418 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 papille 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 
Fic. 52.—The rectal papilla. z. The rectal pouch, showing the four rectal papillee. 
2, A radial section through the axis of one of the papilla. 3. A transverse 
section through the same, more highly magnified. 4. The apex of the outer 
sheath, showing the cuticular setae. 
e, External, and ¢, internal sheath ; , Muscle ; and /, Trachez. 
1869, and Chun published a valuable monograph upon the 
rectal papille of insects in 1876 [164]. 
The External Sheath is plicated longitudinally, and is covered 
by a number of chitinous sete, 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 setz and their relation to the openings or pores in the 
* ‘Mem. du Muséum,’ tom. xx. 
