August 2021 Natural Mystery Answered

We got a good number of answers to last month’s Natural Mystery—and all were correct. Too bad I didn’t post something like this two years ago. I got this question wrong on my 2019 New England Specialist evaluation. But these sharp readers were not fooled. Congratulations to John Arthur, Emily Goldberg, Mike Holtz, Kathy Johnson, Bill Kass, Lin Mulhern, Carol Stiteler & Kirsten Welge for correctly identifying this and sharing your answers with the rest of us.

This is a cicada exuvia—the shed exoskeleton of an annual or “dog day” cicada nymph.

I’ll let Mike Holtz start us off with a simple description of what we see in this photo:

“Cicada nymphs live underground. When they emerge, they cling to a plant or tree trunk, and shed the exoskeleton, which is left clinging there.”

Kathy Johnson gives us a little more info about annual cicadas, and notes that she regularly finds these exuvia in a local garden, under spruce trees:

“Cicada eggs are laid on trees. The young hatch after 6-8 weeks then make their way into the soil to feed on tree & plant roots for 4-8 years (in MN). When the soil reaches a certain temperature, they crawl out of the ground and attach themselves to a vertical plant or tree, where they molt: the exoskeleton splits and the insect pulls themselves out (much like a monarch butterfly does out of a chrysalis). The remaining exoskeleton is what is pictured.”

Emily Goldberg adds:

“Since the nymphs are ground-dwelling and the adults are winged, the nymphs have to climb to an exposed location before they can do their final molt—which apparently takes them a good few hours. I don’t know of any other nymphs in Minnesota that are that large and have the “clinging to trees” characteristic.”

Bill Kass gives us a little more information about identifying cicada nymphs:

“There are several insects that will shed skin out of a larval stage, many that are in this same order of insects, but none are quite this size. Two characteristics of this ‘true bug’ are the very large eyes and front legs/claws used for digging in the soil. Those characteristics along with the robust body tells us this is one of several species of a cicada.”

Kirsten Welge similarly notes:

“The size and shape, including the leading appendages, are a match for the cicadas that emerge each summer. From National Geographic: ‘Cicadas are members of the superfamily Cicadoidea and are physically distinguished by their stout bodies, broad heads, clear-membraned wings, and large compound eyes.’ As this exoskeleton was found in your St Paul backyard, it is an annual cicada (Tibicen spp.). From my reading, I note the famous Brood X periodical cicadas (Magicicada spp.) don’t occur in Minnesota.”

Kirsten also noted that “the adult will survive about 4-6 weeks after emergence.” During this time, as Bill notes, “they will then make a very loud call looking for a mate.”

This call, which is often described in modern times as sounding like the buzz of a power line, is a familiar sound of the “dog days” of summer.

Several of you noted that there is lots of great information about cicadas online. If you would like to read more, here are a few good places to start:

• https://extension.umn.edu/yard-and-garden-insects/cicadas

• https://wildlifeinformer.com/cicada-life-cycle/

• https://www.nationalgeographic.com/animals/invertebrates/facts/cicadas

And for a picture of a mature adult emerging from its nymph exoskeleton, check out: https://www.insidescience.org/news/brood-x-cicadas-have-arrived

Bonus Material

Have you ever wondered where the phrase “dog days of summer” comes from? It’s an eons old expression with astronomical origins. I’ve found a number of pretty good descriptions online, but few take the procession of the equinoxes into account, and get the dates wrong as a result. This is a fun one to research.

You may also be wondering what I put down for this on my New England Specialist eval. That exuva was located close to a pond, and I guessed a dragonfly nymph. Dragonfly nymphs, like this one, have a bit of a different shape and lack the large front legs adapted for digging seen in the cicadas.

Congratulations and thanks again to Bill, Carol, Emily, John, Kathy, Kirsten, Lin & Mike for cracking this Natural Mystery for the rest of us.

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