S-2-4

ODOUR SOURCE LOCATION BY BLOOD-FEEDING DIPTERA: WORKINGOUTSIDE OF THE BLACK BOX

Steven Schofield
Dept. of Biology, Imperial College, Silwood Park, Ascot, Berks., SL57PY, UK


That certain host-originating semiochemicals (i.e. carbon dioxide, 1-octen-3-ol,acetone, phenols, lactic acid etc.) affect biting fly behaviour has beenwell established. Indeed there is much data indicating that some of thesechemicals modify the behaviour of flies such that they locate an odoursource from a distance. The investigation of the process by which thisoccurs, however, has all too often been based on the end-result of an experiment.For instance, increased trap catch in the field and/or upwind displacementin a wind tunnel have been used to infer “attraction”, but do not explicitlydemonstrate this phenomenon. This problem is often exacerbated by the assumptionthat plumes are Gaussian, which is not the case in a natural setting. Instead,the distribution of the odour is filamentous and discontinuous, with theexact nature of a plume being dictated by turbulence, air speed, environmentalcomplexity etc. Ultimately, this means that the detection of odour andthe associated instantaneous wind direction may not provide reliable informationas to the whereabouts of the source, especially in the situation wherethe release point (i.e. the host) moves. The questions raised by theseproblems are immediately apparent: What does an increased trap catch actuallyrepresent? How do flying insects locate an odour source using a complex,intermittent signal? First addressed in the context of the responses ofmale moths to female pheromone, these queries have now been applied tohaematophagous Diptera. Although we are far from a complete understandingof the entire host-orientation behaviour, the available evidence suggeststhat movement towards the host is facilitated by plume structure, distancefrom host, odour flux and odour type. Evidence for these concepts willbe discussed in this paper, with specific examples coming from recent workon tsetse flies (Glossinidae) and stable flies (Stomoxys calcitrans).