Dispersal Polymorphisms in Insects

Abstract

In many insects dispersal by flight occurs prior to reproduction, and the ability to fly is often lost once reproduction begins (37, 92). Thus, dispersal is viewed as an evolved characteristic of a particular stage in the ontogeny of the insect (91-93, 98) rather than a spontaneous response to current adversity. In certain groups of insects, species exhibit polymorphisms that affect flight ability. Variations in wing length and flight muscle development are the most obvious examples. In some species flight may be primarily an adaptation for dispersal, and the proportion of morphs capable of flight may be a reliable measure of level of dispersal. Adaptations for dispersal in animals are for the most part difficult to identify because locomotory move­ ments potentially serve many important functions (or produce many impor­ tant effects). Flight polymorphisms in insects are thus of particular interest in understanding the adaptive significance of dispersal in natural popula­ tions. This review focuses on two aspects of insect flight polymorphisms: how morphs are determined and how polymorphisms are maintained. I do not attempt to document all (or even most) examples of flight polymorphisms but instead consider possible answers to the following questions: Do differ­ ent morphs represent different genotypes, or do they represent alternative developmental patterns produced by a single genotype in response to differ­ ent sets of environmental inputs? What do studies of flight polymorphisms reveal about the evolutionary significance of dispersal? What are the selec­ tive advantages and disadvantages of the normal (winged, not necessarily most common) and flightless morphs in an insect population?