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Mayfly Lifecycle by Mick Hall


One of the most frequent questions to try and fully explain is the sequence of events that occur during a Mayfly nymph’s life and its transformation into the flying insect.  I suppose the best starting place is with the egg, which hatches into a grub, which in turn develops into a nymph.  As the nymph grows it sheds its skin a number of times, anywhere from around twelve to twenty such moults (scientifically known as Instars) before it comes time to hatch out into its terrestrial form.

The lifespan of the nymph can vary from species to species; some have an annual cycle whilst others are bi-annual.  The fact is that the Mayfly’s life is primarily aquatic; it only hatches out into our world to mate.  Another interesting point to note is that we are told that it is the only insect known to have two stages in its adult form, that is the sub-imago (Dun stage) and imago (Spinner).  The key stages for the flyflicker are the free swimming nymphal form and the nymph in its last stage before hatching out into its winged insect form.

The nymph during this period is very vulnerable as it is restless and often has trial runs at breaking through the surface film before it finally emerges.  During this stage in most species the back of the thorax actually turns black as it starts to rot.  This happens to assist the emerging insect to escape the nymphal shuck.  I call this stage the pre-emerger.

The next important stage is the emerging period and as you are probably aware, there are a lot of patterns around to imitate this stage.  Flies like the Shaving Brush, Ash Emerger and the Possum Emerger are just a few.

The next is the Dun or the sub-imago, which is the first stage of the flying cycle.  The dun is still carrying a second skin, which it still has to discard.  We are told that this skin acts as a waterproofing agent, just like a wet suit.  The dun staggers off to the bank side vegetation where it rests and dries out its wet suit.  Interestingly mayflies have a hook-type claw on the tip of each leg.  The Mayfly will embed these hooks into whatever surface that they may be resting on to ensure a good grip.  This in turn allows the spinner to emerge from the dun shuck.  After a period from around an hour in some species to many hours in others, it splits the skin and slowly emerges to what we call the spinner, which is the final stage in the Mayfly’s life.

The adult Mayfly has no functional mouthparts so they do not feed, their sole purpose is to mate and their life is over.  The males are easy to identify, they have larger eyes and they actually grow an extra extension to their forelegs during emergence from dun to spinner and also have a pair of clasps at the base of the abdomen.  The clasps and forelegs are used for grip during copulation, which occurs in flight.  The males tend to hatch out first and on evening they can be seen hovering in columns, rising and falling back in unison.  This activity is often called the Dance of the Mayfly; it is simply performed to attract the attention of the females, which fly into the swarm to select a mate.  After mating the male will often drop to the ground and die.  Rarely, if ever, does it return to the water.

The key to identifying females is their small eyes and shorter forelegs.  After copulation the females head off to develop their eggs, which they lay mostly by dipping their tails on the surface of the stream (in some species they actually crawl or dive back down to the bottom to deposit their eggs).  Once this act is completed, the female often dies on the water where she drifts along with the current, wings outstretched.  It is at this stage that she is known as a Spent Spinner.


I have often thought on how the term spinner came into being and I have traced the term way back to 1784, Anglers Museum by Thomas Shirley, in which the word spinner as far as I can ascertain, was first used.  If anyone can trace it back earlier than that, please let me know.  One thought on how the term came into being is that whilst the spent insect is drifting in the current, the current itself can cause the insect to spin around as it drifts away.


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