Migration and the Coordination of Life-Cycles as Exemplified by Lygaeinae Bugs

Abstract

11.1 Migration and Life-Histories Flight ability has undoubtedly been an important contributing factor for the large success of insects as a group. Although some insects may travel considerable distances by means other than flight, flight is nevertheless by far the most common means for insects to move to new habitats. Migratory flights, by which insects move to new habitats for breeding or diapause, are performed during a limited period in adult life. During this period the insect is behaviourally and physiologically committed to flight, whereas other functions such as feeding and reproduction tend to be suppressed (Johnson 1969,1976; Rankin 1985; Kennedy 1985). In many insects, flight also serves the function of short everyday movements, for example between feeding stations. This kind of flight is often called trivial flight, and the flights tend to be shorter, less straightened out and more easily interrupted by feeding cues etc. Although the two types of flight may be viewed as end points of a continuum, it is often useful to make this distinction when discussing flight in a life-history context. This chapter is concerned mainly with insect flight in its migratory function. In insects, migratory ability is manifested in a highly "compartmentalized" way. The locomotory apparatus, consisting of wings and flight muscles, is located in a separate compartment of the body, the upper part of the thorax. Further-more, this apparatus used in migration is usually fully functional pnly during a short phase in adult life. The migratory phase is preceded by a teneral period and usually succeeded by a period of diapause or reproduction (Johnson 1969). Probably because of this distinct morphological and ontogenetic manifestation of migratory ability in insects, migration has long been included as an important component in the study of insect life-histories (Dingle 1972). It is now generally acknowledged that migratory capability is strongly coordinated with other life-history traits, with regard to both genetic architecture and phenotypic coordi-F. Gilbert (ed.), Insect Life Cycles © Springer-Verlag London Limited 1990 198 Insect Life Cycles: Genetics, Evolution and Co-ordination nation. Migration is hence coordinated with other traits into a life-history syndrome (Dingle 1984, 1985a, b, 1986a, b). This study further elaborates the theme of how migration is coordinated into insect life-history syndromes, using examples from our own studies of life-history variation in a group of seed-feeding bugs of the subfamily Lygaeinae (Heterop-tera: Lygaeidae). Most species of Lygaeinae feed on poisonous plants, such as members of Asclepiadaceae, Apocynaceae and Senecioneae. The insects are usually aposematically coloured in red/orange and black, which makes them easy to observe in the field. Most species are easy to rear in the laboratory on a diet of dry seeds, which makes them good experimental objects. Data on several life-history traits (Table 11.1) are presently available on 13 species, most of which have been collected in the Mediterranean region (Table 11.2). Table 11.1. Life-history traits studied in Lygaeinae bugs. All data refer to rearings of bugs on best-known seed diets at a temperature of 30 ·C and a daylength of 16 or 17 h. This represents optimal temperature as well as diapause-inhibiting conditions. Larvae are raised at standardized low densities, and adults as pairs