POISONOUS PLAXTS 387 



the course in less than an hour. Within two minutes after the application on 

 any part of the stream all mosquito larvse there were dead. 



" I remained several hours afterwards, so as to be able to work alone, dipped 

 and looked carefully on knees and hands all along that stream, but nol one live 

 larva was found, where hundreds could be taken in a single dip before. 



" A few hours after the application the water course was again quite clear and 

 could be used for drinking and washing as before. 



" The vegetation did not appear to have suffered in any way, though that 

 might take longer to show. Most of the other water insects were dead, but a 

 considerable number of water beetles, Hemiptera and dragon-fly larvse withstood 

 the treatment, and I have no doubt but that the percentage of the larvicide 

 solution can be so regulated that it would kill all the mosquitoes and not the 

 tougher predaceous insects. 



" There were no fish in the stream, so I can not say, from personal observa- 

 tion, how it would affect them, but Mr. Trask says they can endure considerable 

 of the larvicide without dying." 



DESTRUCTION OF LARV/E BY SENSITIZATION TO UGHT. 



Dr. Gunni Busck, of Copenhagen, has recently carried out novel experiments 

 in mosquito destruction : 



" I have this fall tried to solve the question of killing mosquito larvae by an 

 entirely different method, on the supposition that it might be possible, through 

 solutions of suitable coloring matter (Erytrosin, Eose Bengal, etc.), to sensitize 

 mosquito larvge so that they would be killed by the sunlight or mere daylight, 

 to which they are directly exposed under natural conditions, when they assume 

 their normal position for respiration at the surface of the water. 



'' It was shown by my experiments that it really is possible to thus sensitize 

 mosquito larvae and that they die in thin solutions of the above mentioned 

 colors, if they are exposed to light, even if only diffused daylight, while they 

 remain alive, pupate and develop to adults in these same solutions, if they are 

 kept in darkness. 



" It was however necessary to use more concentrated solutions of the colors 

 than I had anticipated, and as these colors moreover are rather expensive, the 

 method can at present in no wise compete with the oil methods. The experi- 

 ments will be continued and their scope widened this coming season." 



POISONOUS PLANTS. 



Decoctions and emulsions of an African leguminous plant, Derris uUginosa, 

 have been recommended for larvicidal use, and experiments conducted by Bal- 

 four at the Wellcome Eesearch Laboratories at Khartoum show that it has con- 

 siderable potency. It also kills fish, different species of Derris being well known 

 for this quality, so that even in regions where these plants are native they have 

 only a limited use as insecticides. 



Eecently Daniels (Tropical Medicine and Hygiene, Part 1, 1909, pp. 77-78, 

 120) has advocated the use of the "tuba root," also belonging to the genus 

 Derris, which is used in the East Indies to poison fish, to destroy mosquito larvse. 

 The commonly employed species is Derris elliptica. Other plants are employed 

 in other parts of the world to stupefy fish and some of these might prove to be 

 effective against mosquito larvae. Daniels says of Derris : 



" The roots are crushed and thrown into the water, and the milky fluid from 

 the fresh roots, even in minute quantities, and much diluted, will destroy the 

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