393 



Many of the fcracheoles, however, appear to undergo 

 mainly chemical disintegration; lying, as they often do, 

 closely embrac^'d by the great "fat cells," they seem to be 

 protected from the action of the leucocytes. Their protoplasm 

 becomes finely vacuolated, the lumen disappears, and the tubes 

 gradually fragment. This is especially beautifully seen in 

 some of the great head tracheoles, which disappear at about 

 the time of pupation (fig. 91). The great abdominal tracheoles 

 disappear a few hours earlier, also by chemical disintegration ; 

 frequently, however, if the pupating larvae are examined, 

 rows of leucocytes in the place where the larval tracheoles 

 once were, indicate that phagocytosis of the vestiges of the._ 

 tracheoles has, in the end, occurred. Active phagocytosis may \ 

 be observed at times, however (figs. 87, 88), in places, such | 

 as, for instance, the cavity of the thorax, where they are easily ' 

 accessible to the wandering phagocytes. Several phagocytes 

 may apply themselves to the degenerated tracheole, and, dis- 

 solving parts out of it, gradually absorb it. 



Thus, partly by chemical disintegration and partly by 

 phagocytosis, the whole larval respiratory system, with the 

 exception of the spiral intima of the lateral longitudinal, and 

 anterior and posterior transverse vessels disappears within a 

 few hours after pupation. 



The intima of the lateral stigmatic trunks is shed during 

 moulting. 



The Regeneration of the Tracheal System. 



The regeneration of the imaginal tracheal system has kept 

 pace with the destruction of the larval vessels, and takes 

 place from the ''imaginal nests" at the bases of the stigmatic 

 trunks. At the end of the resting period of the larva, the "H 

 cells composing these "nests," having lain dormant during 

 the feeding period, rise into sudden activity, and proliferating 

 greatly (fig. 89), extend as imaginal tracheal histoblasts in- 

 wards and along the intima of the great tracheal vessels, 

 pushing the epithelial cells which the leucocytes have not 

 removed aside as they advance, and taking up a position 

 between the larval intima and the epithelial cells from which 

 it was secreted (fig. 84). Some twelve hours later the epi- 

 thelium of the larval intima has been completely regenerated 

 (fig. 85). Tliose stigmatic trunks which are to persist in the 

 imago undergo a similar renovation (fig. 89). In the others 

 this does not take place, and they disappear (fig. 92). 



The histoblasts are at first somewhat spindle-shaped as 

 they advance, but they soon spread out and form a thin-walled 

 tube in close contact with the spiral intima. The further 



