Introduction

When and where a female insect lays her eggs or deposits her larvae have a determining influence on her fitness, and the evolution of flight has extended the spatial range over which that choice can be expressed. It has also enabled insects faced with deteriorating environmental conditions to reach more distant sites, where their chances of survival are improved. Thus, the flight capacities and strategies of insects are shaped by the distribution, within and between habitats, of resources on which their own and their offspring's survival and reproduction depend. Some species, including those with which this book is concerned, have evolved the capacity to make flights that have specific behavioural characteristics, usually taking them beyond their current habitat – flights which are defined as migratory on these behavioural criteria (Kennedy, 1985; Gatehouse, 1987a; see also Preface, this volume).

Migration can evolve only when the fitness achieved as a consequence of migration exceeds the fitness that would have been achieved by remaining in the current habitat (Southwood, 1977; Solbreck, 1978). Any such fitness differential must depend on the quality of the current habitat relative to the quality of habitats that can be reached by migration, in terms of their capacity to support reproduction by the migrant and the growth, development and survival of its offspring. It is a consequence, then, of either deteriorating quality of the current habitat, or the occurrence elsewhere of more favourable habitats or, more usually, both (Baker, 1978; Southwood, 1981).