Eocene flower visitation of European Diptera and their potential pollinator role

Pollination by insects, including flies, is commonly a mutualistic interaction, in which both the plant and the insect benefit. Compared to other groups of pollinators or flower visitors relatively little is known about flower visitation by flies (Diptera) during most of the Cenozoic. Until now, the fossil record has provided only a few reliable fossils where flies are preserved with in situ or adhered pollen from flowers they visited before death and fossilization. Here, we report preliminary results from an ongoing study on flower-dipteran interactions during the middle Eocene of Central Europe. We screened 220 Diptera (19 with pollen) from the Middle Eocene of Messel and 20 Diptera (one with pollen) from the Middle Eocene of Eckfeld. The flies were screened for in situ pollen in their digestive system and adhering pollen present on various parts of their exoskeleton (head, thorax, abdomen, leg, etc.). From Messel, we studied, among other Arthropoda, 65 Brachycera (=flies; eleven with pollen) and 93 Nematocera (incl. gnats and midges; six with pollen). Out of the 20 pollen-bearing specimens, one had both gut content and adhering pollen, seven had only gut content pollen, and twelve had only adhering pollen. Each fly had between one and seven different pollen types. The number of pollen types found on the flies suggests both oligo- and polylectic feeding behaviour depending on the taxon. Only a few of the same pollen types (four pollen types) were found on different flies. In total we discovered 19 different pollen types on all the fossil fly specimens. The pollen represents, among others, Araliaceae, Cornaceae, Hamamelidaceae, Juglandaceae, Lythraceae (Decodon), Oleaceae, Rutaceae, Sapotaceae, and Vitaceae (Parthenocissus). Surprisingly, four flies carried Juglandaceae pollen on their exterior, but today, this family is mainly wind-pollinated. The amount of in situ pollen discovered was, in most cases, filling the whole digestive system of the flies, suggesting long and active feeding from anthers of flowers. About half the flies bearing adhered pollen had considerable amounts on their exterior, suggesting they could be potential pollinators of the plants they visited, especially Araliaceae, Hamamelidaceae, and Sapotaceae.