Dr Davy on the Excrements of certain Insects. 233 



so dreaded, so devastating at the moment, may, in their after 

 result, by manuring an exhausted soil, be really beneficial. 

 Whether grubs and caterpillars may also be considered as fer- 

 tilizers by their urinary secretion, remains to be deteraiined 

 by experiment. The instance of the silk-worm is in favour 

 of the conclusion that they are. I have found the urinary ex- 

 crement of another species of large caterpillar, a native of this 

 island, to abound in lithic acid. I shall endeavour to examine 

 more ; and I venture to express the hope, that some inquirer 

 at home may be induced to make similar trials of the excre- 

 ments of the British species, which are so common and abun- 

 dant. 



As there is variety in the composition of the urinary secre- 

 tion of the mammalia, so, probably, if extensive and minute 

 inquiry be instituted into the nature of this secretion in insects 

 (supposing very many of them to possess urinary organs), dif- 

 ferences also of composition will be detected in it. In the in- 

 stance of the common fly, which, in Barbadoes, seems to be 

 the same species as in England, I have not found lithic acid 

 in the urinary excrement, but a substance having rather the 

 properties of urea ; and I have found the same in the liquid 

 excrement of a bee resembling our humble bee.* Should 

 farther trials prove that the urinary organs of some insects 

 secrete urea, it need excite no surprise, considering how nearly 

 urea and lithic acid are allied, — and that if they have not a 

 common base, they differ but little in their composition, and 

 that one is often vicarious of the other. It may be conjectured, 

 too, that they may be found co-existing — a mixed secretion, as 

 in the instance of some of the vertebrata, especially reptiles. 

 I may mention an observation seemingly in favour of this idea, 

 made on a young wasp. Shortly after quitting its cell, it 

 voided a mass of excrementitious matter, quite a cast of its dis- 

 tended intestine ; one portion of this, the faecal part, corres- 

 ponding to meconium, was dark, almost black, and seemed to 

 consist chiefly of thick mucus, coloured by bile, and had an 



* Since the above was written, I found the excrement of another 

 similar insect, voided, when confined under glass, to be composed chiefly 

 of lithic acid and phosphate of lime ; the latter in the largest proportion. 

 VOL. XL. NO. LXXX.— APRIL 1846. Q 



