396 



dorso-lateral regions of the thorax. In the two-day pupa they 

 fuse again with the main tracheal trunks immediately in front 

 of the propodeal stigmatic trunk. 



The tip of the column presents a remarkable frayed 

 appearance (fig. 94) ; this may be a special adaptation to aid 

 the column in forcing its way through the surrounding masses 

 of fat cells. 



The cell columns meanwhile have begun to differentiate. 

 In the growing columns the cells are thick, rather elongated 

 in the direction of growth, and present a clear cytoplasm ; 

 the lumen of the cell column is narrow and devoid of intima. 

 But at about the time of fusion of the post-erior end of the 

 columns with the main trunks the epithelium gradually 

 flattens, transforming the whole structure from a narrow tube 

 into a great air sac (fig. 95). The epithelial cells develop a 

 granular cytoplasm; while in the four-day pupa they may 

 have nucleoli almost as large as those of the degenerating 

 larval cells; a "spiral" intima, is quickly secreted. 



In the early pupa a number of other tracheae have devel- 

 oped from the main longitudinal vessels ; especially prominent 

 are two ventral downgrowths, from which the tracheoles of 

 the wings have been developed. In the early pupa the 

 anterior of these is observed as a thick column of more or less 

 cubical cells, which in their descent have torn off and dragged 

 along portions of the salivary glands as these were under- 

 going phagocytic destruction. A very fine example of this 

 is shown in fig. 88; the great wing trachea is observed with 

 fragments of salivary glands still attached to it; some of 

 the tracheal cells are in a state of great activity and are 

 growing outwards to form tracheoles; more anteriorly lie the 

 larval tracheoles, undergoing disintegration. 



The tracheal trunk to the hind wing is never so prominent. 



While these tracheae and great dorso-lateral air sacs have 

 been developing, the main tracheal trunks have undergone a 

 similar differentiation. The epithelial cells gradually flatten 

 out, and separating from the ''spiral" intima of the larva 

 upon which they have been resting for the last three days, 

 soon form the two great ventral air sacs. A "spiral" intima 

 is quickly secreted. The size of the air sac depends, of course, 

 upon the degree of flattening undergone by the epithelial 

 cells. 



In the defaecating larva a third tracheal system develops 

 (fig. 86), in the form of a pair of outgrowths, ventrally from 

 near the posterior extremities of the main tracheal vessels. 

 They grow out very rapidly; an upper branch supplies the 

 intestine and neighbouring organs, while the main part of 

 the vessel grows downwards and forwards, and undergoing 



