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Ephemeroptera (mayflies) are the most primitive order of pterygote insects. They encompass over 3000 species and over 400 genera constituting about 40 described families (Barber-James et al., in press). Mayflies colonised all kinds of fresh water habitats, from springs to large rivers as well as standing waters such as ponds, dykes and lakes. They are present in all landmass except in Antarctica. The immature stages are strictly aquatic, while the imagos are on the wing. The subimago is a unique stage only found in Ephemeroptera, characterized by possessing functional wings at the penultimate moult. Both imaginal and subimaginal stages are extremely brief (generally from a few hours to a couple of days) as they lack functional mouthparts and are unable to feed. Because of their ancient origin, widespread distribution, and limited dispersal ability, mayflies are excellent candidates for studies of biogeography. The Arabian Peninsula is a priori a rather inhospitable area for aquatic insects as the permanent hydrographic system is very restricted. However, a few natural biotopes such as resurgence of brooklets and pools in wadis constitute suitable habitats for aquatic fauna. Because of the increasing need of water for agricultural and domestic uses, most of these habitats have been modified or destroyed. Artificial constructions such as dams or tanks may constitute substitution habitats for part of the ubiquitous insects living in standing water. Until the 1990’s, the mayfly fauna of the Arabian Peninsula remained almost unknown. Important studies were conducted during the 1990’s by Sartori, Thomas and Gillies on material collected mainly by Prof …