Morphological correlates of flightlessness in southern African Scarabaeinae (Coleoptera: Scarabaeidae): testing a condition of the water-conservation hypothesis

Flightlessness in desert beetles is thought to have evolved either as a response to decreased environmental heterogeneity or directly to reduce water loss. The water-conservation hypothesis rests on three conditions: that spiracular transpiration is greater than cuticular transpiration; that cuticular transpiration rates are lower in desert species; and that changes in body form associated with flightlessness lead to an overall reduction in water loss rates. The extreme form of the morphological-convergence condition suggests that this change in body shape should be most pronounced in desert-dwelling taxa. The morphological-convergence condition was examined using a morphometric analysis of body shape in flying and nonflying dung beetle species from two southern African tribes occurring in arid and mesic habitats. Although the Canthonini have a more rounded body than the Scarabaeini, flightless species in both tribes have a more rounded body than the flying ones, except at the smallest body sizes. This rounding is more pronounced in flightless, desert-dwelling Scarabaeini than in flightless species from more mesic habitats. All three conditions required by the water-conservation hypothesis are met in various beetle taxa, but the hypothesis and its conditions have yet to be tested on a single, monophyletic taxon.