420 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 31 



stream was used both in tEe Chara aquaria and in the controls. In 

 the controls the larvae seemed to develop normally so whatever food 

 was necessary was present in the water added to the Chara aquaria. 

 The results of these experiments are briefly summarized in Table 1.° 



Table 1. — Results of Chara experiments 



Species 



Aedes vexnns 



Culex pipiens 



Culex territans 



Anopheles punctipennis 



Aedes canadensis 



Aedes stimulans - 



Total 



Num- 

 ber of 



larvae 



2,500 

 4,500 

 1,100 

 146 

 800 

 1,250 



10, 296 



Stages of larvae 



Second to fourth 



do -- 



Various 



do - 



Young 



Various 



Time required 



2 to 14 days. 

 Do. 



2 to 15 days. 



3 to 25 days. 



4 to 10 days. 

 6 to 17 days. 



The table is self-explanatory. Not a large variety of species was 

 tested in the experiments, but out of 10,296 larvae there was an 

 emergence of only 897 adults. In the controls our emergence records 

 show that nearlj?^ all the larvae produced adults. What, then, is the 

 factor or factors that prevented these larvae from completing their 

 development ? I thought at first that the plant might produce some 

 lethal substance, judging from the rapid death rate of the larvae. 

 This, however, seemed to be ruled out, as a great abundance of Cy- 

 clops, Daphnia, and other small Crustacea, as well as numerous phy- 

 toplankton, thrived in the experimental aquaria. A factor experi- 

 mented with was the pH values (acidity versus alkalinity). The ex- 

 perimental aquaria and our Chara pounds showed a wide daily cycle 

 of pH values, usually running from a pH of 7.6 to nearly 9.4 each 

 day. The water was always alkaline and following MacGregor 

 (1921) and Senior-Wliite (1926) I thought the changing alkalinity 

 might account for the high death rate. Further experiments con- 

 firmed the results of other workers that pH values have probably 

 little to do with inhibiting larval development. The next factor 

 was the study of the food requirements of the larvae. Could it be 

 possible that Chara ponds and our Chara aquaria did not possess 

 sufficient food for the development of the larvae? 



An intensive search of the literature showed that the main larval 

 food consists of the zooplankton and phytoplankton organisms found 

 in the water. These are swept in by the action of the larval mouth 

 brushes, and everything that can be swallowed is taken in indis- 

 criminately. Along with these substances is swept in a considerable 

 amount of water. Fortunately I had begun an intensive study of the 

 plankton of a typical mosquito-breeding pool and the spring-fed 



Full details may be found in the Amor. Journ. Hygiene, vol. 8, pp. 279-292, 1928. 



