Mayflies are common insects found in almost all freshwater habitats, as well
as some brackish ones. There are over 2 000 named species in 200 genera and
19 families. The adults are soft bodied insects with very short antennae, vestigial mouthparts, two long cerci and usually a long caudal filament at the end of the abdomen. Most adult Mayflies have two pairs of wings however the second pair
are considerably smaller than the first and in some species are absent altogether.
Nymphs have much longer antennae, functioning mouthparts and are aquatic.

     Mayflies date from the Carboniferous and Permian times and are the oldest
of the extant (still living) winged insects, they are unique in being the only insect
order to have a subimago (last non-adult life stage) with wings. Adult Mayflies do
not feed and live very short lives, many species live only for one or two days but
in others the adult life span may be as short as 2 hours or as long as 14 days.
Nymphal life cycles can range between 3-4 week to 2.5 years depending on the species. Mayflies can be easily recognized because they are the only order of
insects to hold their wings pointing straight up, they can not fold them down,
they generally hold their abdomen curved upwards away from the substrate
(what ever they are standing on.)

     Mayflies have attracted man's attention for a long time and one of the oldest accounts of Mayfly biology was published bySwammerdam in 1675, it is an
unfortunate fact that the genus of Mayflies studied by Swammerdam Palingenia sp.
is now extinct in the Netherlands and the rest of Western Europe, a consequence
of the Mayflies general low tolerance for pollution.

Information courtesy of Insect World  http://www.insect-world.com/main/six.html