294 ENTOMOLOGICAL NEWS [Oct., 'l8 



character the plants grow very large. The rootlets become 

 very numerous, forming large heavy masses, and are usually 

 well covered \vith decayed vegetable debris. On one plant 

 taken from such a locality 51 larvae of M. titillans were found 

 attached to the rootlets. 



That the Pistia plant is essential to the existence of M. titil- 

 lans in the larval stage might be well assumed, and is in fact 

 rendered highly probable by the simultaneous increase of both 

 the plant and mosquito in the Canal Zone following the forma- 

 tion of Gatun Lake. However, we strongly suspect that this 

 species may occasionally be found attached to the roots of 

 other aquatic plants. A few larvae closely resembling M. 

 titillans have been found attached to the roots of grasses, 

 sedges, etc., of aquatic nature. These larvae were too small 

 for positive identification and when placed in jars died in a 

 few days. It is possible that they may have been larvae of 

 another species of Mansonia very close to titillans. 



As far as known at present no morbid conditions have been 

 credited to this mosquito. However, should it ever become 

 certain that this species is concerned with the transmission 

 of disease in the Canal Zone, it will then be necessary to take 

 steps for its elimination. This can only be accomplished by 

 clearing the Zone of the water lettuce. When we estimate the 

 number of square miles of this plant that may be found float- 

 ing on the surface, we would on first reflection be justified in 

 considering that its eradication would be an undertaking of 

 considerable magnitude, but in reality it could be accomplished 

 under quite reasonable conditions. The most feasible plan for 

 the destruction of these plants is spraying with an arsenical 

 solution. They are readily killed by arsenical sprays and will 

 decay and sink in from 6 to 8 days after the spray has been 

 applied. This work would require a house-boat, steam launch, 

 pump-boat and about three pangas, or row boats. A crew 

 consisting of one white man and from eight to twelve negroes 

 would be necessary. An outfit of this kind should spray about 

 an average of 35,000 square feet per day when the plants are 

 in good-sized masses. The initial cost of the outfit would be 



