Ephemeroptera
is a group of 2,000 insect species commonly known as mayflies. They are
considered to be part of the clade Uniramia which includes silverfish and
dragonflies, among others. Ephemeroptera and Odonata are the only extant orders
of winged insects in the infraclass Paleoptera. All other insects with wings
are in the Neoptera, and are characterized by a wing articulation (joint) that
allows them to fold their wings back over their abdomens at rest (Carpenter,
1992).
Ephemeroptera
are aquatic insects that often go through many nymph stages (living in water)
and two flying stages (the subimago and the imago). They are the only insects
to have two flying stages, and can be recognized by their three caudal
filaments (tails) at the tip of the abdomen, and a single claw on each leg.
This differentiates them from the closely related stoneflies which have two
tarsal claws. The flying stages are characterized by relatively large
forewings, which are usually kept upright, and reduced or nonexistent hind
wings.
Mayfly Morphology
The first
stage of the life of a mayfly is the nymph (larva), which not only looks very
different from the adult, but lives in the water. When the nymphs hatch from
the eggs, they are less than 1 mm long. They have no gills at first, and their
body shape varies according to habitat. For example, those that burrow (such as
Ephemera) have more cylindrical bodies, whereas those that slide under rocks
(such as Heptagenia) are flatter. Those in the genus Caenis crawl on mossy
stones and vegetation, so they have short bodies with squat legs. Ephemeroptera
nymphs may grow to anywhere from 4 mm to 3 cm long. They are generally
camouflaged against their background. The number of molts a nymph goes through
on its way to becoming an adult does not depend on its nutrition, but the
increase in size that comes with each molt does.
In older
nymphs, gills are found in pairs on each segment of the abdomen (see pictures
below). The gills extend from the sides of the body and are oval-shaped. These
gills beat to control the flow of water through the body, which also controls
the amount of oxygen and salt that flows through the body. Nymphs in still
waters generally have larger gills, and those in running water have smaller
gills; this allows the nymphs of each habitat to get their optimum flow of
water. Not only do the gills function in uptake of water, salt, and oxygen, but
they also send water off at right angles to the body. This is used to mislead
predators. If the water simply flowed out the back of the nymph's body,
predators would know that the nymph was sitting at the beginning of the stream.
However, since they send water away from their bodies at several points, the
nymphs are not as easy to track.
Some signs
of sex can be seen in the last few stages of the nymph, even before it becomes
an adult. At this stage, male nymphs have the beginnings of clasping organs on
the lower portions of their abdomen, with which they hold the female during
copulation. In some species, the males have divided eyes that are two colors.
The upper portion is for seeing movement, and the lower portion is specialized
for seeing details. The females have smaller eyes and oviducts in the lower
abdomen.
When it
comes time for the last nymph stage to molt into a subimago (the first flying
stage), the guts empty out and the mid-gut section fills with air. Often, many
nymphs will then simultaneously let go of their hold on their anchor in the
water and float up to the top. Once they reach the air, the cuticle splits open
on the thorax and the wings come out. This is the time of greatest
vulnerability in their lives as they float on the water before they are strong
enough to fly. The subimago has short hairs on the wings and on the body; the
wings are dull and pigmented. Once it gains some strength, it flies from the
water to some form of shelter such as a tree, long grass, or the underside of a
bridge and molts again within 24 to 48 hours. Thisadditional molt allows the
legs and tails of the insect to grow more. Longer tails give more stability in
flight, and longer legs make it easier for the male to grasp the female in
mating.
The imago
(the final adult stage) has shiny, hairless wings. The longer legs and tails
allow for more rapid flight. The corrugation of the wings protects them by
making them more flexible and therefore less vulnerable to wind damage. The imago
mates and dies within a few hours to a day. (Harker, 1989) This short adult
life is what gives the order its name from the Greek ephemeros meaning
"lasting but a day."
Mayfly nymphs : At left above, is the nymph of a Baetid mayfly, and at right is a Heptageniid nymph. Notice the Baetid has a slender, cylindrical body and small gills on its abdomen. The small gills indicate that it lives in moving water, and the shape of its body makes it well-suited for swimming against the current. The Heptageniid has a broad flat body, better suited for life clinging to the bottom of the stream to avoid being carried away by the current. Notice also that both nymphs lack the wings of the adult stage, and both have the three caudal filaments (tails) characteristic of mayflies. (Click on either of the pictures above for a larger image).
Mayfly Ecology
Mayfly eggs
are eaten by snails and by caddisfly larvae. The nymphs may be eaten by fish,
frogs, birds, flies, or water beetles. The subimagos are eaten by fish, birds,
dragonflies, water beetles, or other predatory insects. For their own
nutrition, mayfly nymphs move over stones and weeds to graze off bacteria. They
may collect from sediments or feed on detritus. Most mayflies are collectors
and scrapers (Edmunds et al, 1976), and most of the consumed bacterial cells
pass through their bodies without being used. They are opportunistic
generalists, meaning that they eat what they can, when they can. Nymphs have
some symbiotic relationships with chironomids that may be commensal. The
chironomid larvae attach to the cuticle of the mayfly and eat the unicellular
organisms that live there. More harmful to mayflies are nematode parasites, who
use nymphs as their hosts by eating their muscles from the inside.
Ephemeroptera
nymphs are usually microhabitat specialists. Each species survives best on a
specific substrate at a certain depth under water with a certain amount of wave
action. For example, Rithrogena generally live in medium to large trout
streams. Ephemeridae burrow into soft areas where flow is slower, or in areas
of lakes and rivers where deposits occur; the particular substrate and burrow
depends on the genus. The primitive habitat of schistonate mayflies is still
water even though most extant mayflies live in running water (McCafferty,
1990). In some areas, succession occurs by different species. For example, in
Utah Epeorus longimanus is followed by E. deceptivus. Some species dominate in
the spring while others dominate in autumn (Edmunds et al, 1976). Some mayfly
nymphs are quite sensitive to pollution and are used to evaluate water
pollution and stream health.
Mating
occurs in a swarm, and at these times there may be such dense clouds of
mayflies in the air near streams that driving becomes impossible. Because their
wings are so fragile, the imagos need calm weather in order to mate. Males
usually swarm very near the water, though swarm formations vary from species to
species. Within the swarm, the insects are always changing positions. When a
female enters the swarm, males try to mate with her. Some float to the ground
while mating, and others continue flying. Once a male has successfully mated,
he will guard the female to make sure that no other male mates with her. The
female then flies to water to lay her eggs. She dips into the water while
flying and releases a few eggs each time. The eggs sink to the bottom and their
surface changes. Some become covered with a sticky substance and some have
adhesive disks. Some species are parthenogenic, meaning that they do not need
sperm to produce fertile eggs (Harker, 1989). The time it takes for emergence
into the subimago form varies depending on temperature -- the milder the
temperature, the earlier the emergence (Edmunds et al, 1976). Many species have
synchronized emergence of subimagos. In these cases, the emergence occurs at a
specific time of day under certain weather conditions (Harker, 1989).
Mayfly Fossil Record
The first
recorded mayfly nymphs are from the Late Carboniferous (Fenton, 1989). In most
places, mayflies are represented primarily by larval fossils (Sinitshenkova,
1990). Presumably, this is because of the very short lifespan of the adult
stage. The siphlonurid form, which is shrimp-like with short caudal filaments
that have lateral hair fringes, is the primitive form (Riek, 1970). However, it
is difficult to classify early fossils in the correct order. The fossil
Triplosoba pulchella is the only insect from the Carboniferous that is
consistently placed in the order Ephemeroptera (McCafferty, 1990).
In recent
years, certain fossils found in Moravia (eastern Czech Republic) and Oklahoma
(central U.S.) previously placed in the order Archodonata have been re-classified
as Ephemeroptera. The Oklahoma fossils are very well preserved with the wing
venation clearly shown. Hubbard and Kukalova-Peck argue that the presence of
three caudal filaments, which is a plesiomorphy, and a well developed costal
brace, which is a uniquely derived character of Ephemeroptera, make it
impossible to place these fossils in any order other than Ephemeroptera. It has
been argued that the presence of segmented tarsi double tarsal claws, traits
which are not found in modern mayfly nymphs, means that the fossils were not
Ephemeroptera. However, this difference merely means that changes have resulted
through evolution since the origin of the group. The simple tarsi and single
tarsal claw must be apomorphies, because the double claws and segmented tarsi
can still be found in the closely related orders Odonata (dragonflies and
Damselflies) and Plecoptera (stoneflies) (Hubbard & Kukalova-Peck,1980).
Early mayfly
adults differ from the Permian also differ from their living descendants.
Fossil imagos of Protereisma from Kansas have functional mouthparts and fore
and hind wings of similar size and shape (Carpenter, 1992). Modern mayflies do
not feed as adults, and have smaller hindwings, or no hindwings at all in some
species. The highest diversity of Ephemeroptera appears to have been during the
Jurassic. Fossils of nine families have been found during this period.
Currently,
the Ephemeroptera are classified in several different ways, depending on who
does the cladistic analysis. According to Riek, this order has six
superfamilies with 14 families. There are also two extinct superfamilies,
Proterismatoidea and Mesephemeroidea, which are the Permian insects that appear
to be mayflies or their precursors. During the lower Cretaceous, there was
extinction and emigration of many mayflies in Brazil -- very few of these have
survived to present day (McCafferty, 1990). The evolution from living in
still-water to living in running water occurred before the Cenozoic Era
(McCafferty, 1990).
