A Functional System of Adaptive Dispersal by Flight

JOHNSON feed, or lay eggs.At exodus, and later, orientation is adapted to.get them out and beyond the birthplace to another oviposition site.As the need to lay eggs grows, flight often seems to be more local, and presumably less widely dispersive (83).Dispersal by other stages is, in general, subsidiary to this.There are, however, important exceptions which will be discussed. BACKGROUND AND PRESENT APPROACH.Different people have seen migration and dispersal as distinct, processes, twice reviewed in these volumes.Williams reviewed a special class of adap tive flight, namely, when insects move unidirectionally en masse, seemingly able to control their direction for relatively long distances (168).He called these migrations.By contrast, he saw dispersal mainly as a passive and ac cidental process as when insects are scattered by wind.This view is still com mon.A broader view of migration expounded by Kennedy was recognized, though not completely accepted, in the second review by Schneider (136).This view regards mass flights, like those of aphids (formerly regarded as passive), as active and adapted to displace populations by wind which con trols the direction of travel; it characterizes migration behaviourally as an accentuation • of locomotor function with a depression of vegetative function.Many classical migrants once thought able to control the direction of their migration, such as locusts and some butterflies, now come into this category of adaptive wind-borne travellers, and when wind takes control it usually disperses, swarm locusts being a notable exception.Also, dispersal occurs as insects drop out en route (153, 156).In this review, therefore, migration and flight adapted for dispersal per se are regarded as synonymous.Nevertheless, insects do disperse accidentally, or (presumably) inciden tally while obtaining food or mates, or by other activities, in a quiet, con tinuous and humdrum way that Elton thought general among animals.An drewartha with insects, that ten dency was phased with the other activities of adult life (4).I t is extremely difficult to measure how much dispersion occurs acci dentally, incidentally, or adaptively, for such movements are usually indis tinguishable in nature.But the mass movements, usually called migra tory, stand out as probably the most important.Therefore if a general, valid, and functional system for all of these movements can be made; it may show up effects of incidental and accidental movements in clearer perspective (I suggest that these are small by comparison with those of adaptive move ments).