EXCRETION 63 



ions in the haemolymph and in the lumen of different parts of the 

 excretory system in Rhodnius have shown that there is in fact a 

 selective excretion of potassium into the lumen of the upper segment 

 of the tubule and a reabsorption of potassium from the lower seg- 

 ment; water and sodium, but not potassium, being reabsorbed in 

 the rectum during the later stages of excretion. 



Rhodnius takes in very large meals of blood (p. 85) and like most 

 blood-sucking insects immediately proceeds to excrete the greater 

 part of the water and salts. This intense 'diuresis' is brought about 

 by a hormone produced by neurosecretory cells in the brain and, 

 particularly, in the fused ganglia of the thorax. The mere injection 

 of saline into the body cavity of Rhodnius does not induce a rapid 

 flow of urine in this way. 



Excretion in other insects 



This conception of a circulation of water through the excretory 

 system is a familiar one in the physiology of birds, and reptiles, and 

 mammals, where the renal tubule itself, the cloaca, and the large 

 bowel all take part in the reabsorption of water. This idea, intro- 

 duced into the physiology of insects, helps to explain many of the 

 properties of their excretory system. 



The simplest system that occurs is that shown in Fig. 10, A. Here 

 the Malpighian tubes are composed of a single type of cell, and con- 

 tain only fluid. As this fluid, mixed with the intestinal contents, 

 passes down the hind-gut, water is absorbed from it - first by the 

 cubical epithelium of the intestine and- finally by the epithelium of 

 the rectum. The material is retained in the rectum until it has been 

 converted into a more or less dry pellet. In some cases, for instance in 

 the human louse Pediculus, the contents of the mid-gut may be re- 

 tained in the stomach so that the hind-gut contains only urine ; under 

 these circumstances, a little pellet of solid uric acid may be formed 

 in the region of the rectal glands, clearly demonstrating the absorp- 

 tion of water by these organs. This arrangement is found in Lepisma, 

 Dermaptera, Orthoptera, Neuroptera, and many beetles. In some 

 cases the active epithelial cells of the rectum are collected into com- 

 pact areas, the 'rectal glands'; in other cases they are evenly spread 

 over the gut wall. 



