ABSTRACT. The zigzagging behaviour of male Plodia interpunctella flying up a plume of sex pheromone was investigated in a horizontal wind tunnel by detailed analysis of the moths' ground tracks, groundspeeds, orientations and airspeeds. The moths ‘homed in’ on the source of the pheromone plume by progressively reducing airspeed and turning more into wind, thereby reducing groundspeed and the distance between track reversals and so narrowing down their zigzags (Fig. 16). Track angles and times between reversals were unaffected. Removing the wind‐borne pheromone plume while a moth was flying along it confirmed that zigzagging can be an anemotactic response to losing the scent rather than a chemotactic response to the plume. For the first 1–2 s after the moth entered pheromone‐free air the zigzagging was indistinguishable from that shown when the plume remained; thereafter it widened progressively until the moths were flying to and fro at c. 90° to the wind. The after‐effect of odour stimulation persisted for many zigs and zags and many seconds (Figs. 4 and 5). Moths flying along pheromone plumes compensated efficiently for differences of wind speed, showing similar distributions of track angles to wind, and of ground‐speeds, in winds of 0.1, 0.2 and 0.3 ms‐1 (Figs. 12 and 13). Groundspeed varied with track angle to wind and this relationship was also similar in the three wind speeds (Fig. 14). This constancy of track angles and groundspeeds was due to the moths both increasing their airspeeds and turning more into wind at the higher wind speeds (Fig. 17). Thus the direction of the apparent movement of the ground pattern beneath the moths varied with wind speed. It is inferred that the moths, although unable to sense the wind directly, are able to compensate for changes in wind speed by integrating the wind‐dependent optomotor input with information about their own airspeed, or with information about their own turning movements. Maintaining some ‘preferred’ relationship between these inputs by adjustments of orientation and airspeed, would then serve to maintain a given combination of track angle and groundspeed independently of wind speed. The preferred relationship is repeatedly re‐set by the changing olfactory input from the pheromone plume, which also controls the switching between left and right of the upwind direction.

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ABSTRACT. On passing from clean air into a homogeneous cloud of sex pheromone in a wind tunnel flying male Adoxophyes orana (F.v.R.) (Lepidoptera: Tortricidae) turned more or less upwind and reduced the time and distance between their switchings of track from one side of the wind line to the other. These responses became adapted under the constant pheromone stimulation in the cloud, thereby arresting upwind progress; but the adapted moths would now ‘lock‐on’ to an added pheromone plume and advance upwind along it. Moths also locked‐on to the border of a pheromone cloud, not by turning back on losing the scent as previously supposed but by initiating the above programme of small‐amplitude, crosswind movements (reversing anemomenotaxis). The onset and cessation of the pheromone stimulus produced anemotactic responses that differed quantitatively within a continuum, not two distinct kinds of response as previously supposed. The behavioural mechanism whereby uniform permeation of an area with synthetic sex pheromone can prevent males from finding females is reconsidered.