Investigations on the Akah river, in the 4th Division of Sarawak, showed 13 species and one variety of Anopheles to be present. A. l. leucosphyrus Dön., the principal malaria vector, was the only species common in houses.

A. l. leucosphyrus has a strong preference for human blood and feeds throughout the night, with a peak of activity soon after midnight. Both fed and unfed mosquitos rest indoors on the walls but the latter become increasingly scarcer later in the night. Most individuals rest low down (62% below 3 ft., 96% below 5 ft.) but only a negligible percentage remains in the house after daybreak. Observations suggest that many females spend quite long periods resting outside the house before entering and that entry is almost entirely through windows and other openings in the walls; very few enter the space below the house. There is also evidence of two separate stimuli, or levels of activation, one bringing the mosquito to the bait and one initiating the act of feeding.

Wet weather was accompanied by an abrupt increase in catches of A. l. leucosphyrus and dry weather by a decrease—this is thought not to be due to any effect on breeding places. During the wet period, females entered earlier and tended to rest to a greater extent before feeding.