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 larve can live and pools and tanks covered in this way 
will be found quite free from larva. Anopheles larve placed in a bowl of water 
covered with a layer of Wolffia 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 larvee the more 
quickly they die, as young larve can pass their tails between the plant grains and 
so get access to the air. Culex larve and Stegomyia larve are also destroyed, 
but the latter live for several hours in the weed-covered water. Where the con- 
tinuity of the layer of Wolffia arhiza is broken by the presence of grass or reeds 
or large leaved weeds, mosquito larve 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 larve, and so is the Nakhoda tank and several pools 
near Parel.” 
Recently 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 Reports,” of the 
Bureau of Manufactures, U. 8S. Department of Commerce and Labor, for 
March, 1909, in a dispatch from Consul-General Richard 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 larve 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 Rhine, 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 
