VI HEXAPODA— METAMORPHOSES 491 



the common racial form' of the Insecta. The distinction between larva and imago is 

 here wanting. The young animal hatched from the egg resembles in all points the 

 sexually mature form, which it reaches by simple growth accompanied by ecdyses and 

 by complete development of the sexual organs. Both young and adult animals live 

 on land and lead the same sort of life. Development without metamorphosis 

 (Ametabole). 



II. The adult insect is, apart from the complete development of the sexual 

 organs, principally distinguished by the possession of wings. In the simplest cases, 

 in the Orthoptera, Corrodentia, Thysanoptera, and most JRhynchota, the larvse lead 

 the same kind of life as the imagines. They change gradually into the imaginal 

 form, growing slowly through numerous ecdyses, while the wings arise and become 

 more strongly developed each time the integument is shed. Gradual meta- 

 morphosis. 



III. In the Oicada the modes of life of the imagines and the larvEe differ. The 

 former live on trees and shrubs, the latter underground on roots, and for this purpose 

 possess strong fore-feet adapted for digging. The transition from the last larval stage 

 to the imago must here he accompanied by a transformation of the fore-legs. Since 

 an intermediate life between that on trees and on the earth is not easily conceivable, 

 and since, consequently, any intermediate form between ordinary feet and digging 

 feet would be purposeless, the transition from the larva to the imago has become 

 du-ect. The last larval stage is then what is called quiescent, i.e. the organisation 

 of the imago develops within the chrysalis at the expense of the accumulated reserve 

 material. Gradual metamorphosis with pupal stage. 



IV. The modes of life of the larvK and imagines of the Epiiemeridce, Odonata, 

 and Plecoptera are very different. The imagines live on land, the larvae have 

 become adapted to aquatic life. In the transition to the imaginal form the tracheal 

 gills are generally thrown off (Ephemeridce and many Libellulidm), the stigmata 

 break through, and the tracheal system becomes holopneustic. In other forms the 

 metamorphosis and the growth of the larvae occur gi'adually (in Ohloe by means of 

 more than 20 ecdyses). Incomplete metamorphosis (Hemimetabole). 



In cases II. III. and IV. the transformation of the larva into the imago is, as a 

 rule, accomplished very gi-adually. The still wingless larva hatched from the egg 

 shows an external organisation like that which the Apterygota possess throughout life, 

 even in their sexually mature condition. 



V. Some Mhynehota are wingless (the Pediculidce, many bugs and the females of 

 the plant-lice), and so are some Corrodentia (the Mallophaga), Orthoptera (various 

 genera and species of Blattidce and Fhasmidce), and Dermaptera. The wingless 

 condition in these forms is derived, as opposed to that of the Apterygota ; 

 they are descended from wing-bearing Shynchota, Corrodentia, Orthoptera, and 

 Dermaptera, but their wings have been reduced in consequence of parasitism or of 

 other habits of life. The larva hatched from the egg has not, therefore, to develop 

 wings. This process is suppressed, and with it the metamorphosis. The young 

 form becomes, by simple growth, the sexually mature imago. In contrast to the 

 original ametabole of the Apterygota we have here an acquired ametabole. 



VI. In contradistinction to the insects as yet mentioned, all others, i.e. the 

 Neuroptera, Panorpata, Trichoptera, Lepidoptera, Diptera, Siphonaptera, Coleoptera, 

 and HymeTwptera, are distinguished by so-called complete^metamorphosis (holometa- 

 bole). A wingless larva is hatched from the egg, which indeed grows and moults, 

 but, nevertheless, always retains the same organisation and undergoes no trans- 

 formation during the larval stage. At the end of larval life, however, when the fat 

 body has become strongly developed by rich nom-ishment, the larva moults and 

 passes over into the differently formed pupal stage. The pupse are very variously 

 shaped, often distinctly segmented with rudimentary extremities and wings, often 



