684 TEXT-BOOK OF ENTOMOLOGY 



accompanied by a reformation of the peritoneal membrane and 

 teenidia. The larval ectotrachea undergoes histolysis, that of the 

 imago being meanwhile formed; the larval taenidia also break up, 

 dissolve, and are replaced by new teenidia which arise from the 

 nuclei of the peritoneal membrane. That the tracheal system in 

 the Muscidae during metamorphosis undergoes a transformation is 

 shown, as Korschelt and Heider claim, by the entirely different 

 shape of the system in the maggot, the pupa, and the fly. The air 

 is admitted to the tracheal system of the maggot, not by lateral 

 openings, but through the two stigmata at the end of the body. 

 On the other hand, the pupa breathes by prothoracic spiracles, while 

 the fly has six pairs of lateral stigmata of the normal structure. 

 There may be in the larva and pupa vestigial closed stigmata, 

 as there are in the thorax of caterpillars, with tracheal branches 

 leading to where were once functional stigmata. These stigmatal 

 branches, as well as some other portions of the tracheal system 

 already observed -by Weismann, seem, according to Van Rees, to 

 function as imaginal buds for the regeneration of the tracheal 

 matrix, while frequently also a regeneration of this epithelium, by a 

 simple repeated division of cells, may be recognized. The disintegra- 

 tion of the tracheal system is accomplished by means of phagocytes 

 in the manner already described. 



The nervous system. The central nervous system passes directly 

 from the larval into the imaginal stage, since it must continue to 

 exercise most of its functions throughout metamorphosis, though 

 it undergoes important changes of form and position. At the same 

 time, certain histological transformations occur which may be re- 

 garded as a histolysis. Such is the destruction and rebuilding in 

 the interior of the organs, which, however, preserve their continuity. 

 Every case of destruction of tissues in the pupa has come to be 

 regarded as a histolysis. 



The problem of the transformation of the peripheral nervous sys- 

 tem is not yet well understood. Although during the destruction of 

 the larval muscles the motor nerves also in part degenerate, in the 

 case of the nerves distributed to the appendages the conditions are 

 different, as these may be recognized in the larva in the form of 

 the nerve-threads which place the imaginal buds in connection with 

 the central nervous system. These threads, according to Van Rees, 

 pass from the larva into the pupa and imago, so that with the farther 

 development of the rudiments of the extremities, only the distal 

 part of the nerves belonging to them are to be regarded as new 

 formations. (Korschelt and Heider.) 



