EXCRETION 65 



clear fluid but the rectum contains an almost dry powder (which is 

 in equilibrium with an atmosphere of 75 per cent, relative humidity, 

 equivalent to supersaturated sodium chloride); the 'perinephric space' 

 around the rectum contains potassium chloride at a concentration 

 exceeding 2M ( A = 8° C). The haemolymph has a freezing-point 

 depression of A = 0-75° C increasing to 1-5° C under dry conditions. 

 There is clearly a steep gradient of increasing concentration in pas- 

 sing from the haemolymph to the lumen of the rectum. 



A similar arrangement is met with in the larvae of Lepidoptera 

 (Fig. 10, C). But here the lower part of each Malpighian tube con- 

 tains solid granules of uric acid, oxalate, &c. ; and this suggests that, 

 as in RJiodnius, the Malpighian tubes themselves assist in reabsorbing 

 water. 



In many Diptera (the mosquito, Muscid flies, &c), the Mal- 

 pighian tubes may contain solid uric acid throughout their length 

 (Fig. 10, D). The tubes consist apparently of but a single type of cell, 

 so that the mechanism for the precipitation of uric acid must be 

 different from that in Rhodnius. In the rectum, a further absorption 

 of water probably occurs ; but these insects lack the power of really 

 drying the excrement. Conditions are somewhat similar in certain 

 Hymenoptera. In other Hymenoptera, the excretory system is like 

 that of the flea (Fig. 10, E). It never contains any solid uric acid 

 because sufficient water is retained in the capacious rectum to keep it 

 in solution ; but it is none the less probable that this fluid and also 

 salts are reabsorbed, and circulate again through the system. 



Finally, in those many dipterous larvae in which the Malpighian 

 tubes contain carbonates, these are generally confined to the upper 

 part of the tube (Fig. 10, F); but the mechanism of their secretion is 

 unknown. 



These are just a few of the arrangements of the Malpighian tubes 

 and hind-gut, which will serve to illustrate the general principles of 

 excretion in insects so far as these are known at present; many other 

 arrangements exist. And it should be pointed out that many other 

 hypotheses as to the function of the rectal glands of insects have 

 been put forward from time to time. Some of these are certainly in- 

 correct; but others are probably subsidiary functions in particular 

 species. 



