The Spring Migration of the Bushfly (Musca vetustissima Walk.): Evidence of Displacement Provided by Natural Population Markers Including Parasitism

Abstract

In an earlier paper Hughes (1970) presented data on the occurrence of bushflies (Musca vetustissima Walker) along a diagonal transect across south-eastern Australia on several occasions following the winters of 1967 and 1968, together with the physiological agecharacteristics of the flies in the samples collected. The pattern of occurrence of the flies and their 'age' distribution could be most readily interpreted by inferring that, in each year, relatively 'old' flies were moving southwards to re-occupy areas where bushflies had died out during the cold winter. On the basis of (i) the occurrence of flies throughout the winter in southern Queensland, (ii) an observed large-scale displacement of flies associated with the warm strong north-west winds of an anticyclonic system, and (iii) the apparently regular occurrence of such wind systems over the region in spring, an annual long-distance displacement of bushflies by northerly winds was proposed as an integral part of the species' life system in south-eastern Australia. Subsequently, attempts were made to collect more definite information on bushfly movements. The recapture of flies marked before they were released was tried, but demonstration of long-distance displacement by this method proved to be impractical with the staff and facilities available. The chance of recapturing a marked fly fell so rapidly with distance from the release point (cf. Norris 1966) that the experiments only showed movements on the wind over trivial distances. For the purpose of the analysis of the population dynamics of the bushfly being made at Canberra, it was essential only to know of the occurrence, the timing and magnitude of any spring immigration of bushflies. Details of their displacement across New South Wales, although of great interest, were of considerably less importance. From this specialized viewpoint a simple way to verify, at least partly, the migration hypothesis would be to recognize flies which had immigrated into the Canberra study area. Natural markers were sought which could be used to distinguish flies from sources outside the area. Once again it proved impractical to identify individual immigrant flies. There were, however, occasional marked changes in the abundance of bushflies in the Canberra study