420 THE ALIMENTARY CANAL OF THE IMAGO. 



importance in relation to the physiology of the tracheal 

 system. 



The Secreting Cells. These 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 tracheae 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 

 papillae. The branches of these nerves have no sheath, and 

 resemble the visceral nerves of the insect in other parts 

 (Fig. 52, /). 



Morphology. There cannot be the slightest doubt that the 

 rectal papillae are developed from the epithelium of the proc- 

 todaeum. 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 papillae. They are universally regarded as respi- 

 ratory in the Dragon - flies (Libcllula), 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- 



