Another primitive trait of the mayflies are the subimago; no other insects have this winged yet sexually immature stage. A few specialized species have females with no subimago, but retain the subimago stage for males. The reasons the subimago still exists in this order could be that there hasn't been enough selection pressure to get rid of it; it also seems specially adapted to do the transition from water to air. The male genitalia are not fully functional at this point.
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Subimago of Leptophlebia marginata Mayflies are hemimetabolous (they have "incomplete metamorphosis"). They are unique among insects in that they moult one more time after acquiring functional wings; this last-but-one winged (alate) instar usually lives a very short time and is known as a subimago, or to fly fishermen as a dun. Mayflies at the subimago stage are a favourite food of many fish, and many fishing flies are modelled to resemble them. The subimago stage does not survive for long, rarely for more than 24 hours.
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The final moult of the nymph is not to the full adult form, but to a winged stage called a subimago that physically resembles the adult, but which is usually sexually immature and duller in colour. The subimago, or dun, often has partially cloudy wings fringed with minute hairs known as microtrichia; its eyes, legs and genitalia are not fully developed. Females of some mayflies (subfamily Palingeniinae) do not moult from a subimago state into an adult stage and are sexually mature while appearing like a subimago with microtrichia on the wing membrance. Oligoneuriine mayflies form another exception in retaining microtrichia on their wings but not on their bodies.
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Like related species, the Eastern Green Drake has a fully aquatic larval stage. This is followed by a phase of a flying sexually immature adult called the subimago. Finally, there is a short-lived sexually mature adult stage.
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Modern mayflies have eliminated all the instars between imago and nymph, except the single instar called subimago, which is still not (at least not in the males) fully sexually mature. The other flying insects with incomplete metamorphosis (Exopterygota) have gone a little further and completed the trend; here all the immature structures of the animal from the last nymphal stage are completed at once in a single final moult. The more advanced insects with larvae and complete metamorphosis (Endopterygota) have gone even further. An interesting theory is that the pupal stage is actually a strongly modified and extended stage of subimago, but so far it is nothing more than a theory.
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Subimago of L. marginata Leptophlebia marginata, the sepia dun, is a species of mayfly in the family Leptophlebiidae. It is native to Europe and North America where it is distributed widely near lakes, ponds and slow-moving streams. The larvae, which are known as nymphs, are aquatic.
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Ephemera vulgata can be told in both adult and subimago stages from the rather similar green drake (Ephemera danica) by its duller colour and slightly smaller size. The wings are more heavily veined and the upper side of the abdomen has pairs of dark lateral markings on each segment.
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Ephemera danica can reach an imago size of in males, while females are larger, reaching . This mayfly, with its characteristic markings and three tails (Cerci), is the most commonly seen of British Ephemeridae. Imago wings are translucent with dark veining, while in subimago they are dull and yellowish with brown veins. Moreover, forelegs and the tails of the spinners are very much longer than in duns.
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Their immature stages are aquatic fresh water forms (called "naiads" or "nymphs"), whose presence indicates a clean, unpolluted environment. They are unique among insect orders in having a fully winged terrestrial preadult stage, the subimago, which moults into a sexually mature adult, the imago. Mayflies "hatch" (emerge as adults) from spring to autumn, not necessarily in May, in enormous numbers. Some hatches attract tourists.
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One reason for this could be that the modification of the abdominal appendages into male copulation organs emerged later than the evolution of flight. This is indicated by the fact that dragonflies have a different copulation organ than other insects. As we know, in mayflies the nymphs and the adults are specialized for two different ways of living; in the water and in the air. The only stage (instar) between these two is the subimago.
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They are unique among insect orders in having a fully winged terrestrial adult stage, the subimago, which moults into a sexually mature adult, the imago. In 1853, Walker first described about two species of mayflies from Sri Lanka. Then 1858, Hagen documented 8 more species from the country. The most useful taxonomic and other ecological aspects of mayflies of Sri Lanka came during 1960s and 1970s by Peters (1967) and Peters & Edmunds (1970).
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In Florida, the adults emerge at the end of April or in early May, and in South Carolina, in the first half of June. When ready to transform, the nymphs swim to the surface, split their skin, transform into the subimago winged form and fly off, all in the course of ten to twenty seconds. The exuviae (nymphal skin) float away downstream. Their emergence is synchronised and the males emerge first, about one and a half hours before sunrise.
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Male subimagos moult but the females, which emerge soon after the males, remain in the subimago form. Having moulted into the adult form within about five minutes of emergence, the males patrol a stretch of river about long, seeking out females with which to mate. They continue to do this until they drown, having fallen into the water from exhaustion. Upon emergence, the females mate, deposit their eggs into the water and die within the course of about five minutes.
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Subimagos are generally poor fliers, have shorter appendages, and typically lack the colour patterns used to attract mates. In males of Ephoron leukon, the subimagos have forelegs that are short and compressed, with accordion like folds, and expands to more than double its length after moulting. After a period, usually lasting one or two days but in some species only a few minutes, the subimago moults to the full adult form, making mayflies the only insects where a winged form undergoes a further moult.
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In more primitive fossil forms, the preadult individuals had not just one instar but numerous ones (while the modern subimago do not eat, older and more primitive species with a subimagos were probably feeding in this phase of life too as the lines between the instars were much more diffuse and gradual than today). Adult form was reached several moults before maturity. They probably didn't have more instars after becoming fully mature. This way of maturing is how Apterygota do it, which moult even when mature, but not winged insects.
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The distant ancestor of flying insects, a species with primitive proto-wings, had a more or less ametabolous life-cycle and instars of basically the same type as thysanurans with no defined nymphal, subimago or adult stages as the individual became older. Individuals developed gradually as they were grew and moulting, but probably without major changes inbetween instars. Modern mayfly nymphs do not acquire gills until after their first moult. Before this stage they are so small that they need no gills to extract oxygen from the water.
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These sink to the bottom and hatch after 45 days, the nymphs burrowing their way into the sediment where they spend two or three years before hatching into subimagos. When ready to emerge, several different strategies are used. In some species, the transformation of the nymph occurs underwater and the subimago swims to the surface and launches itself into the air. In other species, the nymph rises to the surface, bursts out of its skin, remains quiescent for a minute or two resting on the exuviae (cast skin) and then flies upwards, and in some, the nymph climbs out of the water before transforming.
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For example, the flies known as "emergers" in North America are designed by fly fishermen to resemble subimago mayflies, and are intended to lure freshwater trout. In 1983, Patrick McCafferty recorded that artificial flies had been based on 36 genera of North American mayfly, from a total of 63 western species and 103 eastern/central species. A large number of these species have common names among fly fishermen, who need to develop a substantial knowledge of mayfly "habitat, distribution, seasonality, morphology and behavior" in order to match precisely the look and movements of the insects that the local trout are expecting. Izaak Walton describes the use of mayflies for catching trout in his 1653 book The Compleat Angler; for example, he names the "Green-drake" for use as a natural fly, and "duns" (mayfly subimagos) as artificial flies.
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