418 MOSQUITOES OF NORTH AMEEICA 



A short statement regarding the practical use of water plants occurs on pp. 1 

 and 3 of the 4th volume of Theobald's Monograph of the Culicida of the World. 

 This statement may 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 keep- 

 ing mosquitoes from laying their eggs on water. He shows tliat tanks covered 

 with this green flat weed never contain larvse of Culiddce, 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 larvae 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. 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 

 larvse of Anopheles hifurcatus 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 larvse in natural and artificial 

 collections of water. 



" The little Lemna arrhiza, or the Eootless 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 Mosquito and House 

 Fly," published in Dragon-Flies vs. Mosquitoes (The Lambom Prize Essays, 

 N'ew York, 1890). Mr. Beutenmiiller states that Mr. L. P. Gratacap, of the 

 American Museum of Natural History, suggested the increase of fresh-water 

 algae as deterring the progress of mosquito larvae 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 

 algae, which, in such rod-like forms as Oscillatoria, will float through the Avater 

 and by its intermixed and diffused stipes embarrass the development and move- 

 ments of the mosquito larvae." In fact, mosquito larvae are frequently found in 

 the presence of algae and it is well known that Anopheles larvae thrive where algae 

 are abundant. One of us (Knab) has found larvae of Culex tarsalis abundant 

 among a dense growth of algae and larvae of Uranotcenia 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 : 



