In a recent paper by the writer (Bull. Ent. Res., xxi, 1930, pp. 201–256) it was shown that the apparent fly density varied according to season. It has now been found that these seasonal variations in the fly density can be correlated with the evaporation rate.

The accompanying graph shows the mean monthly evaporation rate, and the mean monthly apparent fly density. The evaporation rate figures have been obtained from a Livingston atmometer at the Kikori Entomological Station. The fly figures have been taken from the North-East Kikori Round, because this fly-round is not subjected to large game movements and hence is considered to be the best for demonstrating the correlation between fly density and season.

The graph deals with a period of eighteen months, and it is clear that a relationship does exist between the two curves.

In June and July 1929, the fly density was at its maximum. Hence it is reasonable to postulate that, at this time, the evaporation rate was suited to the requirements of the fly community.