176 THE BIOLOGY OF INSECTS 



various imaginal rudiments, however, are formed from 

 organs or from groups of cells in the larva whose tissues have 

 arisen from the segmenting egg in the course of embryonic 

 development. The comparison of a pupa to a *' second 

 egg " is therefore fanciful, and the life-history of the butterfly 

 or bee after hatching must be regarded as a specialised and 

 curiously modified form of growth. In all such insects the 

 hidden development of the wing-buds, lying apparently 

 within the body, affords, as D. Sharp (1898) pointed out, 

 a definite character by which these metamorphic or holo- 

 metabolous insects (Endopterygota) may be separated from 

 those hemimetabolous insects, undergoing little or com- 

 paratively sHght transformation and exhibiting outward 

 wing- rudiments early in their life-history (Exopterygota). 

 We find here the essential distinction between *' incomplete " 

 and " complete " metamorphosis among insects. 



The comparison of a butterfly with its caterpillar 

 demonstrates that the adult insect or imago diflfers from its 

 larva not only in structure but also in its manner of Hfe, 

 and this aspect of the study of insect transformation appeals 

 to those interested in questions of Hfe-relations and the 

 organism's adaptations to meet the conditions under which 

 it has to exist. The winged adult flies while the larva 

 crawls ; the butterfly sucks nectar from flowers while the 

 caterpillar eats solid pieces of plant tissue, leaves, roots, or 

 wood, which it bites off with its strong mandibles. As the 

 student of the biology of insects reviews a series of larvae 

 belonging to various types, he becomes convinced that in 

 each case the form of the larva is adapted to its habitation 

 and manner of feeding ; its differences from the adult 

 correspond with different life-relations. 



It has already been pointed out that although the cater- 

 pillar differs markedly in aspect from its parent butterfly, 

 its body is built essentially on the same general plan. There 

 is agreement in the number of segments, in the regions 

 (head, thorax, and abdomen) into which they are grouped, 

 in the relative positions of the various systems of organs. 

 The difference between a newly-hatched caterpillar and a 



