1. The behaviour of A. gambiae and A. melas in houses is described in relation to possible use of pyrethrum and DDT.
2. Mass spraying of isolated villages with pyrethrum in kerosene four times a week reduces the day-time catch of Anopheles to one-third. The sporozoite rate of mosquitos in sprayed villages remains much the same as before treatment.
3. Observations in experimental huts with window traps attached show that pyrethrum exerts a repellent effect on many hungry Anopheles for a day or two after spraying, provided blood meals can be obtained easily in adjacent untreated houses.
4. Where all available huts occupied by man are sprayed, there is no great repellence to hungry mosquitos, which continue to feed in large numbers even in huts sprayed every day.
5. In untreated huts about 20 per cent. of the blood-fed females leave the house at dawn after feeding. In huts sprayed daily the proportion increases to about 80 per cent.
6. The fall in the house catch in villages sprayed 4–6 times a week is not due to any great reduction in the mosquito population, but to a shift from indoor to outside resting places.
7. As intense house spraying with pyrethrum fails to bring about any substantial reduction either in the mosquito population or in their infectivity, it is not likely to be of great anti-malaria value in West Africa.
8. Mass spraying of village houses with 5 per cent. DDT in kerosene produces a dramatic fall in the day catch of resting Anopheles in houses. The catch remains very low for 4 weeks after treatment and it is still fairly low after 2 months.
9. Observations in experimental huts (fitted with window traps) treated with DDT in kerosene show that mosquitos may enter and feed within a few days of treatment. By the second week after treatment large numbers of Anopheles may feed every night in treated huts.
10. There is no evidence of mosquitos succumbing to the effects of DDT in kerosene inside the hut. Nearly all the Anopheles feeding in such huts leave after feeding and show no appreciable mortality in the following 48 hours.
11. The DDT in kerosene has a marked residual irritant effect on Anopheles, driving them out of the house after they have fed, and preventing mosquitos resting long enough on treated surfaces to absorb a lethal dose of DDT.
12. The few days complete protection from biting mosquitos which follows spraying inside the house with DDT in kerosene, is shown to be due mainly, if not entirely, to the repellent effect of the heavy dose of kerosene which accompanies the DDT.
13. Treatment of all rooms in an isolated village reduced the day catch to nil during 5 weeks after treatment. In outside resting places beside the village, bloodfed and gravid Anopheles, of which 2 per cent. had sporozoites in the salivary glands, Were taken regularly during this period.
14. The apparent elimination of mosquitos from houses following treatment of rooms with DDT in kerosene is due to a complete shift from inside to outside resting places, on account of the residual irritant, but not lethal, effect of DDT-in-kerosene treated surfaces.
15. The sharp fall in the house catch of Anopheles following treatment Of West African village houses with DDT in kerosene can not, therefore, be accepted as evidence of mosquito reduction or effective control.