Ordinal species richness in insects-a preliminary study of the influence of morphology, life history, and ecology

Abstract

In an effort to understand why some groups of insects, especially beetles, are much more diverse than others, analyses were carried out on the role of morphological, life-history, and ecological factors on the species richness of various orders, as quantified by the number of extant species they each contain. The influence of body size, the form of the wings and thorax, the mode of development, parasitism, the ability to accumulate resources for egg laying at both juvenile and adult stages (ovigenic flexibility), diet range, and habitat choice were examined using comparative methods that control for phylogeny. Hierarchical multiple regression revealed that about 10% of the variance in insect species richness occurred within subclasses, 34% within superorders, and 54% within or below ordinal level. The evolution of wings and mode of development were key differences between the subclasses, and two superorders, each containing a single species-rich order and several species-poor ones, were responsible for most of the variation at superordinal level. After controlling for phylogeny, and on the basis of minimal adequate statistical models, the rate of cladogenesis declined with time, and was higher in orders with a broad range of larval diets. The number of species in each order was most closely correlated with larval diet breadth, the proportion of species able to fly, physical protection of the hindwings by the forewings at rest, the proportion of species that are parasitic, and the age of the order. Evidence from the literature on adult diet and ovigeny index, supported by measurements of the width of the thoracico-abdominal junction, indicates that the food processing ability of adult Diptera, Lepidoptera, and Hymenoptera is subject to morphological constraints limiting the contribution that the adult diet makes to the accumulation of the protein reserves needed for egg formation. Adult Coleoptera are free of such constraints, and thus exhibit the broadest range of both larval and adult diets and inhabit the widest range of habitats. Beetles can thus accumulate resources for egg production as larvae, adults, or both, as a consequence of their unique adult morphology. The key adult features involved are (1) the protective elytra and thickened exoskeleton, (2) the flexible jointing of the prothorax with the pterothorax, (3) the wide, rigid post-thoracic junction with the abdomen allowing enlargement of the gut for the storage, passage, and efficient digestion of a wide range of particulate foods, and (4) the biting mouthparts with which to process them.