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ABSTRACT. The body angle and airspeed of free‐flying Drosophila hydei were measured and found to be inversely related. Values for the forces produced by a fly in level flight were calculated from the measured body angles, using published information from tethered fliers. The drag varied linearly with airspeed. The measured groundspeed of free fliers increased with starvation, not because of the change in weight and thus in the power‐to‐weight ratio, but through behavioural regulation of power output.

This communication reappraises the behavioral evidence concerning insect flight toward a point source of wind-borne odor in the light of meteorological information not yet considered in this context. The horizontal tracks of puffs of smoke from a generator in the open air were videorecorded and found to continue along nearly straight lines from the source for at least 25 m, while the shifting wind direction caused the plume formed by the succession of puffs to "snake" to and fro. It is inferred from this and much previous work that within such a distance the wind will be aligned on the source of any wind-borne odor wherever the odor can be detected. This being so, a strategy of finding the odor source by flying roughly upwind on meeting the odor, but holding station against the wind with or without casting across it on losing the odor (odor-modulated anemotaxis), seems likely to be highly adaptive, whereas a strategy of flying along the plume ("odor-trail following") seems unlikely since it would often take the flier in "wrong" directions and would be more disrupted by turbulence.

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