Dispersal and Interspersal of Blowfly Populations

Abstract

Three experiments are described in which separately marked groups of blowflies were released at different points, and the degree and speed of interchange assessed by trap recoveries over a period of days. Two types of flight were recognized in the flies dispersing from the initial concentration, viz, dispersal- and intespersal-flying. The dispersal, or exodus, flight from the center is apparently unidirectional, and either fast or sustained, since considerable distances were covered in short intervals. One timed flight over 200 yd, was at a speed of at least 7 m.p.h. Interspersal-type, or random, direction, may involve less, or no, flying net displacement; an individual flight is possibly more open to interruption than with the dispersal type, and the direction of a renewed flight apparently independent of the preceding flight. Evidence of reversal of direction was obtained. A greater proportion of a dispersing group suffer considerably displacement than would be expected from random flying, and it is suggested that the majority of a group, a released concentration or grouped emergences, for example, exhibit the exodus type of first flight. The direction of the exodus flight appears to be largely independent of the vegetational or edaphic environment. Evidence is adduced that the normal range of effective attraction of trap baits is very limited, and it is suggested that this is also true of the naturally occurring sources of attraction in the habitat. Exodus fliers whose radial paths do not come within the influence of ground zones of high local attraction will not have their flight interrupted, and may achieve unidirectional flight-lengths limited only by physiological ability. The normal duration of free-flight appears to be less than 15 min. but it is suggested from field and laboratory evidence that some individuals are capable of exceptionally prolonged single flights. Interspersal-type flying is responsible for a high degree of intermingling of populations from separate major habitats. In one experiment with 4 habitats from 1/4 to over 1 mile apart, over half of the population of each, as judged by trapped samples, consisted of flies which had come from the other 3 habitats. It is suggested that where concentrations of dispersing flies have been observed in trap samples from areas with high densities of native flies, this effect is not due to the exodus paths having been deflected towards such foci, but simply to the greater tendency to interruption of flight by exodus fliers passing over or close to the foci. Where a fly whose exodus flight is interrupted by ground stimuli reverts to interspersal-type flying, its flight being now random will be across as well as along the dispersal radii; the observed groupings of marked flies will therefore be augmented by interspersive movement.