With the filling up of Gatun and Mira Flores Lakes, water lettuce (Pistia stratiotes, Linné) and water hyacinth (Eichornia crassipes, Solms.) 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' paper) refer to the changes in the mosquito fauna brought about by the rapid increase in Pistia. Both Mansonia titillans. Walker, and Aëdomyia squamipennis (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.