DEVELOPMENT. 203 



Insects, like Frogs, disperse as adults, because of the 

 difficulty of the medium, aerial locomotion being even more 

 difficult than locomotion by land, and implying the highest 

 muscular and respiratory efficiency. The flying state is attained 

 by a late metamorphosis, which has not yet become universal in 

 the class, while it is not found in other Tracheates at all. 

 Peripatus, Scorpions, and Myriopods become sexually mature 

 when they reach the stage which corresponds to the ordinary 

 less-modified Insect-nymph, with segmented body, walking- 

 legs, and mouth-parts resembling those of the parent.* 



The Caterpillar is not, as Harveyf maintained, a kind of 

 walking egg ; it is rather the primitive adult Tracheate modified 

 in accordance with its own special needs. It may be sexually 

 immature, imperfect, destined to attain more elaborate develop- 

 ment in a following stage, but it nevertheless marks the stage 

 in which the remote Tracheate ancestor attained complete 

 maturity. Where it differs from the primitive form, hatched 

 with all the characters of the adult, the changes are adaptive 

 and secondar. 



The Genealogy of Insects. 



To construct from embryological and other data a chart of 

 the descent of Insects, and of the different orders within the 



* It is possible that the curious cases of agamogenetic reproduction of the larvae 

 of Aphis, Cecidomyia, and Ckironomus are vestiges of the original fertility of 

 Insect larvre. 



f "Alia vero semen adhuc imperfectum et immaturatum recludunt, incrementum 

 et perfectionem, sive maturitatem, soris acquisiturum ; ut plurima genera piscium, 

 ran*, item mollia, crustata, testacea, et cochleae : quorum ova primum exposita sunt, 

 veluti origines duntaxat, inceptiones et vitelli ; qui postea albumina sibi ipsis circum 

 circa induunt ; tandemque alimentum sibi attrahentes, concoquentes et apponentes, 

 in perfectum semen atque ovum evadunt. Talia sunt insectorum semina (vermes ab 

 Aristotele dicta) qute initio imperfecte edita sibi victum quserunt indeque nutriuntur 

 et augentur, de eruca in aureliam ; de ovo imperfecto in perfectum ovum et semen." 

 De generatione, Exc. II., p. 183 (1666). Viallanes justifies this view by applying 

 it to the histolysis and regeneration of the tissues in Diptera. But these remark- 

 able changes are surely secondary, adaptive, and peculiar, like the footless maggot 

 itself, whose conversion into a swift-flying imago renders necessary so complete a 

 reconstruction. 



J The reader is recommended to refer to Fritz Miiller's Facts and Arguments for 

 Darwin, especially chap. xi. ; to Bal four's Embryology, Vol. II., chap, xiii., 

 sect. ii. ; and to Lubbock's Origin and Metamorphoses of Insects. 



