Experiments were carried out on the flight pattern of unfed female mosquitoes in an area of the Gambia adjacent to mangrove swamps. The mosquitoes were trapped in flight by the use of ramp-traps as they approached natural or simulated baits. The traps were arranged in series in four avenues of cleared bush, radiating out at right angles to each other. The baits were installed either at the centre of the cross formed by the avenue or at the outer end of one avenue. Trapping was carried out at six ranges simultaneously, extending from 5–80 yd (4·5–73 m) distance from the bait. The baits were either two small calves or carbon dioxide emitted from a cylinder at a rate equivalent to the expiration rate of the animals. A series of catches with no bait was also made.

In an inland area of the Gambia the density of flying mosquitoes at successive distances from a pair of bait-calves or an equivalent source of carbon dioxide was measured with ramp-traps to determine the maximum range at which orientation towards the bait was initiated. Thirty-six traps were operated at intervals of 7·5 m up to a distance of 60 m from the bait, on 66 nights. Anopheles ziemanni Grünb., Mansonia uniformis (Theo.) and Aedes spp. were responding to the presence of the baitcalves from a distance of 22·5–30 m; with carbon dioxide as bait the same species showed a clear response at 15 m but, with the doubtful exception of M. uniformis, not at 22·5–30 m. M. africana (Theo.) responded to both calf and carbon dioxide baits at 15 m but not at more distant ranges. Culex thalassius Theo. and C. univittatus Theo. showed a response at 7·5 m but not at any greater distance. C. decens group showed no response to either bait even in the nearest traps (7·5 m). The parous rate in Mansonia spp. varied with distance from the bait. Similar variation in the proportion of females infested with mites was not detected.

Mosquitoes flying at low levels over open farmland were sampled by means of electrical suction traps. These were set up at nine levels from ground level up to 6 m. From the vertical profiles obtained it was possible to recognise three patterns of behaviour: (1) a low-flying group with relatively very high densities below 1 m, comprising Mansonia (Mansonioides) spp., Aedes spp. and some species of Anopheles; (2) an intermediate group with densities rather evenly distributed at the lower levels but declining above 2-4 m, comprising A. funestus Giles, A. gambiae Giles and Culex neavei Theo.; (3) a high-flying group with catches at 6 m greater, or much greater, than at 1 m, composed of C. antennatus (Becfcer), C. thalassius Theo. and C. poicilipes (Theo.). For all species, catches after 23.00 h showed an increase in the proportion of mosquitoes taken in traps at the lower levels, this being most marked at ground level and 0-5 m. No influence of either moonlight or wind speed could be detected to account for this. Biting catches on human baits showed a generally similar pattern to suction-trap catches, although differences between baits at 1-m intervals at the higher levels were less than with unbaited traps.

The Polovodova ageing technique was applied to females of Anopheles darlingi Root in the northern part of Mate Grosso, Brazil, in April 1978; 1·5% of the females had oviposited at least four times and were old enough to be potential vectors of malaria. The oldest female dissected had oviposited six times. The mosquito has a resting period of at least 24 h between oviposition and the next blood-meal. There was a pronounced peak of biting activity, consisting largely of nulliparous females, at dawn and dusk.

Field experiments were conducted in the Gambia to measure the convergence of mosquitoes towards single bait-animals (calf or man) or to an equivalent source of carbon dioxide, the mosquitoes being caught in ramp-traps. Catches of unfed females of Anopheles melas Theo. fell off steeply with increasing distance up to 15 yd from animal bait; with carbon dioxide as bait, raised catches were only recorded out to 10 yd. Catches of other Anopheles spp. with calf-bait gave similar results to those for A. melas. A positive effect of the calf on densities at 15 yd was obtained for Culex tritaeniorhynchus Giles but this was less clear-cut in the case of C. thalassius Theo.; with carbon dioxide as bait, raised catches of both species were recorded at 10 yd but not at 15 yd. Both types of bait gave raised catches of the C. decens group only at 5 yd. It is concluded that A. melas and other Anopheles spp. were detecting and responding to the single animal bait from 15–20 yd, C. tritaeniorhynchus and C. thalassius from 10–20 yd, and the C. decens group from 5 yd. In all except the last named group, as also for the other Anopheles spp., on which data are lacking, the range of attraction of carbon dioxide was less than 15 yd. Olfactory cues were therefore held to be responsible for long-range orientation in these three species of mosquitoes, the effect being most marked with A. melas.