Nitrogen Excretion 



199 



Fig. 34. Uric acid in mg./gm. dry wt. of nephridium of snails. Modified 

 from Needham." (s) summer, (w) winter. 



is significant, and two aquatic species excrete some uric acid (Table 30). 

 Needham argues that the mode of excretion is related to the water supply in 

 embryonic life; since turtles usually lay their eggs in moist sand where diffu- 

 sion of NH3 would be too slow to avoid toxicity and yet where storage of uric 

 acid is unnecessary, urea is sufficiently nontoxic. Other reptiles are uricotelic 

 (see below); hence a careful correlation of nitrogen excretion with life history 

 in turtles might throw light on the origin of uricotelism in vertebrates. 



Uricotelic Animals: Insects. Most insects are too restricted in their water 

 supply tor them to excrete ammonia, or even urea, in quantity. The urine 

 formed in the malpighian tubules of insects is rich in uric acid and may even 

 be a mass of uric acid crystals. '"^ Negligible amounts of nitrogen are lost in 

 other forms by adult insects. In the bug Rhodnius, for example, immediately 

 after a blood meal there is considerable urea; later the uric acid is greatly in 

 excess. ■*'' Blowfly larvae which are eating meat give off much ammonia; this 

 is probably formed in the gut and passed out with feces, and hence is not a 

 true metabolic excretory product. At pupation ammonia excretion stops and 

 uric acid becomes the main product. Some of the uric acid from protein break- 

 down is converted to allantoin (Table 30). '^' ^" Insects rid themselves of 

 much waste material at the time of molting. 



