59 THE RECTAL PAPILLAE. 



The second set are the terminations of the main tracheee. After giving 

 off the lateral branches these run directly into the central cavity, where 

 they become tortuous, and anastomose with each other, giving off com- 

 paratively few small vessels in proportion to their size, and forming a 

 network which fills the central cavity, but none of their branches pierce 

 its investing membrane anywhere. Fig. 9 represents the central cavity 

 with its trachece, with a small portion of the lateral branches and their 

 terminations amongst the glandular structure. 



The gland cells are large about l-300th of an inch in diameter and 

 slightly angular by mutual pressure. Each contains a granular nucleus 

 about one-third the diameter of the cell. 



That these organs secrete uric acid seems probable both from 

 ^ their structure, and from the fact that uric acid is found in abun- 

 dance below the recto-colic valve, and not above it. 



I believe the central cavity is continuous with the visceral 

 cavity, and that it affords a means of bringing the circulating 

 fluid into almost immediate contact with the secreting cells, a 

 fine structure -less membrane only being interposed and still 

 further I believe I am justified in asserting that the circulating 

 fluid is expelled from, and a fresh supply is drawn into, the cen- 

 tral cavity by a rhythmic muscular act. 



In the female fly, the rectal papillae lie fortunately between the 

 second and third rings of the ovipositor when that organ is ex- 

 serted, where it is sufficiently transparent to allow of the papillae 

 being seen during life. I have repeatedly observed a movement 

 of the kind I have described. I believe this is the explanation 

 of the radiating bands of muscle at the base of the papillae. They 

 probably open the central cavity, at the same time pressing 

 upon the contents of the papilla, and, assisted by the muscular 

 wall of the rectum, which contracts at the same instant, not 

 only expel the contents of the central cavity, but also press the 

 secreted fluid through the minute pores in the calyx into the 

 cavity of the rectum. The central cavity is probably refilled by 

 the elasticity of the calyx itself. 



With regard to the urinary secretion, that which is passed by 

 the insect when it first emerges from the pupa case, is a semi-solid 



