The house-entering behaviour of nocturnal mosquitoes was studied in The Gambia. Mosquitoes were captured as they attacked man in the open and in experimental huts which comprised 1.8 m cube frames with corrugated iron roofs and plywood walls of various heights. Catches of all species were similar in the open and in a roofed, but unwalled, hut frame. The mosquitoes taken in catches in unwalled huts and others with wall heights of 0.6, 1.2 and 1.7 m (giving an 8 cm eaves-level entry slit) fell into two categories. The first group, which included the endophilic species Anopheles gambiae Giles s.l., An. melas Theobald and Mansonia spp. were only slightly affected by increasing wall height, but the second group, including the exophilic mosquitoes Aedes spp., An. pharoensis Theobald, Cx poicilipes (Theobald) and Cx thalassius Theobald showed a very marked progressive exclusion. In comparisons of catches in two huts with 8 cm entry slits at eaves or ground level, large numbers of An. pharoensis found access through the ground level entry but not at eaves level. No consistent difference could be demonstrated for other species. It is concluded that the house-entering behaviour which distinguishes endophagic mosquito species includes at least two distinct responses: flight upwards to eaves level and the passage from outside to indoors. It is also suggested that house entry as a component in host-seeking behaviour and indoor resting are distinct, but not necessarily exclusive, behavioural traits.

The vertical distribution of mosquitoes was investigated at Keneba in The Gambia, using flight traps and suction traps set up over open ground. Flight traps were used to sample three levels, ground to 1*37 m, 1-45-2-82 m, and 2-90-4-27 m. Suction traps were mounted in two series of experiments at three levels, 0-68, 2-13 and 3-51 m, and in a third series at four levels, 0-91, 3-05, 6-10 and 9-15 m. Most species, including Anopheles melas Theo., A. squamosus Theo., Aedes punctothoracis (Theo.), the Culex decens group, C. tritaeniorhynchus Giles, Mansonia africana (Theo.) and M. uniformis (Theo.), were most common near the ground, with catches declining progressively with height. C. thalassius Theo. was frequent at all levels, although the largest catches were taken at 9-15 m. Only C. neavei Theo. and C. weschei Edw. were more common in the higher traps. Although larger catches of A. melas and C. tritaeniorhynchus were taken during moonless periods, the numbers of other species were unaffected by lunar phase. In general, the presence or absence of moonlight had little effect on vertical distribution. However, C. thalassius and the C. decens group were shown to be more attracted to flight traps in moonlight, and it is concluded that visual responses were involved. A limited series of four-level, human-bait catches, with the catchers stationed at the same heights as the suction traps of the third series, produced results similar to those of the suction trap collections.

The attractiveness, to mosquitoes, of human subjects breathing normally and wearing a breathing apparatus which removed 95·5% of expired carbon dioxide were compared in the field. Significantly fewer mosquitoes approached the subject with a reduced carbon-dioxide output. Species differences were apparent in the degree of reduction of mosquitoes at the bait, as measured by the ratio of numbers attracted to subjects without and with the breathing apparatus. From the results of trials in the Gambia, West Africa, this ratio gave values of 0·67 for Mansonioides spp., 0·46 for Anopheles gambiae Giles/A. melas Theo. and 0·19 for Culex thalassius Theo., while complementary work in southern England gave a value of 0·34 for Aedes cantans (Mg.)/Ae. annulipes (Mg.). A reduced carbon-dioxide output had no effect on the proportion of mosquitoes attempting to feed once within close range of the host. These findings confirm that carbon dioxide is a long- or medium-range attractant.

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