PLANTS AND MOSQUITO CONTROL MATHESON 



421 



Chara pond. This investigation "^ soon showed a greater variety and 

 a much higher density of plankton organisms in our Chara pond 

 and aquaria than that found in a typical mosquito-breeding pool. 

 If plankton organisms are the food of mosquito larvae, here then 

 was an abundant food supply and yet they died amidst plenty. 

 However, I have never fully believed that plankton organisms con- 

 stitute the entire larval food but that the larvae probably obtain 

 most of their food from substances in solution in the water and 

 from decaying organic wastes. My assistant was in the midst of test- 

 ing this theory, and his preliminary results indicated that my thesis 

 held true for the si^ecies with which he was experimenting.' If 

 plankton does not constitute the main food of mosquito larvae but 

 rather substances in solution and organic wastes, then the question is 

 does the Chara remove these substances in solution so rapidly as to 

 starve the larvae? Decaying organic matter is largely absent in 

 Chara ponds, as one of their marked characteristics is the crystalline 

 clarity of the water. I could find no way to test all these suppositions 

 experimentally. 



There was still another factor which always excited my curiosity, 

 more especially since the publication of Cleveland's work in defaun- 

 ating the intestinal tract of termites with oxygen. As Chara gives 

 off during the day an immense number of tiny bubbles of oxygen 

 I felt fully convinced that the larvae must sweep hundreds of them 

 into their intestines. What effect could the presence of oxygen 

 have on the digestive processes of the larvae ? To test this I devised 

 a rather simple apparatus. Pure oxygen was passed, under pres- 

 sure, through Berkefeld filters N and W and entered the water in 

 bubbles almost as tiny as those given off by the Chojra. Plate 1, Fig- 

 ure 1 shows the apparatus in operation. Two cylinders are being 

 treated with oxygen while two others serve as controls. The same 

 water and food were supplied to all the jars. This experiment was 

 conducted for a considerable time and a brief summary is presented 

 in Table 2. 



Table 2. — Tests with vxygen 



" The results were published in detail in the American Journal of Hygiene, vol. 11, 

 pp. 174-188, 1930. 



'Full account by Ilinman, Amor. Journ. Hygiene, vol. 12, pp. 238-270, 19.'{0. 



