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3. Mayflies: subimago to imago .... ?

CHARACTERISTICS OF THE SUBIMAGO
The subimago of a mayfly is vastly different from the larva, whereas the
subimago and adult mayflies are quite similar and in a few species can be
difficult to tell apart. There is now little doubt that the subimagoa nd adult
represent two separate instars. In early publications, Lameere (40) and
Needhamet al (52) argued that the change from subimagot o adult was only
delamination of the outer layer of cuticle. This idea was refuted by Ide (29),
who showed critical cuticular differences as well as significant dimensional
510 EDMUNDS & McCAFFERTY
differences of some structures in the two forms, and by Taylor & Richards
who showed that complete cuticles exist in both forms and result from
complete molting cycles that include apolysis and ecdysis.
All male mayflies and most females molt from subimago to adult

Surface emergence would theoretically be possible once the subimago was
preadapted, or exaptated, to being unwettable because of surface microtrichia.
Two situations in the evolution of mayflies could have been related to a
shift from an ancestral land emergence to surface emergence, which is now by
far the predominantb ehavior. Ani ncrease in predatory pressure on land could
have favored surface emergence. A consideration of the kinds of habitat
transitions amongth e families of mayflies, in light of their relationships and
principal aquatic habitats, however, strongly suggests that the behavioral shift
was related to a shift from primarily still- or quiet-water habitats to runningwater
habitats where larval exit from the water would be problematic.

Wouldn't this imply that mayflies are a 'more recent evolution'....

Conclusion
Because of above mentioned, we can conclude that the extant mayflies remain many primitive characters. They are relics of primitive Pterygota. Those make them the good materials for explaining the origin and evolution of Insecta.

How do you guys know all this stuff???

Scharfer(1975) suggested that the functional genitalia and wings of mayflies do not be mature simultaneously is a relic of ancient double moult: the wings occur first , after the second moult the genitalia are functional. While in other insects, they occur in only one moult. The selection pressure did not affect the ephemerous beings, the mayflies, greatly.

Wing articulation and wing position
Without the flexing wing sclerites and muscles, Mayflies hold their wings vertically upwards when they are at rest, can not flex their wings backwards on the abdomen like the Neoptera.
This character is often cited as primitive one by many Entomologists. But according to Kukalová-Peck, this is not true.

John, I don't think that "primitive" and "elegant" are mutually exclusive. After all, I doubt that mayflies could have survived for so long without some kind of evolutionary elegance. And the unique "dun" stage is a large part of what makes them a premier dry-fly insect and so cherished by fly fishers.

Finally, I notice that this thread started with a "why" quesiton. Science doesn't deal too well with whys. It deals more with hows.

As I understand it, selection pressures are influences (from predators or environment, for example) that allow certain changes/adaptations to be promoted in evolutionary development. I assume that the line means that those pressures have not yet been sufficient to force mayfly evolution into a single molt that would produce a winged and sexually mature adult in both sexes.