Life history attributes of the rare mayfly Siphlonisca aerodromia Needham (Ephemeroptera:Siphlonuridae)

Over a 3-y period, we examined the biology of late-instar nymphs, adults, and eggs of the rare predaceous mayfly S. aerodromia at Tomah Stream, Maine, to identify life history attributes associated with the mayfly's movements between the stream channel and the bordering floodplain. Eggs were deposited in the stream channel in June and hatched in November and December. Embryonic development occurred in 47-92% of eggs from unmated females. With rising water levels following snowmelt in March or April, nymphs moved from channel to floodplain, where most nymphal growth and development took place. Sex ratios of nymphs in the floodplain were female biased (1 M:1.4 F in 1991 and 1 M:2.1 F in 1992). Nymphs molted to the final instar earlier in 1991 than in 1992 and male nymphs molted to the final instar before female nymphs in 1991 but not 1992. Time in the final instar decreased as the season advanced. Seasonal emergence was protandrous and lasted 10-11 d during late May and early June; timing of seasonal emergence in 1991, 1992, and 1993 was related to maximum air temperatures and persistence of standing water in the floodplain during May. Emergence of subimagos was female biased in 1991, but male biased in 1992. Diel emergence was from 0700 to 1700 h (EST) and occurred only when water temperature was >11??C.