493 



existed. It may, of course, be said that insects of a kind 

 which now undergo no metamorphosis may have done so at 

 an earlier period; there is, however, no need to make this 

 assumption, for, as I shall show later, the evolution of meta- 

 morphosis has been a necessary consequence of the specializa- 

 tion which these early generalized orders have since undergone. 



In order to trace the stages through which metamorphosis 

 has been evolved it will be necessary to describe, very briefly, 

 the main features of the postembryonic developments of a 

 number of insects, in so far as we know them. As many 

 of the accounts are not very reliable, and as no metamorphoses 

 have been investigated from this point of view, the comparison 

 is less complete than it ought to be. The insects are arranged 

 in what I regard as ascending degrees of metamorphosis. The 

 reason for this will be clear later. It is very interesting, at 

 the same time, to observe that the order is also approximately 

 that of increase of specialization. 



(1) The Aptera. — These emerge from the <egg in practic- 

 ally the adult condition. In Machilis the eye is believed to 

 continue to develop. Sexual organs undoubtedly ripen during 

 post-embryonic life. 



(2) Orthoptera and Hemiptera. — These usually emerge in 

 a condition which, though like that of the adult, is somewhat 

 more generalized, e.g., the thoracic segments have not yet 

 markedly differentiated. During post-embryonic life there is 

 a gradual growth of the wings; the insect moults several 

 times, but only at the last moult do the vtdngs appear free on 

 the surface. Sexual organs undergo a parallel development. 

 In the wingless forms post-embryonic life is of the . apteran 

 type. 



(3) Odonata. — Partially developed wings clearly visible 

 through the integument, even in early instars. Legs always 

 very well developed, resembling those of adult. "The internal 

 metamorphosis begins considerably before the hatching of the 

 insect; the larva refuses to feed and is restless. Hypoderm 

 cells proliferate, causing the larva to appear tense and swollen. 

 The wing muscles grow greatly and increase the size of the 

 thorax. New elements form rapidly in the eye.'' — Tillyard 

 (1917). The larva then leaves the water, and the moult dis- 

 closes the adult insect. Nothing is known of the cellular 

 changes underlying the process, but they are evidently of the 

 highest interest. 



(4) Coleoptera (type: Galeruca, examined by Poyarkoff). 

 — Larvae emerge with typical insect head and mouth 

 appendages. Legs present, but much more reduced than in 

 Odonata. Wings never clearly seen in larva. Division of 



