An experiment is described in which G. tachinoides in an isolated secondary focus was marked with distinct colours during thirteen successive weeks and the fate of the groups studied by recaptures. The population was known to be declining from moderate density to great scarcity through the dry season and the wastage in the colour groups on the average proved to be 11½ per cent, per diem, rather more, probably, than the wastage in unmarked flies. Following Jackson's method of assessment the population is shown to fall from 6,000 at the end of October to 400 by mid-January. The ratio of the boy-hour rate to these numbers is worked out for each collection and it is found that when the boy-hour rate is above 50 it is not relative to population, but below 50 the rate falls as the population, allowing for the influence of weather on rate of catching.

It is shown that under the conditions of the experiment there is an almost complete change of fly population each 5 or 6 weeks, though an occasional fly may survive 11 weeks. The proportion of male flies in the catches fell with increasing age from an initial 57 per cent, to 23 per cent, in the 6th week and no male fly surviving this period was caught.