INFLUENCE OF ENVIRONMENT 237 



The intestines were filled with brown desiccated matter from the bottom 

 of the pond. Both young and old larvae had about the same sort of in- 

 testinal contents, and the evidence seems to be that food is taken at 

 random. If undisturbed the larvae would constantly move their mouth 

 parts, creating a current in the water. The operation is continuous ex- 

 cept when some large particle is seized, when the movement of the mouth 

 parts is stopped long enough for the particle to be chewed and swal- 

 lowed or else cast aside. It seems therefore that food is taken as it passes 

 the mouth at random ; that part which can be crushed is crushed, but 

 small cysts are apt to pass through unharmed. The latter might happen 

 with small flagellates and diatoms if the available food is abundant, 

 otherwise the protoplasm is simply digested out of the shells of the dia- 

 toms and the small flagellates digested beyond recognition. 



"It is conceivable that the kind of organisms present in the water is 

 dependent upon the chemical constitution of the water. A comparison of 

 chemical analyses of the different samples (Table IV) shows that in the 

 water where C. pipiens were breeding 219 parts per million of CO3 as 

 CaCOs and ninety-one parts per million of CI were present, while in the 

 sample where C. pipiens and A. canadensis larvae had their habitat, 

 practically no CI or CO3 was found. However, the former was distinctly 

 alkaline (pH 7.5) and the latter distinctly acid (pH 6.1). In the latter 

 case the water was collected from a small brooklet in a small patch of 

 woods, and plenty of decaying leaves and small branches of the oak 

 trees standing thereby had dropped in the brooklet. The water was col- 

 ored, indicating possible presence of tannins, but unfortunately no test 

 for tannin was made at the time. 



"In the case where A. vexans was breeding, 132 parts per million of 

 CO3, with no CI, was present, the sample being distinctly alkaline (pH 

 8.0). In the water where P. ferox was found appreciable amounts of Al, 

 Fe, SO4 were present; also some CI. The SO4 amounted to more than 

 0.25 per cent. Nearly all the iron present was in the ferrous state. The 

 water was yellow, and although no sediment was present, the guts of the 

 larvae contained large quantities of ferric hydroxide. It is very likely 

 that the soluble iron was precipitated by the fluids secreted from the 

 larval bodies into their guts. The remarkable thing was that these lar- 

 vae were able to live and survive in this strongly acid water. The total 

 acidity found by titration was 836 parts per million, and the hydrogen- 

 ion concentration proved to be pH 2.9. 



"In the pool from which no breeding has been reported for several 

 years the food supply was plentiful, and nothing indicates from the 

 chemical analyses why no breeding should occur. 



