EXCRETION 61 



carbonates will tend to precipitate. It is worth noting that carbon- 

 ates are present in the phytophagous Agromyzid Acidia, but not in 

 the parasitic genus Cryptochaetum; and they are said to be absent in 

 aquatic larvae from very acid waters. (It may be noted in passing that 

 similar deposits of lime ('calcospherites') may occur in the fat body 

 of some insects.) 



Excretion in Rhodnius 



Insects are essentially terrestrial animals. Consequently, the main 

 problem with which their excretory system is faced is the elimination 

 of all these substances, and particularly the elimination of nitrogen, 

 in such a way that they are able to conserve their limited supplies of 

 water; and it is instructive to consider the excretory system from this 

 point of view. It will be convenient to start by describing the excre- 

 tory system of the blood-sucking bug Rhodnius prolixus, which has 

 been studied in considerable detail, and then to compare with this 

 some other insects. 



The excretory system of Rhodnius consists of four very long 

 Malpighian tubes, opening into a large rectal sac (Fig. 9, A). Each 

 tube is made up of two distinct segments : a translucent upper seg- 

 ment, about two-thirds of the whole, the lumen of which contains 

 only a clear fluid, and an opaque white lower segment stuffed with 

 spheres of uric acid. The histological structure of these two segments 

 is quite different; the one changes abruptly into the other; and im- 

 mediately below the junction the granules of uric acid appear in the 

 lumen (Fig. 9, B). The explanation of these facts which has been 

 suggested is that the upper segment of the Malpighian tube is secret- 

 ing a solution of sodium or potassium acid urate, and the lower 

 segment reabsorbing both the water and a large proportion of the 

 base (sodium or potassium - perhaps in the form of bicarbonate) 

 leading to a precipitation of the free uric acid. For it can be shown 

 (i) that the contents of the upper segment OH = 7-2) are definitely 

 more alkaline than those of the lower segment OH = 6-6); (ii) that 

 neutral red added to the blood of the insect is secreted into the lumen 

 by the cells of the upper segment and reabsorbed again by the cells 

 of the lower segment; and (iii) that if two ligatures are applied to 

 the tube near its lower end at a time when the uric acid spheres have 



