THE chironomid, Polypedilum vanderplanki Hint., breeds in small pools formed in shallow hollows on unshaded rocks in Horthern Nigeria and Uganda. The hollows are often only 5-9 in. deep, and during the rainy season they may fill and dry several times. When the pools dry up the larvæ also dry up and remain in this condition until it rains again, when, in about 1 hr. or so they resume feeding. The degree to which the larvæ dry up depends upon the relative humidity of the air. When moisture content was plotted against relative humidity, it was found that at 0 per cent relative humidity it was less than 3 per cent, at 60 per cent relative humidity it was 8 per cent, and at higher humidities the curve rose more steeply. In saturated air the moisture content altered from 3 to 33 per cent within 6 hr. at 25° C. The figures for moisture content are based on the assumption that the larva contains no water when its weight is constant at 106° C. The capacity to tolerate repeated dehydration is clearly of selective value to this species, and in the laboratory larvæ have survived as many as 10 dehydrations to a moisture content below 8 per cent. Between dehydrations they were allowed to feed in water, usually for 1-4 days.