Mayfly Spinners?
You gotta love spinners. For one thing spinners are unique to the order Ephemeroptera or mayflies. That’s right, no other group of insects, out of the million or so species currently known, have a stage similar to a mayfly spinner.
Spinners, technically called the imago, are the second and final winged stage of mayflies. The first winged stage is known as the dun or sub-imago stage. Duns typically pop up on the water’s surface during a mayfly hatch, and after a few brief seconds they fly off the water, assuming of course that a trout, or swallow, or willow fly catcher, or any number of other bug hungry critters don’t intercept them first. Those that survive land on streamside foliage where they sit and do nothing but wait until they have the uncontrollable urge to molt one more time. This final molt is when the reproductively immature duns turn into sex crazed spinners.
The actual time from dun to spinner varies with species, but it is never particularly long. On the short side the dun stage may last only thirty minutes before it molts into the spinner. On the long side it may be three or four days. Twelve to 24 hours is most common.
Spinners truly have only one thing on their mind - sex. And in this case sex is not just their final act, but their only act, a situation born out of the fact that mayfly spinners (and duns too for that matter) have no mouthparts or digestive organs. Eating therefore is off the table so to speak. This means they won’t live long, and just like the dun stage, a spinner’s life can be as brief as an hour or two or, for those that reach true old age, a max of four or five days. So to make sure another generation follows them they can’t get distracted with anything but mating and laying their eggs.
Male spinners start the process by forming large mating swarms in the air. At times these swarms are truly epic in size with millions of male spinners floating ten, twenty, or thirty or more feet above the water. This rather rhythmic up and down dance is how male spinners fly in their large swarms. At some point a female spinner flies into the swarm of males where she is greeted immediately by any number of horny suitors. The male who arrives first is rewarded. Actual mating takes place in the air, and is completed in about as long as it takes the pair to fall through the air twenty or thirty feet.
Males depend on their eyesight to be the first to see a female when she enters the swarm. That’s why males have such huge eyes (note in the photo above, the large pale blue balls with a small dark spot in each, are the eyes.) Females in contrast have small dark eyes.
After mating females of most species begin laying eggs almost immediately by gliding down to the water’s surface. When her abdomen touches the water a small round cluster of eggs immediately falls off. The female then lifts off the water, rises a few feet, then glides back to the water where another cluster of eggs are released. This will be repeated a half dozen times or so until all the eggs are laid or the female is swiped out of the air by a darting swallow or sucked on the surface by a waiting trout. Once the eggs have all been laid the female dies floating on the surface with her wings flat on the water straight out from her body. The males also die after mating and many may fall to the water, but they also get blown over land before falling out of the sky.
That’s pretty much their story. All male mayfly spinners have very large eyes compared to females. They are also often a different size and color than their female counterpart of the same species, though this is not always true. When it comes to imitating spinners, fly fishers should be very careful to note the size and color of the females more so than the males, since the majority of adults on the water will be the egg laying females. Trout become very selective to spinners when there is a large number of them on the water. And since the spinners are dead trout are in no hurry to feed on them. To be successful your flies need to closely match the naturals and be presented on the surface without any drag. Often easier said than done!
There are other animals that die after mating. Pacific salmon are a good example of that. But salmon adults live one to several years before they become obsessed with mating. Mayfly adults have taken this much further, or should I say shorter, for they have just a few hours to mate and lay their eggs.
