62 



INSECT PHYSIOLOGY 



fig. 9. — A, excretory system of Rhodnius: schematic; B, detail of Mal- 

 pighian tubes at junction of upper and lower segments; arrows indi- 

 cate the circulation of water and base; C, two ligatures applied to 

 Malpighian tube three hours after meal ; D, the same 24 hours later. 

 (Modified after Wigglesworth) 



Is, lower segment of Malpighian tube: lumen full of uric acid spheres; nig, mid-gut; 

 r, rectum ; rg, rectal gland ; us, upper segment of tube 



been washed out by a recent meal, then uric acid appears above the 

 upper ligature and below 1 the lower ligature, but between the liga- 

 tures there is no uric acid, nor is there any distension of the tube 

 (Fig. 9, C, D). 



Thus it is suggested that, in Rhodnius, there is a continuous circu- 

 lation through the excretory system of both water and of base, the 

 same water and base being used over and over again to carry uric 

 acid from the body. 



It is not unlikely that the rectum, also, assists in the process of 

 reabsorbing water; for around the point of entry of the Malphigian 

 tubes there is a ring of large epithelial cells constituting a so-called 

 rectal gland; and we shall find that there is evidence in other insects 

 that the function of the rectal glands is the absorption of water from 

 the excrement. Simultaneous measurements of the concentration of 



1 This has doubtless been derived from the other Malpighian tubes, and 

 entered from below. 



