420 THE ALIMENTARY CANAL OF THE IMAGO. 
importance in relation to the physiology of the tracheal 
system. 
The Secreting Cells—tThese are the largest cells in the imago. 
Each cell contains a giant nucleus. The protoplasm is very 
reticular towards the external part of the papilla, and stains 
feebly ; towards the central cavity and the adjacent edges of the 
cells it stains more deeply, and the reticulations are much less 
distinct. The part of the cell towards the external sheath 
appears to be vacuolated by drops of secretion. 
The Cavity of the papilla has a feeble reticular connective 
tissue uniting the trachez with the internal sheath. It contains 
leucocytes (blood corpuscles) and blood. 
Nerves.—Chun and Leydig both describe a large nerve which 
enters the papilla. I can, however, trace no nerve into the 
papilla, but the thoracic ganglion gives off a median nerve 
from its posterior extremity, which extends to the posterior 
part of the abdomen and divides into two branches. These 
run one on either side of the rectum, and give branches to its 
muscular coat; they also supply the generative organs. Each 
gives several branches to the radiating muscles of the rectal 
papilla. The branches of these nerves have no sheath, and 
resemble the visceral nerves of the insect in other parts 
(Fig. 52, 7). 
Morphology.—There cannot be the slightest doubt that the 
rectal papillz are developed from the epithelium of the proc- 
todeum. In the young stage of the pupa they first appear as 
a thickening of this epithelium. In many insects they retain 
this character, and have neither an internal cavity nor a 
muscular base. Moreover, they are identical in structure with 
the so-called rectal gills of the Dragon-flies (see Chun’s descrip- 
tion [164]). 
Function.—Authorities are divided with regard to the function 
of the rectal papilla. They are universally regarded as respi- 
ratory in the Dragon-flies (Libellula), and I think it in- 
dubitable that they have a secondary respiratory function in 
these insects, as the tracheal tuft is prolonged beyond the 
secreting epithelium into a whip-like process, which is con- 
