Urinary Excrement of Insects. 19 



tension of one kind, and favouring the growth of other kinds. 

 In illustration of their excrement acting as manure, I may 

 mention that I have seen in this island, a field of many acres 

 of sweet potatoes that was laid bare in a night by the inva- 

 sion of caterpillars ; in the morning scarcely a leaf was left, 

 and the caterpillars had disappeared ; but they had deposited 

 where they had thus voraciously fed, abundance of their ex- 

 crement, almost darkening the ground ; and shortly, the plants 

 vegetated afi'esh and vigorously, and a good crop of roots 

 was obtained. The illustration of their promoting the 

 growth of various species of plants commingled, is best wit- 

 nessed in flower-beds, and in wild natui'e, especially within the 

 tropics, where, under favourable circumstances, the powers 

 of vegetation are so great, and where, without some check, 

 such as the one alluded to, the plants of most rapid and vigor- 

 ous growth would deprive all others, feebler gi'owing, near 

 them, of nourishment, and would starve them to death. 



I have endeavoured to detect the urinary organs in the 

 catei'pillars of the hawk-moths. Their large size was favour- 

 able to the inquiry ; but I cannot say that I have been per- 

 fectly successful. On each side of the intestinal canal, there 

 is a large quantity of yellowish matter, in which, examined 

 under the microscope, ai'e to be seen innumerable minute 

 tubes, some of them, no doubt, tracheae, others probably ovi- 

 ducts, and some I apprehend performing the function of se- 

 creting urine. I am led to this conclusion in consequence of 

 finding that by digesting the yellow matter in very dilute ni- 

 tric acid, traces of lithic acid are obtained in evaporating the 

 solution formed, and heating the residue. A small glandular 

 mass near the anal extremity of the intestinal tube, whicli I 

 suspected might be the urinary organ, similarly treated, yield- 

 ing only negative results. 



IUrbadoes, March 8, 1848. 



