DEVELOPMENT OP THE IMAGO. 369 



during larval life, so that it is no longer capable of performing its 

 functions and therefore disintegrates, while another part of the 

 rudiment remains from the first in an undeveloped condition, 

 persisting as the imaginal disc, in order, during the pupal stage, 

 to undertake the regeneration of the organ. 



It should here be pointed out that this remarkable method of development 

 of the organs of the imago, although most marked in the Insecta, is also found 

 indicated in other animal groups. We find repeatedly that, instead of the 

 gradual transformation of a larval organ into the corresponding adult organ, 

 another course is entered upon, the larval organ being destroyed or degenerating, 

 and the corresponding organ in the adult appearing anew as a rudiment. We 

 refer here to Vol. ii., p. 312, where the disappearance and reappearance of limbs 

 during the metamorphosis of the Crustacea is described. A similar phenomenon 

 was mentioned in connection with the Acarina (pp. 104 and 105), in which a 

 partial destruction and a reconstruction of the internal organs occurs. Where 

 the distinction between the larval and the imaginal form of an organ is very 

 marked, the latter mode of development may even appear as a simplification 

 of the ontogenetic process. 



Although Swammerdam had already shown that the limb-rudiments 

 can be recognised under the integument of the larvae of holometa- 

 bolic insects, our more detailed knowledge of the changes connected 

 with the pupal stage is due to the researches of Weismann (No. 129) 

 in connection with the ontogeny of the Diptera. The fact that later 

 students of this subject, among whom should be named Ganin, 

 Viallanes, Kunckel d'Herculais, Kowalevsky (No. 112), and Van 

 Kees (Xo. 121), restricted their investigations to the same order, 

 accounts for our being most familiar with the processes of meta- 

 morphosis in the pupa of the Dipteran family Muscidae. Our 

 description will therefore chiefly refer to this family. But since, 

 as is easily seen, we have in the Muscidae the most complicated and 

 the most modified ontogenetic conditions, we shall often have to take 

 as starting-points the simpler formative processes found in other 

 Holometabola, such as the Nematocera (Corethra), the Hymenoptera, 

 and the Lepidoptera (Weismann, Ganin, Dewitz, etc.). It should be 

 mentioned that our knowledge of the subject is still very incomplete, 

 and only the principal features can be regarded as established. We 

 have, especially, no knowledge as to how far the conditions of the 

 inner metamorphosis ascertained as prevailing in the Muscidae occur 

 also in the other groups of Insecta, although it must be regarded as 

 probable that similar processes take place in the pupae of the 

 Lepidoptera, Hymenoptera, and perhaps also of the Coleoptera. 



We shall consider these ontogenetic processes under two heads, 



2 B 



