Diptera van de Engelsmanplaat Bijdrage tot de kennis van eilandfauna’s

1. 2400 specimens were collected on Engelsmanplaat during 24 days in the period from the middle of May until the beginning of August 1975. This small, sandy, island is situated between Ameland and Schiermonnikoog. It is inundated several times a year. The 2400 specimens belong to more than 83 species. More than 1700 specimens attributed to 46 species were identified with certainty. 2. Judging from the literature 93% of the 1700 specimens seem to be able to develop in biotopes as present on the Engelsmanplaat. Three species, Fucellia maritima Hal., Rhicnoessa grisea Fall., and Helcomyza ustulata Curt. were present in such a high number, that it is quite reasonable to suppose they can survive on Engelsmanplaat during the whole summer. Together they form almost 90% of the total number of specimens collected. 3. It seems that small Diptera were primarily brought to the island in a passive way in a weathertype with a lot of thermals, but that bigger species on the contrary came in a more active way on days with calm weather. 4. Since only a small number of characteristic species has been collected in comparison with the number known of the larger islands in the Waddenzee, that are not inundated several times a year as Engelsmanplaat, and since most islands biotopes have a poorer Diptera-fauna than comparable biotopes on the mainland it is suggested that dispersion by wind is more exception than rule. 5. Arguments are provided the wrack-zone in three sub-zonations of which each may have its own biocoenose.