66 BRITISH LEPIDOPTERA. 



existence they commence to grow rapidly and replace the earlier 

 structures, and, although the two processes of histolysis and histo- 

 genesis are mutually dependent on each other (the former supplying 

 the material which is utilised by the latter), they do not go on side by 

 side, although, to a certain extent, contemporaneous. During the 

 quiescent period preceding pupation, many of these discs enlarge and 

 grow rapidly, whilst, at the same time, there is a destruction of the 

 larval tissues, due to the activity of the phagocytes, certain of the 

 larval organs and the fat-body forming a creamy mass, the imaginal 

 discs resisting the leucocytes and living on the nutrient matter thus 

 produced by the dissolution of the fat-body and these larval organs, 

 and, as the last steps in the destruction of the larval organs only take 

 place after those of the imago have assumed their definite shape and 

 size, it follows that the connection of the organs in question remains, 

 in most cases, entirely continuous. 



It is, perhaps, not quite correct in Lepidoptera (although somewhat 

 difficult to avoid in treating this technical subject in general terms) to 

 speak of the imaginal discs of legs, antennae, and other organs that 

 exist in the larva. The point of development of the imaginal organ is, 

 in reality, the larval organ, not a disc existing throughout larval life 

 as a structure separate from that of the larva, e.g., the larval leg is 

 modified, some of its parts are destroyed by histolysis, but the 

 embryonic cells that form the imaginal leg are derived from the larval 

 leg, and so on. Where any other suggestion appears to be implied it 

 mast be considered rather as a weakness of diction than any wish to 

 assert the contrary. 



It might be remarked here, that the old idea of the imago existing 

 in the larva, and its appearance after various outer shrouds have been 

 successively cast off, is not so very different from the real truth as it 

 was represented to be when the first novelty of a more definite idea of 

 the processes involved still affected us, and tended to warp or numb 

 our appreciation of the actual facts. The imago does exist in the 

 larva, even in the very young larva, not only potentially but actually. 

 The imaginal parts are all represented either by actually similar 

 structures, or by imaginal discs. These may be merely a few cells of 

 no definite structure, certainly none similar to the structures they 

 represent, and into which only they are capable of developing, and are 

 recognisable only by their special anatomical relationships. Develop- 

 ment goes on, to some extent, during larval life, but it is, however, 

 only at its end, and in association with the change to pupa, that they 

 assume forms recognisable as those of the fully developed organs. 

 During the pupal stage, after a period of quiescence, both outwardly 

 and physiologically, in those species that hybernate (or activate, &c), 

 the full development of the several organs occurs. The store of 

 material that nourishes the developing tissues is chiefly that contained 

 in the fat-body, but some material is provided by the histolysis of 

 larval structures that are no longer required and disappear. Though 

 there are thus whole organs that are fully developed in the imago and 

 do not exist in the larva, except as imaginal discs, there are hardly 

 any complete organs in the Lepidoptera that wholly disappear at this 

 stage, although, of many, the modifications are so profound, that, to 

 describe what takes place as a complete histolytic destruction of them, 

 is not much exaggerated. The alimentary canal, for instance, remains, 



