Mayflies (order Ephemeroptera), insects favored as food by freshwater fish and as models for the artificial lures of fly fishers, live double lives. During their immature (nymphal) stages, they pursue an underwater career for 1 y or 2 y, molting their cuticular exoskeleton on numerous occasions as they grow. As in other insects, their future wings grow only slowly, but, as maturity approaches, the organs of flight grow rapidly, and the adult insect emerges from the water in a completely different form than the nymph. All winged hexapods (Pteryogota) undergo a metamorphosis of more or less this kind, but mayflies are unique among living insects in having more than one winged developmental stage (1), the final imago or adult instar being preceded by a short-lived subimago (called by anglers a “dun”) (Fig. 1). A paper in PNAS by Kamsoi et al. (2) now presents a molecular analysis of the hormonal and cellular control of metamorphosis in a model ephemeropteran species, Cloeon dipterum, casting fresh light on the evolutionary significance of the mayfly subimaginal stage.
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