6. Insect development and life histories
In this chapter we discuss the pattern of growth from egg to adult — the ontogeny — and life histories of insects. The various growth phases from the egg, through immature development, to the emergence of the adult are dealt with. Also, we consider the significance of different kinds of metamorphosis and suggest that complete metamorphosis reduces competition between conspecific juveniles and adults, by providing a clear differentiation between immature and adult stages. Amongst the different aspects of life histories covered are voltinism, resting stages, the coexistence of different forms within a single species, migration, age determination, allometry, and genetic and environmental effects on development. The influence of environmental factors, namely temperature, photoperiod, humidity, toxins, and biotic interactions, upon life-history traits is vital to any applied entomological research. Likewise, knowledge of the process and hormonal regulation of molting is fundamental to insect control.
Insect life-history characteristics are very diverse, and the variability and range of strategies seen in many higher taxa imply that these traits are highly adaptive. For example, diverse environmental factors trigger termination of egg dormancy in different species of Aedes although the species in this genus are closely related. However, phylogenetic constraint, such as the restrained instar number of Nepoidea (Box 5.5), undoubtedly plays a role in life-history evolution in insects.
We conclude the chapter by considering how the potential distributions of insects can be modeled, using data on insect growth and development to answer questions in pest entomology, and bioclimatic data associated with current-day distributions to predict past and future patterns.