

 young of most insects pass through marked changes after 

 hatching and the greatest degree of change is shown by 

 members of the most highly specialised orders. From the 

 facts surveyed in this chapter it will be apparent that the 

 insect larva, even of a type so degraded as the muscoid 

 maggot, is not a precociously hatched embryo, but a modifi- 

 cation of the type of structure displayed by the developed 

 insect. Whenever the larva differs markedly from the 

 imago we find that it lives and feeds differently from the 

 latter, and we conclude that there must have been specialisa- 

 tion not in one direction only, but in two. The imago 

 shows high elaboration in the form of jaws, wings, sense- 

 organs, while the larva, even if degenerate, is also itself 

 specialised in correspondence with a mode of life widely 

 divergent from that of the winged insect. The degree of 

 divergence between the adult and the larval structure and 

 life necessitates a corresponding degree of reconstruction 

 at the crisis of development marked by the pupal stage, 

 involving a resting period in the life-history during which 

 the reconstructive processes can be carried out. 



In the series of gnibs, belonging to beetles and other 

 insects, illustrating increasing divergence between larva and 



