66 INSECT PHYSIOLOGY 



Excretion in aquatic insects 



When insects come to live in fresh water they are no longer com- 

 pelled to be so careful of their water supply. Water is taken up in 

 quantity with the food, or it may enter through permeable regions of 

 the body surface, such as the 'anal papillae' of mosquito larvae; and 

 the Malpighian tubes produce a copious clear urine. In the mosquito 

 larva this does not appear to have resulted in any notable change in 

 nitrogen metabolism : the fat body may at times become laden with 

 uric acid, and if the uptake of water is restricted, as when the larva 

 is kept in slightly hypertonic salt water, solid deposits of uric acid 

 may appear in the lumen of the Malpighian tubes. On the other 

 hand, certain aquatic insects, such as Sialis, Trichoptera, Odonata, 

 Dytiscus, and Notonecta have become 'ammonotelic' in their larval 

 stages and eliminate almost all their nitrogen as ammonia. They 

 revert to a 'uricotelic' condition (uric acid excretion) during the 

 pupal stage in preparation for terrestrial life. 



Regulation of osmotic pressure and ionic composition of the haemo- 

 lymph 



Life in fresh water raises new problems in the physiology of 

 excretion. The continuous excretion of water is liable to deprive 

 the insect of essential ions such as chloride, potassium, and sodium. 

 Some aquatic larvae, such as Sialis, have a body covering which is 

 highly impermeable to salts and there is presumably a very active 

 recovery of salts in the rectum before the urine is discharged. Other 

 larvae, such as that of the mosquito Aedes, not only reabsorb 

 chloride, sodium and potassium through the columnar epithelium 

 lining the hind-gut, but they also have 'anal papillae' which can 

 absorb the traces of chloride, sodium and potassium found in fresh 

 water and so maintain actively the normal ionic composition of the 

 blood. The 'gills' of many other aquatic insect larvae have similar 

 functions. 



In terrestrial insects (Tenebrio, Schistocerca, Caliiphora, &c.) an 

 active reabsorption of water and of inorganic ions by the rectal 

 glands fulfils the same need to keep constant the osmotic pressure 

 and the ionic composition of the haemolymph. 



