Abstract.During oviposition, femalePotamarcha congener(Rambur) (Libel‐lulidae: Odonata) hover over water and face towards the margins of temporary pools. They repeatedly scoop up drops of water with the posterior end of the abdomen and flick them together with eggs towards the bank. Embryonic development is complete in 7–8 days at 28d̀C but hatching can be delayed for up to 80 days if the eggs are kept in air at 100% r.h. Hatching is triggered by oxygen lack. In the laboratory, perfusion with gases containing oxygen at partial pressures below 5.3kPa causes hatching without submergence in water. Eggs exhibit considerable variability of hatching threshold: this is partly due to age but it also occurs regardless of age among the eggs of a single clutch and probably has adaptive value. The eggs of two other species tested,Pantala flavescensandOrthetrum pruinosum, show no hatching response to oxygen