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ANOPHELES BREEDING AMONG WATER LETTUCE— A NEW HABITAT. 



By James Zetek, B.A., 

 Entomologist, Ancon, Panama Canal Zone. 



With the filUng up of Gatun and Mira Flores Lakes, water lettuce {Pistia stratiotes, 

 Linne) and water hyacinth {Eichornia crassipes, Sohns.) rapidly increased in numbers 

 until large areas became completely covered by them. Masses of these plants would 

 become detached and float about in these lakes. It was found necessary to destroy 

 these " floating islands," not because they were serious mosquito habitats, but 

 because they interfered with navigation and the operation of the spillways. The 

 water lettuce is the habitat of the very specialised larvae of Mansonia, of which 

 titillans is the commonest member. Knab (1913) and Busck (comments appended 

 to Mr. Knab's paper) refer to the changes in the mosquito fauna brought about by 

 the rapid increase in Pistia. Both Mansonia titillans. Walker, and Aedomyia 

 .bguamipennis (Lynch- Arrib.) Theo., have been collected by me in very large numbers 

 since 1912, and to-day Mansonia is the dominant species caught in the army barracks 

 on the west side of the canal. Knab expressed the opinion that perhaps measures 

 would have to be taken to destroy the Pistia habitats ; now that we have found 

 Anopheles larvae in them, this prophesy comes closer home. 



The only question involved is how much of a menace are these floating islands 

 to onr towns. It is my belief they are a menace to the towns on the west side 

 of the canal, and if allowed to float toward Paraiso and Pedro Miguel, or close to 

 Gatun, would be a source of danger there. It has been shown conclusively that our 

 common malarial Anopheles do fly more than a mile (Zetek, 1915) ; in fact, it appears 

 that they will fly as far as they must in order to get food. Should Las Cascades 

 become a negro settlement, the dangers from malaria would be increased. I doubt 

 if we can depend upon screening and mosquito-catching indoors to control the 

 Anopheles and malaria at such a settlement. At any rate, the idea seems to be 

 getting firmly established that the sanitation of the Panama Canal Zone is so efficient 

 that there is virtually little danger from the Anopheles, and as a result our people 

 are becoming somewhat neghgent. It wiU depend mainly upon the cumulative 

 evidence gathered by the District Sanitary Inspectors, whether or not these floating 

 islands will need to be controlled. 



That this evidence may be available and accurate, it is very necessary that the 

 sanitary inspectors make it their practise to take ample field notes. These should 

 contain as complete a statement of the particular habitat, date and locaUty added 

 thereto, and specimens of the larvae should be sent to the laboratory for the accurate 

 determination of the species. At the close of each month a general summary of 

 these notes should be made, a copy of which should be sent to the entomologist. 

 It is a regrettable fact that much valuable information is lost or buried in the heads 

 of observers. 



In 1918 I found Anopheles larvae among water lettuce at Juan Mina, a citrus 

 plantation about five miles up the old Chagres River. On 21st November 1919, 

 ]^Jessrs. Picket and Tolar, sanitary inspectors at Pedro Miguel, Dr. D. P. Curry, 



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