Large Stonefly

2021 June 20

During the first couple of weeks of June, I kept seeing these good-sized insects (nearly an inch long) fluttering along over the road that we live on. And on June 15, 2021, I finally caught one. Well, not “caught”, exactly. OK, it had landed on the road and died, so all I had to do was pick it up. Anyway, here it is:


It is standing on a newspaper, the text is the standard newspaper font if you would like a size reference. Since it was dead, it was easy enough to flip over for a ventral shot:


Here is a closeup of the legs. You can see that the legs are not very differentiated from one another. While the front legs are smaller and the hind legs are bigger, they all have the same relative proportions and the same joint configuration, which isn’t really typical of most other insects.


There are no mandibles that I can see in this shot of the underside of the head, and it doesn’t really look like it has a functional mouth at all. This is pretty clearly one of the insects that does all its eating as a juvenile, and the adult form just coasts on stored fat while it mates, lays eggs, and dies.


The eyes are decent size, but not excessively so.


The wing veins are pretty pronounced, and we can also see that it has four wings that are all nearly the same size and shape.


This is pretty clearly a stonefly, order Plecoptera. I’ve seen stoneflies before, but they have always been little guys that come out first thing in the spring as the snow is melting. This is easily the biggest one I’ve seen. I expect it is one of the Common Stoneflies in the family Perlidae.

These are one of the insect orders that have aquatic nymphs, which crawl out of the water and then split open to release the winged adult. It is believed that this is the original lifestyle of the ancestral insect, and orders that split off a very long time ago (like stoneflies, dragonflies, and mayflies) have all retained the semiaquatic lifestyle. This one was probably a nymph in the little stream that runs alongside our road.

Stonefly nymphs are popular fishing bait, so you are likely to see them in baitshops. You can distinguish them from mayfly nymphs by counting the tails tendrils at the tip of their abdomen. Stoneflies have two tails, while mayflies have three.

Supposedly, the ability of stoneflies to survive in a stream means that the water quality is good. So, I guess our little stream is probably pretty clean as these things go.

3 Responses
  1. June 25, 2021

    What are the vein-like structures on the wings?

  2. June 27, 2021

    KT: The wing veins are the main support structures for the wing membranes. When insects emerge as adults, their wings are wet and floppy. They make them stand up by pumping fluid into the veins at a high enough pressure to stiffen them, and then wait for the membranes to dry out and harden in their final shape. The fluid is then drained from the wing veins to save weight, leaving the veins as tubular stiffeners for wings that would otherwise resemble plastic wrap as far as stiffness.

    All winged insects have these wing veins, but the stoneflies have unusually prominent ones.

  3. July 20, 2021

    Wow! That’s way cool. Thanks for the info.

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