26 



in many full-grown larvae. The permanent tracheae of the wing appear 

 usually during the third instar, the main trunks even then agreeing 

 with what afterwards form the nervures of the wing ; but they are not 

 filled with air until the time of pupation. These trunks give rise in 

 the pupae to a new system of fine tracheae, which replace those that 

 have been active during the larval period. The imaginal legs, like the 

 wings, are developed within hypodermal sacs, usually (but not always) 

 remaining there until the end of larval life. In the Lepidoptera the 

 imaginal leg grows from a disc, the part agreeing with the tarsus 

 of which only enters the larval leg ; consequently, if mutilation of a 

 larval leg occurs, the only portion of the imaginal leg that can be 

 injured is such part of the outgrowing disc that will form the imaginal 

 leg that is, at the time of injury, contained in the larval leg. The 

 view that the leg of the lepidopterous imago is entirely contained 

 within the leg of the larva has been alleged to be erroneous, and should 

 perhaps be abandoned. Similarly the maxillae, labial palpi, and other 

 imaginal structures are formed from imaginal discs, the resulting 

 organs being very different in some instances from their morphological 

 equivalents in the early stages. 



Many modifications take place in the development of the Hymen- 

 optera and Diptera, although it may be fairly said that the general 

 principle underlying the development of the imago during the larval 

 and pupal stages is in general very similar. 



If now we summarise the facts connected with metamorphosis as 

 observed in Coleoptera, Lepidoptera, Diptera, and Hymenoptera, and 

 possibly all holometabolous insects, we find the following general 

 conditions to exist. All and each of the external organs of the imago, 

 and those of the internal organs that have an ectodermal origin, 

 originate in single cellular masses called imaginal discs (or buds) which 

 are already present in the larval (or even the later embryonic stages). 

 There appear to be imaginal discs for each part of the body — head, 

 legs, wings, ovipositor, and the different sections of the alimentary 

 canal — that of the head being compound, and containing elements 

 which give rise independently and individually to the various organs — 

 antennae, maxillae, palpi, etc. Simultaneously with the formation of 

 the imaginal organs by histogenesis, the larval organs under the 

 influence of the leucocytes undergo a process of histolysis or de- 

 struction, the histogenetic process going on side by side with the 

 histolytic, so that there is no break in the continuity of the organs, 

 the final destruction of a larval organ only taking place with the 

 complete formation of the corresponding imaginal organ. 



It is quite clear that the sharp division of the holometabolous 

 insect into larval, pupal, and imaginal stages is only applicable to the 

 external body, and is not maintained in the processes of internal 

 development, which form a continuous series of transformations in 

 the case of each organ. 



Although on the whole the form of the larva, pupa, and imago are 

 kept distinct, and respond to the condition of their separate en- 



