Bodenheimer (Animal Life in Palestine, 1935) has this to say:
The larvae of the Ephemerid flies (Ephemeridae), of which only a very few species have been bred in Palestine, develop in the water. Polymitarcys sp. develops in the muddy beds of rivers etc. The larvae of Cléon dipterum, on the other hand, are found in stagnant waters. Mass flights of these smaller species have never been observed. Most of our species are relatively small, so that even mass-flights may easily be overlooked. The author, however, witnessed a mass-flight of the gigantic species Palingenia jordanica (25mm body - 70 mm cerci-length) on the lower Jordan, on an early April morning. Between 4-5 AM the larvae appeared on the water surface in large numbers and the adults left for a short flight. At 7 AM everything was over and the recently hatched adults after having copulated and oviposited were found everywhere, dead or dying.