30 PREVENTIVE AND REMEDIAL WORK AGAINST MOSQUITOES. 



in diameter. From the under surface hang one or more roots, which never fasten in 

 the soil, but derive their nourishment from the water. Its reproduction, mainly by 

 division of the frond, is so rapid that in a short time (usually before July 1) it com- 

 pletely mantles quiet waters, notably sheltered ponds and ditches without percepti- 

 ble flow. Its extraordinary abundance, often covering whole acres of shallow water, 

 makes it an efficient protection from mosquito breeding. Wherever this plant forms 

 a complete covering no larvae have been found. Such places should never be treated 

 with oil, for nature has provided a far more lasting and equally effective protection. 

 It is probably impossible for a mosquito to lay her eggs on lemna-covered water. 

 Even should larvae wander in from adjacent waters, they would be unable to reach 

 the surface for air, and would thus soon become asphyxiated. Larvae of Culex pun- 

 gens, injected by means of a pipette beneath the lemna in the jar * * -5^ died in 

 less than an hour. Where the lemna mantle is not complete, but presents interspaces 

 of open water, larvae of both Culex and Anopheles will usually be found in small 

 numbers only, for lemna waters are apt to harbor the various predaceous water-bugs 

 in great numbers. 



In considering these duckweeds it should be pointed out that mos- 

 quito larvse other than Anopheles are often found in waters well 

 covered by them. Both Dr. H. G. Dyar and Mr. Fpederick Knab 

 have made this observation. 



SMUDGES AND FTJMIGANTS. 



Hunters and campers have been in the habit of using almost any- 

 thing that will make a dense smoke as a smudge to drive away mos- 

 quitoes. In Bermuda, fresh cascarilla bark is burned for this pur- 

 pose, and elsewhere other green bark and vegetation. For household 

 use, however, a number of different substances have been tried. 



PYRETHRUM OR CHRYSANTHEMUM. 



For many years finely ground pow^ders knowTi as Pyrethrum pow- 

 der. Chrysanthemum powder, Persian insect powder, or Dalmatian 

 insect powder have been used to kill insects. They became famous 

 for their insecticidal effects long before their composition was known. 

 Their use seems to have originated in Asiatic countries beyond the 

 Caucasus Mountains. The powder was sold at high price by the 

 inhabitants and was brought by merchants to Russia and western 

 European countries. The nature of the powder was kept a secret 

 until the beginning of the last century, when an Armenian merchant, 

 Mr. Jumtikoif, learned that the powder was obtained from the dried 

 flowerheads of certain species of composite plants of the genus Pyre- 

 thrum growing abundantly in the region now known as '^ Transcau- 

 casia." The son of Mr. Jumtikoff began to manufacture the article 

 on a large scale in 1828, and since then the pyrethrum industry has 

 steadily grown and now the export in dried flowerheads in that part 

 of the country is very important. 



The species grown co*mmercially in the Transcaucasian region is 

 Pyrethrum roseum. The species grown in Dalmatia is P. dnerarise- 



