Field Observations On Swarming and Mating in Anopheles Gambiae Mosquitoes in Tanzania1

Based on field observations, the swarming behaviour of two, closely related, mosquito species, Anopheles gambiae s.s. and An. arabiensis, is described and compared. Males, captured from swarms, are identified by electrophoresis, using a biochemical key for the members of the Anopheles gambiae group of species. The swarms are composed of males and are found between the houses of rural villages at sites with open ground and a free view of the sky. Swarming takes place at dusk, in a period of about 20 minutes during which the light intensity decreases from about 55 to 1 lux. Matings in, or near, the swarms are recorded. Spacial nor temporal differences are found between swarms of the two species and evidence is given for a case of mixed swarming in an area where they coexist. It is concluded that the observed genetical isolation between sympatric populations of An. gambiae s. s. and An. arabiensis can not be fully explained by their swarming behaviour and that other, as yet unidentified, factors must play a role.