The Role of Carbon Dioxide in Host-Finding by Mosquitoes (Diptera: Culicidae): A Review

\ud From a critical review of the literature, it is concluded that the role of carbon dioxide in host-seeking by mosquitoes comprises two distinct actions. Firstly, it acts as an attractant’, orientation towards the host being mediated by kinesis and optomotor anemotaxis. When tested in the absence of moving air currents, orientation to the source is not possible and only the kinetic or activating effect is manifested. Moreover, in the absence of other host factors, sustained flight takes place only in response to intermittent pulses of carbon dioxide; this response is not elicited in uniformly permeated airstreams. Secondly, carbon dioxide has a combined action with warm moist convection currents\u

The Effect of High Fences on the Dispersal of Some West African Mosquitoes(Diptera: CuUcidae)

Many "West African mosquitoes fly at low levels when crossing open country. Experiments were therefore conducted in the Gambia on the effect of high fences on their dispersal. In the first experiment, a circular mosquito-proof fence 2-9 m high and with a radius of 18 m was constructed. Catches inside on human bait of Mansonia spp. were about 60% lower than catches outside the fence, whereas catches of Anopheles spp. in the two sites were not significantly different. In unbaited suction traps, catches of Mansonia inside the fence were significantly lower in traps 0 and 0-5 m above the ground, but not at to 3 m, than in traps at similar levels outside. This suggested that, after flying over the fence, mosquitoes had not resumed normal flight levels by the time they were trapped in the centre of the circle. In a second experiment, a much larger fence was erected, 6 m high and with a radius of 65 m. Catches were conducted in experimental huts in the centre of .the enclosed area and at a distance of 70 m outside it when the fence was erected and again after its removal. Catches of Mansonia spp. on human bait in the hut inside were 46% of the total catch in the two huts when the fence was erected and 48% after its removal; catches of Anopheles spp. {A, gambiae Giles and A. funestus Giles) were 43% with, and 53% without, the fence. In calf-baited light-traps, catches of Mansonia were 41% of the total with, and 38% without, the fence, and those of Anopheles 58 and 44%. Thus the presence of the 6-m fence had no significant effect on the density of mosquitoes in .the centre of the enclosed area, and it is concluded that mosquitoes were flying freely over it. Due to (he prevailing low winds in the area, passive transport of mosquitoes over the fence by the wind was not ’thought to be important.\u

Field Experiments With a Wind Tunnel on the Flight Speed of Some West African Mosquitoes (Diptera: Culicidae)

An open wind tunnel, down which air was blown from over a bait animal, was used in the field in the Gambia to measure the flight speed of host-seeking mosquitoes. Insects were trapped on an electrocuting grid fitted halfway up the tunnel. As the speed of air movement through the electrocuting grid was increased from 0-5 m/s, catches of Mansonia spp. fell off steeply, reaching negligible levels above 1-4 m/s. At air speeds lower than 0-5 m/s, catches at the grid were greatly reduced. Similar results were obtained for Anopheles ziemanni Grimb. and other species of Anopheles. It is concluded that the maximum flight speed of hostseeking females of all these species was in the range 4-1-8 m/s

The Vertical Distribution of some West African Mosquitoes (Diptera,Culicidae) Over Open Farmland in a Freshwater Area of the Gambia

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

Occurrence of Rhithrocloeon (Baetidae : Ephemeroptera) in west Africa

#Rhithrocloeon n'était connu auparavant que des torrents de l'Afrique de l'Est. L'imago male d'une nouvelle espèce, #Rhithrocloeon elouardi, est décrite de Côte d'Ivoire. (Résumé d'auteur