74 JAMES ZETEK. 



Assistant Chief Health Officer, and I, made an inspection of floating islands at 

 Gamboa, C. Z., both in the canal prism and across in the bayous ; we went as far 

 as Empire. Mr. Pickett previously reported the presence of Anopheles larvae 

 in these islands at this place, and that they were going southward toward his station. 

 Our joint inspection revealed larvae of Anopheles albimanus, Wied., and 

 A. tarsimaeulatus, Goeldi, to be plentiful among the leaves of the water lettuce. 

 An unidentified Culeoc was also present. 



This particular habitat is very favourable to the Anopheles. The larvae are 

 afforded much protection from the schools of young fish that usually feed upon 

 insect larvae. They are also protected to some extent from predaceous insect 

 larvae, such as those of dragon-flies. Shade is another important factor in this 

 habitat, and this means a more even temperature, and an almost total absence of 

 the direct hot rays of the sun. Wave action does not disturb these larvae very 

 much ; but the most important advantage to them is the exceptionally favourable 

 presence of oxygen, and this certainly must be effective in prolonging the life of 

 the larvae and pupae. This factor is very easily explained. The Pistia, being 

 a green plant, gives off oxygen. Its leaves are at or just above or below the surface 

 of the water. Therefore this oxygen enriches the local atmosphere available to 

 the larvae, and some of it is undoubtedly taken up by the water and made use of 

 by the larvae through their cutaneous respiration. 



During the summer of 1911, while engaged by Dr. S. A. Forbes to study the 

 Anopheles at Havana, Illinois, a similar condition was found to exist, excepting 

 that in place of Pistia the plant in question was the tiny Lemna. This plant covered 

 the surface of the water in the bayous, ponds and quiet recesses of the rivers. The 

 Chatagua grounds at Havana were invaded each year by Anopheles, but their source 

 was not known until we found them in and among the floating islands of this Lemna. 

 These islands broke away from the almost continuous mass across the river from 

 Havana and by current and wind were swept across this deep body of water and 

 came to rest along the shores of the Chatagua grounds ; here the larvae developed, 

 pupae formed and adults emerged in rapid succession. 



There is one important aspect of this sort of habitat which must not be over- 

 looked. It is that wind and current detach portions of this floating vegetation 

 and sweep it across deep bodies of water to new localities, where were it not for this 

 exceptionally good vehicle these larvae would be unable to cross the barrier of deep 

 water. 



Searching through the hterature available at our laboratory, I flnd that Ingram 

 and Macfie (1917) found larvae of Anopheles costalis, Lw., and A. marshalli, Theo., 

 in exactly the same sort of habitat, composed of the same species of Pistia, at 

 Christiansborg, near Accra, West Africa. Associated with them were : Aedomyia 

 africana, Nev.-Lem., Mimomyia splendens, Theo., Culex quasigelidus, Theo., and 

 Mansonioides africanus, Theo. 



In a previous paper, in which he discusses the limitations of kerosene as a larvicide, 

 Macfie (1917) states (p. 278) that Mansonioides africanus flies over one mile and that 

 its breeding places can be located with fair accuracy, inasmuch as they are composed 

 of Pistia. On page 278 he states it is useless to hope for a total abolition of such 

 mosquitos by ordinary oihng, and that a layer of kerosene (p. 294), no matter 



