420 MOSQUITOES OF NORTH AMERICA 



grains float on the surface of the water just like a scum of bright green seeds. 

 When it is growing in large amount so as to form a continuous layer on the top 

 of the water no mosquito larvse can live and pools and tanks covered in this way 

 will be found quite free from larvse. Anopheles larvae placed in a bowl of water 

 covered with a layer of Wolfjia arhiza immediately assume an almost vertical 

 position, hanging head downwards in a manner quite unnatural to them. After 

 a short interval they sink to the bottom dead. The larger the larvge the more 

 quickly they die, as young larvas can pass their tails between the plant grains and 

 so get access to the air. Culex larvas and Stegomyia lar^^se are also destroyed, 

 but the latter live for several hours in the weed-covered water. Where the con- 

 tinuity of the layer of Wolfp.a arhiza is broken by the presence of grass or reeds 

 or large leaved weeds, mosquito larvae are enabled to exist without difficulty, and 

 it would appear therefore that before this plant could be used with success as a 

 protection for tanks, the latter would have to be cleared of other growths. The 

 round tank at the Mint which is usually covered with a good growth of Wolffia 

 arhiza is free of mosquito larvae, and so is the Nakhoda tank and several pools 

 near Parel." 



Eecently the idea of cultivating such aquatic plants with practical ends in 

 view in regard to anti-mosquito work has been taken up in German colonies in 

 Africa, It is stated in the " Monthly Consular and Trade Eeports," of the 

 Bureau of Manufactures, U. S. Department of Commerce and Labor, for 

 March, 1909, in a dispatch from Consul-General Eichard Guenther, of Frank- 

 fort, that the director of fisheries at Biebrich, Dr. Bartmann, had found a duck- 

 weed-like plant of the genus Azolla to be especially well adapted to this use ; and 

 at the instance of Dr. Bartmann experiments were made at the malaria station at 

 Wilhelmshaven. It was found that in a short time the growth of the plant 

 covered the experimental waters with a layer of about 6 centimeters ; this suffo- 

 cated all the mosquito larvae below and prevented the parent insects from de- 

 positing eggs in the water, Consul-General Guenther states that several years 

 ago Director Bartmann communicated this method to the Mosquito-Destroying 

 Commission at Eltville on the Ehine, which has used it repeatedly with good 

 success. 



One of the officials who took part in the question of mosquito extermination in 

 the German African colonies was far from enthusiastic regarding the practical 

 use of this plant, although it had been advertised on all sides in Europe and in 

 this country. In his opinion, it may possibly be of some use in special places, 

 but, so far as experiments have gone, it appears that the plants will not grow in 

 dense or even moderate shade and therefore they are of no use in the tropical 

 forests where there are large and small pools of water, the very places where it is 

 most needed. Moreover, the Azolla plants do not stand any great cold nor do 

 they stand short seasons, for which reasons their use is excluded from highland 

 and northern regions. Further, they will not grow in brackish water and can 

 not be utilized along sea-coasts, and, still further, in case of drouth they all 

 perish and thus necessitate the restocking of pools and swamps that have 

 been dry. 



So positive were the statements published in the United States, as to the 

 value of the Azolla plant to prevent mosquito-breeding, that one species was 



