Why Do Mayflies Lay Eggs on Dry Asphalt Roads? Water-Imitating Horizontally Polarized Light Reflected from Asphalt Attracts Ephemeroptera

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At sunset, Kriska et al. (1998) observed that individuals of several mayfly (Ephemeroptera) species swarmed and mated above and landed on dry asphalt roads (Fig. 22.1A-C) and shinyblack plastic sheets used in agriculture (Fig. 22.1 D-I) in the immediate vicinity of ephemeropteran emergence sites (mountain streamlets), and that after copulation the females laid their eggs en masse on dry asphalt roads instead of laying them on the water surface. The behaviour of male and female mayflies swarming and mating above asphalt roads is similar to that above water surfaces. Previous descriptions of ephemeropteran swarming, mating and egg-laying (reproductive) behaviour have misinterpreted this phenomenon: (1) It has generally been assumed, that asphalt roads serve as “markers”1 for swarming mayflies. (2) Oviposition by mayflies on asphalt roads has been simply explained by the shiny appearance of wet roads which may lure the insects like the surface of real water bodies. The first explanation, however, cannot apply to the observed egg-laying on asphalt roads, because mayflies never oviposit onto objects serving as markers. The second interpretation cannot explain why egg-laying by Ephemeroptera also frequently occurs on totally dry asphalt surfaces.