Insect life-cycle polymorphism: Current ideas and future prospects

Abstract

Polymorphisms are important components of insect life cycles and interact with responses to the environment. The forces selecting these adaptations can be estimated, through ecological correlations and mathematical modelling, especially from the ways in which different species budget time, space, and energy. Several general conclusions emerge from work to date: similar environmental challenges can be solved by insects in many different ways; similar responses can evolve independently in different species; responses evolve in combination; a single response may contribute to many functions in one species; the same response may serve different functions in different species; trade-offs between potentially interdependent traits are not inevitable; selection is a long-term process; and the nature of environmental variation is the key to understanding life cycles. These generalizations provide several focal points to help steer future work. Because the range and predictability of seasonal conditions largely govern the continuous process of life-cycle evolution, detailed assessments of habitat conditions and their variability are needed alongside biological studies. It is fair to say that for every life cycle, controlled by external cues, that we might view as potentially reasonable, some insect can be found to illustrate it. Consequently, additional elementary experiments, or yet other examples of responses already well established, are not required. Rather, further understanding of insect life-cycle polymorphism requires multifaceted studies (in carefully chosen individual species and habitats) that have been planned from the broadest possible perspective. Such multiple approaches are most likely to reveal fundamental principles underlying the evolution of the complex, integrated and polymorphic life cycles of many insects.