Odour Plume Shape and Host Finding by Tsetse

Plotting the movement of air through typical tsetse habitats in Zimbabwe revealed that it is liable to change direction by over 90° within a few metres. In winds of <1 m/sec, there was a negative correlation between windspeed and the wind’s tendency to meander, whether in thick bush or out in the open. And at the mean windspeed of wet season mopane woodland (0.3 m/sec), the wind changed direction by c. 15°/sec. Accordingly, host odour does not move downwind in simple trajectories that tsetse flies could easily follow. Even 5 m from a source, odour (modelled with smoke) approached a notional tsetse fly for a quarter of the time from >90° away from the true source direction. Also, air turbulence at common tsetse resting sites generates much nonsense information about the “true” direction of any odour-bearing wind. The suggested answer to how tsetse manage to find distant, invisible hosts, in spite of this confusing information from the wind, is that they progress by a biased random walk which is the outcome of their upwind anemotactic responses to odour-bearing wind and the proportion of the time for which the wind does blow in the “correct” direction. Computer simulation shows that this might work in principle.RésuméL’analyse du mouvement de l’air dans des habitats typiques de tsé-tsé au Zimbabwe révèle que celui-ci est susceptible de changer de direction de plus de 90° sur quelques mètres. Dans le cas de vents <1 m/sec, une corrélation négative est observée entre la vitesse du vent et sa tendance à sinuer, que ce soit en brousse épaisse ou en terrain dégagé. Pour la vitesse moyenne du vent dans les bois de mopanés en saison des pluies (0.3 m/sec), le vent change de direction de 15°/sec. Conséquemment, l’odeur de l’hôte au fil du vent ne se déplace pas en trajectoires simples que les tsé-tsé pourraient suivre facilement. Même à 5 mètres d’une source d’odeur, celle-ci (modélisée par de la fumée) approche une mouche tsé-tsé imaginaire avec un écart de plus de 90° par rapport à la direction exacte pendant le quart du temps mesuré. De plus, les turbulences dans les gîtes de repos habituels des tsé-tsé génèrent beaucoup d’information erronée (“non-sens”) sur la “vraie” direction des vents porteurs d’odeurs. A la question de savoir comment les simulies réussissent à trouver des hôtes invisibles à distance malgré cette information déroutante, les auteurs suggèrent une progression au hasard biaisée, résultat de leurs réponses anémotactiques vers le vent porteur d’odeurs d’une part, et de la proportion de temps pendant laquelle le vent souffle dans la “bonne” direction d’autre part. La simulation par ordinateur montre que ce schéma peut marcher dans le principe.

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