Mayfly eggs have a variety of attachment structures that enable them to adhere to submerged objects or to the substrate. Differences in egg morphology have enabled the construction of identification keys, purely on the basis of eggs. This has provided a useful complement, not only to studies of phy-logeny, but also to taxonomy, since identification of female adults by means of external characters is often difficult.


Most nymphs hatch at temperatures in the range of 3 to 21°C. However, in the North American Hexagenia rigida, the nymphs hatch successfully between 12 and 32°C and even at 36°C if incubation is started at lower temperatures. In Tricorythodes minutus, nymphs hatch between 7.5 and 23°C, but mortality is least at 23°C. Hatching success is variable, ranging from over 90% in several Baetis and Hexagenia species to less than 50% in the Heptageniidae studied. Excluding the few ovoviviparous species, the total length of the egg development period varies from a week in H. rigida to almost a year in Parameletus columbiae. Temperature is the major factor determining the length of the period of egg development in mayflies. There is no indication that photoperiod influences egg development time. Ovoviviparity is rare in the mayflies and is restricted to the Baetidae. In North America, a number of species in the genus Callibaetis are ovoviviparous.

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