4i8 



THE ALIMENTARY CANAL OF THE IMAGO. 



a large tracheal vessel, after dividing into several branches, 

 enters the papilla and forms a tuft of anastomosing vessels in 

 its interior, from which numerous capillaries pass outwards 

 through the inner sheath, and ramify and form a network 

 between the epithelial cells. 



Historical. Although the rectal papillae of several insects 

 were known to and figured by Swammerdam [4] , and attracted 

 the attention of Lyonet * and Treviranus [161], the first 

 description of their structure in the Blow-fly appeared in 

 Leydig's text-book on histology in 1857. I described them in 



FIG. 52. The rectal papilla;. /. The rectal pouch, showing the four rectal papilla-. 

 2. A radial section through the axis of one of the papilla;, j. A transverse 

 section through the same, more highly magnified. 4. The apex of the outer 

 sheath, showing the cuticular setoe. 



e, External, and /, internal sheath ; ;//, Muscle ; and tr, Tracheae. 



1869, an d Chun published a valuable monograph upon the 

 rectal papillae of insects in 1876 [164]. 



The External Sheath is plicated longitudinally, and is covered 

 by a number of chitinous setae, which are exceedingly numerous 

 towards the apex of the papilla. These were described by 

 Leydig, and also by Chun, who failed to observe the grouping 

 of these setae and their relation to the openings or pores in the 



* ' Mem. clu Museum,' tom. xx. 



