422 ANNUAL REPORT SMITHSONIAN" INSTITUTION, 1931 



These experiments, and many others which were tried, indicate 

 that the oxygen present in the water in small bubbles may be ingested 

 by the larvae and may bring about a high death-rate. The larvae in 

 the oxygen-treated cylinders appeared irritable, and though their 

 intestines were filled with apparently normal food yet their growth 

 was slow and they gradually dropped to the bottom and died. There 

 was great difficulty in preventing the oxygen bubbles from accumu- 

 lating at the surface. Wliether or not larvae could break through 

 the surface film with their air-tubes remains unsolved. This may 

 prove the real reason for the death of the larvae. However, the 

 presence of the oxygen in the intestines (not proved, except by 

 inference) may interfere with the digestive functions and bring 

 about the death of the larvae. It may be recalled that Cleveland 

 (1925) brought about the death of termites by defaunating their 

 intestinal tracts by means of oxygen. In this case the oxygen killed 

 the commensals (Protozoa) which converted the eaten wood into 

 substances capable of being digested by the termites. AVhat oxygen 

 may do in the intestinal tracts of mosquito larvae remains to be 

 solved. 



In addition to the aquaria experiments with Ohara a long series of 

 tests was conducted with wooden tubs sunk in the ground (pi. 5, 

 fig. 1), with wooden tanks divided into compartments (pi. 6, 

 fig. A) so that there would be Chard growths separated from the 

 ordinary water by screens, and also to provide a method of testing 

 Chara in still as compared with running water. The results of 

 these experiments ^ were uniformly successful and gave additional 

 evidence that Chara fragilis in some way prevented the develoj^ment 

 of the larvae. 



The second problem, do females normally oviposit in pools con- 

 taining vigorous growths of Chara^ was also tested experimentally. 

 A series of aquaria filled with a growth of Chara fragilis was set 

 up in our greenhouse (pi. 4, fig. 1). An abundant supply of the 

 adults of two Culex species {C. p/'piens and C. territans), Anopheles 

 pu7ictipennis, and Aedes aegyjM (the yellow fever mosquito) was 

 at all times present in and about our experimental quarters. The 

 results of these experiments are shown in Table 3. 



*A full account of these experiments may be found in the Araer. Journ. Trop. Med., 

 vol. 9, pp. 249-266, 1922. 



