INSECT METAMORPHOSIS 



have been derived from a common ancestor, as seems 

 almost certain, then the original larvae must have been 

 all alike, and they must have had approximately the 

 structure of those larvae of the present time that depart 

 least from the structure of the adult. Therefore it is 

 evident that many larvae of the present time have some- 

 how acquired certain embryonic characters. We may 

 suppose, therefore, either that such larvae have had a 

 retrogressive evolution into the embryonic stage by 

 hatching at successively earlier ages, or that certain 

 embryonic characters representing ancestral characters 

 but ordinarily quickly passed over in the embryonic 

 development, have been retained and carried on into the 

 larval stage. The latter view seems the more probable 

 when we consider that no larva has a purely embryonic 

 structure, and that those larvae which have embryonic 

 features in their anatomy present an incongruous mixture 

 of embryonic and adult characters. 



We may, therefore, finally conclude that the larva of 

 insects with complete metamorphosis represents the nymphal 

 stage of insects with incomplete metamorphosis; and that 

 the structure of the larva has resulted from a suppression of 

 the peculiarly adult characters, from an invagination of the 

 wings, a loss of the compound eyes, the retention of certain 

 embryonic characters, and a special development of the body 

 form and the organs suited to the particular mode of life of 

 the larva. By allowing for variations in all these elements 

 that contribute to the larval make-up, except the two 

 constants — the invagination of the wings and the loss of 

 the compound eyes — we may account for all the variety 

 in form and structure that the larva presents. 



While, in general, the larva remains the same in struc- 

 ture from the time it is hatched until it transforms to the 

 pupa, there are nearly always minor changes observable 

 that are characteristic of its individual stages. In 

 Chapter I we encountered the case of the little blister 

 beetle that goes through several very different forms dur- 



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