Did you actually observe them emerging, i.e. sitting on the water's surface with wings in an upright position, or did you observe a mating swarm? I ask that because your description of the tail length would be more characteristic of the spinner than the dun stage.

Roger

From: FiveWeight

No, I didn't see them emerging, they were probably in a mating swarm. But the way you phrased that question makes it sound like they change form between the emergence and spinner fall. Is that right? I thought a spinner was just a dying dun. Doesn't a spinner have same proportions and colors, but lay with wings extended outward and body on the water plane rather than upright? Sounds like there's something more that I don't know here.

FiveWeight-

Good question. Most mayfly species, both male and female, as mature larvae (nymphs), undergo metamorphosis, escape their external skeleton, emerge from their aquatic environment, and seek temporary refuge as a winged sexually-immature adult, called a dun (subimago).

Some time later, generally from less than an hour to several days, primarily depending on species, these sexually-immature adults undergo a second metamorphosis, once again escaping their external skeleton, thereby transforming to a winged sexually-mature adult, called a spinner (imago). Incidentally, mayflies are the only insects having two winged-life-stages.
 
 The dun stage differs from the spinner stage in a number of ways. Some of the more obvious are: the wings, which transform from dull and opaque to hyaline (shiny and transparent); the body color, which is generally brighter in the spinner; the length of the abdomen, which is generally shorter in the spinner; and the length of the tails, which are generally longer in the spinner.
 
 Neither the dun nor the spinner stages have functioning mouthparts, and as such, are unable to eat. This doesn't present a challenge, given their ephemeral life expectancy. However, they are also unable to drink, so must seek shade from the sun to conserve their bodily fluids, so they will last long enough to complete the mating ritual.
 
 When the time is right, the male spinners form a mating swarm, which the female then enters to select a mate. After mating, the female deposits her eggs, either dropping them from some distance above the water, or flying along, and dipping the tip of her abdomen in the water, or by diving beneath the water and affixing them to vegetation. This oviposition behavior is dependent on the species involved.
 
 The spinner fall (to which you refer) is the expiration of the female spinners involved in egg-laying, when they no longer have the strength to fly, or even to hold their wings upright.
 
 Hope this answers your question. However, if it triggers other questions, just ask.