Introduction

Most insect species are subject to either predictable or unpredictable habitat deterioration and have evolved life-history traits that enhance survival when habitat quality is inadequate for normal development and reproduction. In certain species, individuals enter a state of dormancy or quiescence within the habitat and remain in this stage until conditions improve at some later date. In others, individuals emigrate to more favourable sites. These options, broadly defined as the ‘here-later’ and the ‘there-now’ strategies (Southwood, 1977; Solbreck, 1978), involve considerable physiological changes, which are initiated by an array of external cues. In cases where habitat deterioration is of a predictable or seasonal nature, parameters such as day-length, temperature or precipitation serve as token stimuli. In contrast, cues such as the absence of suitable food, oviposition sites or mates, as well as sudden changes in abiotic conditions, may serve to induce the physiological modifications necessary to cope with unpredictable changes in habitat quality (see Chapter 10, this volume, for additional information).

The classic scenario relating migration to the reproductive status of the migrant is the ‘oogenesis-flight syndrome’ (Johnson, 1969), with migration being undertaken by individuals that are sexually immature or between reproductive cycles. Migration and reproduction are perceived as mutually exclusive processes. However, in a number of species known to undertake migratory flight, juvenile hormone (JH) is necessary for both flight and oogenesis (see Rankin, McAnelly & Bodenhamer, 1986, and references therein).