This paper reports studies carried out in two small contiguous ponds, created by damming a small stream called Caño Garzón. The mosquito fauna in the two ponds was found to be the same, so the ponds are treated as a unit in the analysis of data. The species breeding most commonly in the ponds were Anopheles rangeli, A. strodei, and A. triannulatus. Larvae of A. darlingi, A. albitarsis, A. argyritarsis and A. pseudopunctipennis were occasionally found.
Data are given for adult catches in a stable trap maintained at the ponds. The chief species caught were A. rangeli with a peak in June, and A. strodei and A. triannulatus with peaks in January. Data are given from a trap run at another location (El Chircal) at the same time, to show that this seasonal shift in fauna is general for the region.
Larval density was measured by number of larvae per dip. Since the number and places of dipping were standardized, this corresponds also to the number of larvae per unit of time spent in dipping. The peaks of larval abundance corresponded with, or slightly preceded, the peaks of adult abundance.
The species of larvae in the pond corresponded very closely with the species of adults caught in the trap, following identical seasonal trends. The percentage of larvae of different species among the first-instar material was checked weekly for a period of nine months and was found to correspond very closely with the percentage of different species among contemporaneous fourth-stage larvae. In other words, there was no indication of differential mortality among species breeding in the pond to account for the seasonal change in fauna.
Data were kept on the percentage of larvae found in different instars of growth throughout the period of study. Fluctuations in the percentage of larvae reaching fourth instar were not great, though there was a tendency for mortality to be highest when the larval population was most dense, indicating the sort of compensating mechanism that would be expected. The larval instar ratio is considered to be a valuable index of the favorableness of conditions in a given habitat for larval survival. This ratio remained remarkably similar through the year, despite the great changes in larval density and species composition, which is taken to indicate that these changes depended on events outside of the aquatic habitat.
Details are given of temperature records in the ponds. Temperature conditions were remarkably constant throughout the year, and consequently this factor can be of little importance to the larvae. The data are, however, interesting in view of the paucity of such figures for midtropical aquatic habitats.
Measurements were made of the organic content of the water, with the oxygen consumed from permanganate as an index (the Tidy figure). This showed that soluble materials in the water were greatly reduced with the onset of the rains, which would be expected. There is no indication, however, that the amount of change touches the critical limits of tolerance of the species breeding in the ponds.
The literature on seasonal fluctuation of anophelines in the tropical environment is briefly reviewed. The general impression is that our knowledge is inadequate for the interpretation of this basic ecological phenomenon. The present study and the studies reported in the literature seem to indicate that more attention should be paid to the effect of environmental factors on adult populations; that the specific limiting factors controlling anopheline distribution may operate in the adult, rather than in the larval, stage.
Villavicencio Field Laboratory, Villavicencio, Colombia.