18 Dr Davy on the 



a reddish hue, it was found to be owing to the presence of a 

 little purpurate of ammonia. Under the microscope, crystals 

 were detected in it also of lithic acid. 



The excrement of the hawk-moths on quitting their pu- 

 paria, was a turbid fluid of a reddish-brown colour, with a 

 sediment of a fawn colour. The sediment, which was very 

 abundant, consisted chiefly of granules of from to o o o to ts o o iy 

 of an inch in diameter, as seen under the microscope, and 

 was composed of lithate of ammonia. In the turbid fluid 

 hippuric acid was detected, and also a trace of purpurate of 

 ammonia, and of lithate of ammonia. 



Urea was sought for both in the excrement of the papi- 

 lio and sphinx caterpillars, and in that voided on the acquir- 

 ing of the imago state, but without well-marked results ; in 

 one or two instances, there were appearances rather indica- 

 tive of its presence ; and I think it probable, that were larger 

 quantities of the excreted matter to be examined than I 

 had an opportunity to collect, it would be found to be a con- 

 stituent part, at least occasionally. 



The existence of hippuric acid in the urine of the leaf-eat- 

 ing caterpillars may be considered as pointing to an analogy 

 between the secretion in them and in the herbivorous mam- 

 malia. The quantity of urinary excrement formed in the 

 pupa stage of the insects under consideration, and voided on 

 their quitting this stage, is remarkably lai'ge. It may be 

 conjectured to be derived from elements obtained from cer- 

 tain organs of the caterpillar, in its state of transition ; and 

 the large quantity of nitrogenous matter, especially of lithic 

 acid which it contains, seems in favour of this conclusioji. 



This composition of the excrement of catei'pillars, and of 

 that voided in the assuming of the perfect form of insect, 

 leads, in considering what pai't they may perform in the econo- 

 my of Nature, to the conclusion, that they are not altogether 

 destructive, and that, on the whole, they may be more use- 

 ful to the vege!,able kingdom than injurious ; as by manuring 

 where they have depastured ; and by feeding on the leaves of 

 some plants, as they commonly do in preference to others, 

 (one species of caterpillar mostly choosing for its food the 

 leaves of only one species of plant), checking thereby the ex- 



