DETEREENT TREES AND PLANTS. 29 



graph of the Culicidse of the World." This statement may well be 

 quoted : 



Major Adie, I. M. S. (Ind. Med. Gaz., xxxix, June, No. 6, 1904), brings considerable 

 evidence to bear on the benefit of Lemna minor as a means of keeping mosquitoes from 

 laying their eggs on water. He shows that tanks covered with this green flat weed 

 never contain larvae of Culicidse, 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 enclosed 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 larvse 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 to me. ^^^ly not deliberately promote the growth of Lemna minor in all 

 unavoidable collections of water to prevent the propagation of mosquitoes?" 



This same green plant grows freely in England, and I have noticed a similar occur- 

 rence here. A pond close to my house was frequented by numbers of the larvae 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 

 larvae 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 larvae 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 L. minor. 



An early suggestion as to the practical use of water plants occurs 

 in Mr. William Beutenmiiller's essay on the ''Destruction of the mos- 

 quito and house fly," published in Dragon-Flies v. Mosquitoes, The 

 Lamborn Prize Essays, New York, 1890. Mr. Beutenmtiller states 

 that Mr. L. P. Gratacap, of the American Museum of Natural His- 

 tory, suggests the increase of fresh-water alg^ as deterring the 

 progress of mosquito larvae in the water and as affecting their destruc- 

 tion 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 number of fronds of Oscillatoria in the Central 

 Park lakes, in New York City, have had a deterrent effect on the 

 propagation of mosquitoes in the lakes. A largely disseminated 

 mass of algae floating through the water by its intermixed and dif- 

 fused stipes he thought would seriously embarrass the development 

 and movements of the mosquito larvae. 



The duckweeds were considered by Dr. H. P. Johnson in an appen- 

 dix to Smith's New Jersey Report for 1902,^ and by virtue of the 

 actual small-scale experiment tried, these observations are printed in 

 full. 



While most forms of aquatic vegetation promote the breeding of mosquitoes, the 

 Lemnaceae, or duckweeds, are unfavorable, and in many waters almost or even wholly 

 prevent it. These tiny plants consist merely of a floating frond, resembling a minia- 

 ture lily-pad. It is circular or more frequently lobate;! and three to six millimeters 



a Rep. Ent. Dept. N. J. Agr. Coll. Exp. Sta. f. 1902, pp. 565-566. 



