Wettability and contaminability of insect wings as a function of their surface sculptures

The wing surfaces of 97 insect species from virtually all relevant major groups were examined by high resolution scanning-electron-microscopy, in order to identify the relationships between the wing microstructures, their wettability with water and their behaviour under the influence of contamination. Isolated wings with contact angles between 31.6 degrees and 155.5 degrees were artificially contaminated with silicate dusts and subsequently fogged until drops of water (''dew'') formed and rolled off. The remaining particles were counted via a digital image analysis system. Remaining particle values between 0.41% and 103% were determined in comparison with unfogged controls. Some insects with very unwettable wings show a highly significant ''self-cleaning'' effect under the influence of rain or dew. Detailed analysis revealed that there is a correlation between the wettability and the ''SM Index'' (quotient of wing surface/(body mass)(0.67)) with values ranging from 2.42 to 57.0. Furthermore, there is a correlation between the ''self-cleaning'' effect and the SM Index, meaning that taxa with a high SM Index. e.g. ''large-winged'' Ephemeroptera, Odonata, Planipennia, and many Lepidoptera, have very unwettable wings and show high particle removal due to dripping water drops. The ''small-winged'' insects, such as Diptera and Hymenoptera, and insects with elytra, such as Blattariae, Saltatoria, Heteroptera and Coleoptera, show completely opposite effects. This is clearly a result of the fact that species with a high SM Index are, in principle, more restricted in flight by contamination than species with a low SM Index which can also actively clean their own wings. The wings primarily serve a protection function in insects with elytra, so that the effects of contamination are probably of minor importance in these insects. Copyright (C) 1996 The Royal Swedish Academy of Sciences.