THE INTERNAL STRUCTURE OF THE LEPID0PTER0U3 PUPA. 67 



but is altered almost beyond recognition by the disappearance of 

 glands and muscles, and by the diminution of its length. Certain 

 mouth-parts also disappear and so on. 



The changes that take place in the Lepidoptera are certainly not 

 so extreme as in some of the higher Diptera, where, after the larva 

 has done feeding and before the stage corresponding to the lepidop- 

 terous pupa is assumed, extreme histolysis of the larval tissues takes 

 place before the imaginal discs show any signs of development. Some 

 observers have even asserted that the state here reached is comparable 

 with that which takes place in the ovum, and that the imaginal parts, 

 even as discs, do not exist before this stage is reached. This does 

 not seem to be at all likely, notwithstanding that the Diptera do 

 afford many surprises to those whose views are based on a study of the 

 less highly specialised orders. Lowne says, " As all the discs preserve 

 an embryonic character, it is probable they are all present in the young 

 larva or even in the embryo, as distinct groups of cells." In the 

 Mwcidae, it would appear that all muscles, for instance, disappear by 

 histolysis before the imaginal discs have made any obvious advance. 

 In the Lepidoptera, on the other hand, certain muscular structures 

 persist throughout. 



Most of the changes that take place in the pupal period and 

 culminate in the development of the imago, are in active operation, at 

 least, during the final (quiescent) period of larval life. It is during 

 this period that the greatest changes occur in the alimentary canal, 

 the nervous system, &c, and, in many respects, the modifications 

 now cause the organs to approach more nearly to the imaginal struc- 

 tures than the larval from which they have originated. Many of the 

 internal organs represent, at this time, the imaginal organs already in 

 an advanced state of development, whilst others have yet to be almost 

 entirely broken down by histolysis and the new imaginal structures 

 built up from the imaginal discs already present, the latter, as already 

 stated, obtaining their nutrient material from the histolytic products 

 present in the pupa. As a general statement, we may assert that, 

 during the quiescent larval stage, histolysis is predominant, in the 

 pupal stage histogenesis. The " imaginal discs " remain quiescent 

 wbilst the other tissues are being disintegrated without sharing in the 

 disintegration, and then commence a career of development, although, 

 as we have already pointed out, this is much less true of Lepidoptera 

 tban of the Muscidae. 



Although this chapter is nominally one on the internal structure 

 of the pupa, it becomes absolutely necessary to consider the previous 

 development of the organs that are found in the pupa, before it is 

 possible to comprehend the nature of the internal organs, or to under- 

 stand the further changes that take place in them, in the pupal stage. 

 We are especially indebted to Gonin for our knowledge of the part of 

 our subject that immediately follows. 



In the adult larva, the true legs offer no vestige of an imaginal 

 disc, but they contain a great number of embryonic cells. These cells, 

 round and fusiform, are almost always ranged about a nerve or trachea, 

 sometimes independent, and, at others, retained to the peritoneal 

 sheath. They appear to be formed by the proliferation of this sheath. 

 Some thus contribute to the elongation of the tracheal branch or 

 nerves, others, detaching themselves, constitute the leucocytes of the 



