Complex Mayflies

Mayfly- Click to enlarge

I have been under the impression that the presence of mayfly larvae in a stream is an indicator of high quality water with good oxygenation.  In some cases that is true, but the whole story is more complex.

There are 15 families of Mayflies found in Missouri,  all lumped in the order Ephemeroptera.  The name reflects their ephemeral existence because of the mouthless adults life span of a few hours to two days.
We tend to think of their presence in streams as an indicator of good oxygenation because some species with fixed gills require flowing water with adequate oxygen content.  It turns out that some other species have developed motile gills to tolerate lentic* (still) water in ponds and lakes as well as low oxygen conditions.

The larvae that we look for in macroinvertebrate sampling will molt from 12 to 45 times over two weeks to two years before reaching adulthood.  Even then they are not done, as the last molt before adulthood is quite different from a human teenager.  It comes out as a "dun", a dull colored, sexually immature winged creature which then molts into the adult capable of mating.  No other insect has this intermediate form.

Swarm hatch on truck

Although many mayflies deposit their eggs in masses by dipping their ovipositor in the water as they fly, others drop them singly or from the air over water.  When the eggs hit the water they begin to develop immediately.  A female may drop as many at 500-3000 or more.  If they drop them all at one time, there may be a swarm hatch all at once, sometimes dramatic in number.

Mayfly nymphs usually live on detritus or algae which they scrape off the stream bottom.  For this reason, well oxygenated streams with clean silt free bottoms are especially important as the nymphs fulfill a reluctant role near the bottom of the food chain. While some have evolved to withstand low oxygen and stagnant waters, the presence of lots of nymphs in lotic* (moving) water is usually an indicator of good quality water.  Trout fishermen (the top of the food chain?) also use Mayflies' presence to help them choose which fly to tie on.  You gotta love them.


The Missouri Stream Team newsletter has all this information and a lot more.  Just go to their
facts site  and download #14.

*Words for the day:
Lentic ecosystems are still waters such as marshes, ponds, and lakes. 
Lotic ecosystems have flowing water such as springs, creeks and rivers.
Now don't you feel better?