418 MOSQUITOES OF NORTH AMERICA 
A short statement regarding the practical use of water plants occurs on pp. 1 
and 2 of the 4th volume of Theobald’s Monograph of the Culicide of the World. 
This statement may be quoted: 
“Major Adie, I. M. 8. (‘ Ind. Med. Gaz., xxxix, June, No. 6, 1904), brings 
considerable evidence to bear on the benefit of Lemna minor as a means of keep- 
ing mosquitoes from laying their eggs on water. He shows that tanks covered 
with this green flat weed never contain larve of Culicide, whilst others at the 
same time of year are full of them. 
“ As a test he ‘cleared certain areas near the banks of all Lemna and en- 
closed them with light floating structures, which were fixed enough to resist the 
winds—in fact made experimental pools. I was pleased,’ he says, ‘ to find in due 
time plenty of Anopheles larvee in these pools. This seemed to prove that Lemna 
acts as a mechanical obstruction to the process of egg-laying, and a very obvious 
method of prevention occurred tome. Why not deliberately promote the growth 
of Lemna minor in all unavoidable collections of water to prevent the propaga- 
tion of mosquitoes ?” 
“ This same green plant grows freely in England, and I have noticed a similar 
occurrence here. A pond close to my house was frequented by numbers of the 
larvee of Anopheles bifurcatus and A. maculipennis every year. Two years ago 
its surface became smothered with Lemna minor, Linn., and Lemna arrhiza, 
Linn., no Anopheline larve could then be found. As this was the only breeding 
ground near, both species have practically died out. 
“This small yet widely distributed genus of floating plants evidently has a 
very marked effect upon the frequence of Culicid larve in natural and artificial 
collections of water. 
“The little Lemna arrhiza, or the Rootless Duckweed, occurs in Asia, Africa, 
South America and Europe, and apparently has the same effect as the larger 
LL. minor.” 
An early suggestion as to the practical use of water plants occurs in Mr. 
William Beutenmiiller’s essay on the “ Destruction of the Mosquito and House 
Fly,” published in Dragon-Flies vs. Mosquitoes (The Lamborn Prize Essays, 
New York, 1890). Mr. Beutenmiiller states that Mr. L. P. Gratacap, of the 
American Museum of Natural History, suggested the increase of fresh-water 
algee as deterring the progress of mosquito larve in the water and as effecting 
their destruction before they can rise to the surface of the water to breathe. Mr. 
Beutenmiiller, considering the suggestion important, stated that he believed 
“that the vast numbers of the fronds of Oscillatoria in the Central Park lakes 
[in New York City] have had a deterrent effect upon the propagation of mos- 
quitoes in those localities. The requisition here is a largely disseminated mass of 
alge, which, in such rod-like forms as Oscillatoria, will float through the water 
and by its intermixed and diffused stipes embarrass the development and move- 
ments of the mosquito larve.” In fact, mosquito larve are frequently found in 
the presence of alge and it is well known that Anopheles larvex thrive where algez 
are abundant. One of us (Knab) has found larve of Culex tarsalis abundant 
among a dense growth of alge and larve of Uranotenia under the same cir- 
cumstances. 
The value of duckweeds was considered by Dr. H. P. Johnson in an appendix 
to Smith’s New Jersey Report for 1902, and by virtue of the actual small-scale 
experiment tried, these observations are printed in full: 
