Evolution of flightlessness in Scarabaeoidea (Insecta, Coleoptera)

Flight clearly confers exceptional mobility on an insect, and conversely the lack of night greatly restricts the area over which an insect can search for mates, oviposition sites and food. Therefore it is reasonable to expect that selection will favour the retention of flight in a highly variable environment and that non-migratory life histories will most likely evolve in very persistent habitats. Environments with a higher than expected incidence of flightlessness amongst a wide variety of scarabaeoids are temperate highland forests in the tropics, mountains, deserts, islands, termite nests and cold regions. The flightlessness may be in either sexes or only the female and may be as a result of wing reduction or it may be bt behaviourally or physiologically induced. In some taxa flightlessness appear's to be a random occurrence with a few species out of many in the taxon nightless whereas in others there is clearly a phylogenetic propensity for flightlessness with all or many species flightless. Flightlessness in some taxa is inexplicable in terms of current distribution and historical explanations must be advanced. The habitat with a predicted high incidence of flightlessness but which, in fact, has low incidence amongst scarabaeoids, is that in tropical forests. In addition, intimate phoretic associations with other animals, although relatively common and highly specialised, yield none of the expected evidence of flightlessness. It is proposed that biologically simple communities, in addition to stable habitats, contribute toward flightlessness. Flightlessness may evolve in one of two ways amongst scarabaeoids; where only females are nightless, the selective pressure to trade-elf flight against reproduction is paramount: where it occurs in both sexes, small microhabitats or high population density which increase the chances of sexual encounters, are overriding.