Wing shape versus asymmetry as an indicator of changing environmental conditions in insects

In insects, the fluctuating asymmetry of bilaterally symmetrical traits has been suggested as an indicator of environmental stress because asymmetry is expected to increase when stressful conditions disturb the normal development of organisms. However, the extensive literature on asymmetry-stress associations is indeterminate. Here we contrast changes in asymmetry with changes in an alternate stress indicator, the shape of insect wings. The development of wing shape involves numerous genes that act throughout egg-to-adult development, so stresses that act at a specific time could alter shape in specific ways. Shape changes, as measured by the Procrustes technique, were considered in five data sets: exposure of Drosophila melanogaster (Meigen) to multiple stresses involving ethanol, low nutrition and cold shocks; exposure of a chironomid (Chironomus tepperi (Skuse)), a blowfly (Lucilia cuprina (Wiedemann)), and lightbrown apple moth (Epiphyas postvittana (Walker)) to pesticides; and development of C. tepperi under saline conditions. All these conditions influenced viability and development time. In none of these cases was a change in symmetry of wing size or wing shape detected. In contrast, in four of the five data sets there was a change in wing shape. These results suggest that wing shape may be altered more commonly by stress than trait asymmetry. Wing-shape monitoring may be useful in detecting stressful environmental conditions during development, at least under controlled conditions.