Mayfly Naiads (aquatic nymphs)

2010 November 20

Back when we were crayfish-hunting in the middle of May, we also caught this by accident while turning over stones looking for crayfish:

This is an aquatic insect nymph, or “naiad”[1]. At the time, I thought it was a stonefly naiad, but then I found out that stoneflies only have two cerci (the long appendages coming out of the end of the abdomen), while this one has three. That means it’s a Mayfly, and quite possibly a Stream Mayfly, which are commonly found under submerged rocks.

These are detrivores, which hide under rocks and rustle up any decaying bits of organic matter they can find.

The gills run along the edge of the abdomen, and I gather that an expert can distinguish types partially based on the shape of the gills.

In addition to this relatively big one, we also found this smaller one, which was only about a quarter the size. It also has the three cerci (so it’s another mayfly), but I don’t think it is is the same species: the head is narrower, the abdomen is wider, it holds its cerci differently, and it has an orangeish-brown stripe down its back that the larger one lacks.

One thing that I think is interesting is, here we have two different species of mayfly. They occupy the same niche, they eat the same things, they are subject to the same predators, and they were even under the same rock. So the question is, why are there two species? Why not just one? What made them diverge into two different species while living in the same place and having practically everything in common? Well, in the case of mayflies, it was probably a matter of timing. When mayflies emerge as adults, they generally only last for a couple of days (and I gather that a few species only last for a matter of hours)[2]. So, they can only mate with the ones that come out at the same time. If they miss the window by even a day or so, it is too late. So, if we originally have one mayfly species that emerges on, say, June 1 every year, some will be a bit early, and will mate with the other early-birds, while others will emerge late and mate with the other late-risers. Then their offspring will also tend to be early or late. Over time, and assuming that there is no selective pressure giving preference to a particular emergence date[3], we will end up with an early population that emerges on, say, May 30-31 that never even encounters the late population that emerges on June 2-3. And once that happens, interbreeding between the early and late populations stops, and they are free to drift off and evolve in their own way, even though, as naiads, they live right next to each other.

[1] Ah, terminology derived from greek mythology, don’t you just love it? Naiads were a sub-category of Nymphs, who were associated with freshwater springs and streams. If one is going to call insect nymphs “naiads” just because they live in water, why stop there? Why not call insect nymphs that infest trees Dryads, or those that live in mountainous areas Oreads? I suppose it is poetic and all, and a good way for 19th-century taxonomists to show off their classical educations, but the name doesn’t actually tell you anything directly. I would really prefer naming schemes that were less poetic and more informative.

[2] “Perhaps old mayflies sit around complaining about how life this minute isn’t a patch on the good old minutes of long ago, when the world was young and the sun seemed so much brighter and larvae showed you a bit of respect. Whereas the trees, which are not famous for their quick reactions, may just have time to notice the way the sky keeps flickering before the dry rot and woodworm set in. What matters isn’t so much how long you actually live, as how long it seems.” – Terry Pratchett

[3] Of course, there are things that enforce particular emergence dates. For example, if they emerge too early, there is a risk of getting killed by a late frost, while if they emerge too late, the migrating insect-eating birds will be on hand to eat them all. Sometimes timing really is everything.

4 Responses
  1. November 20, 2010

    Love the Terry Pratchett quote!

  2. November 21, 2010

    Thanks, Anne. The quote is from the introduction to “Truckers” (the first of the books in his “Bromeliad” trilogy), which is about a species of tiny people who live in and around human habitations and only live a few years, but are so fast that we hardly ever see them. As I recall, they were actually aliens who had gotten marooned here, and we meet them originally living in a department store. He was explaining why we didn’t need to feel sorry about their short lifespans, because a month to them *seemed* just as long as a year does to us.

    I may have the quote a bit wrong, since it is from memory (I can’t find the book at the moment), but that’s pretty close.

  3. BioBob permalink
    June 23, 2011

    Top larva/nymph is indeed order Ephemeroptera family Heptageniidae – they get most of their nutrition from scraping algae off the rocks.

    Bottom larva/nymph is family Ephemerellidae possibly genus Ephemerella or Eurylophella – they feed on algae and also shred leaves, detritus, and will snag a chironomid if they can.

    Stream insects segment the habitat by selecting different microhabitats. You will generally find more of those Ephemerellids in leaf packs and woody debris and Heptageniids on/under larger smooth rocks. If you look closely at the mouthparts, you will see that the Heptageniids sweep the surface of rocks for food while that type of Ephemerella chews up stuff like a grasshopper.

    There actually should be many more mayfly species present and if the water stays cold enough, a stonefly predator species at least to feed on them.

    Mayflies have species with either 2 or 3 cerci. Some stonefly larva have such small cerci that you need a scope to see them. A good way to tell the difference is that stoneflies usually wiggle sideways like a shark swimming and mayflies wiggle up and down like a whale when they swim.

    Nobody uses Naaid anymore. The Needham types at Cornell tried to start that term back in the 1910s & 20s but it didn’t catch on. Some folks use nymph for the hemimetabolous bugs and larva for holometabolous but others call them all larva. It doesn’t much matter, to most since they are common names and the scientific ones exist and are precise enough to suit.

  4. BioBob permalink
    June 23, 2011

    ah, forgot to mention….

    Many of the temperate zone mayflies use facultative parthenogenesis. Females will mate if they can but will lay their eggs (which will almost all hatch unfertilized) even if they miss out on their fun.

    Also, there is almost always more food in their habitat than population to eat it for most mayfly species. Temperate stream populations are usually controlled more by abiotic factors like flooding, drought and microhabitat abundance than food itself.

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