Evidence for downwind flights by host-seeking mosquitoes.

THE behaviour of flying insects which make use of airborne chemicals in their search for food, oviposition sites or sexual partners can be separated into two phases, the search flight bringing them into contact with the attractant plume and the approach flight leading them to its source1,2. The second phase is characterised by upwind orientation in response to the appropriate chemical stimuli3–6. Mosquitoes are known to fly upwind as they approach a warm-blooded host7–9. For most insects relatively little is known of the pattern of the preceding search flight which Haskell2 described as ‘wandering’, but where the wind is light it may be upwind1. Experiments in West Africa have shown that the majority of mosquitoes entered flight traps from the downwind side, that is they appeared to have been flying upwind, regardless of the presence or absence of a host (ref. 10 and W. F. Snow, unpublished). Here we report experiments that unexpectedly provide evidence for the opposite type of behaviour.