Evolution of flightlessness in Scarabaeoidea (Insecta, Coleoptera)

Flight clearly confers exceptional mobility on an insect, and conversely th e 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 en vironment and that non-migratory life histories will most likely evolve in very persistent habitats. Environments with a higher than expected incidenc e of flightlessness amongst a wide variety of scarabaeoids are temperate hi ghland forests in the tropics, mountains, deserts, islands, termite nests a nd cold regions. The flightlessness may be in either sexes or only the fema le and may be as a result of wing reduction or it may be bt behaviourally o r physiologically induced. In some taxa flightlessness appear's to be a ran dom occurrence with a few species out of many in the taxon nightless wherea s in others there is clearly a phylogenetic propensity for flightlessness w ith all or many species flightless. Flightlessness in some taxa is inexplic able in terms of current distribution and historical explanations must be a dvanced. 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, a lthough relatively common and highly specialised, yield none of the expecte d evidence of flightlessness. It is proposed that biologically simple commu nities, in addition to stable habitats, contribute toward flightlessness. F lightlessness may evolve in one of two ways amongst scarabaeoids; where onl y females are nightless, the selective pressure to trade-elf flight against reproduction is paramount: where it occurs in both sexes, small microhabit ats or high population density which increase the chances of sexual encount ers, are overriding.