Saturday, September 12, 2009

Cicada shells


Almost every summer I receive several questions about strange bugs sitting motionless on fence posts, tree trunks, and other upright objects. I thought I would do an entry here to explain the mystery.

The descriptions of the creature that people give to me vary from a “cross between a crayfish and a beetle,” to “gnome-like,” and all mention the large claws on the “front end.” No one has ever seen anything like it, and it is no wonder. Normally, the creature they are seeing lives fairly deep underground.

I have taken to calling these bugs “former insects,” since the objects being seen are not entire insects but the cast exoskeletons (“skins”) of cicadas, family Cicadidae. Scientists call these shed skins “exuviae,” all that remains behind when the insect molts from the nymph stage into an adult. Dragonflies, damselflies, stoneflies, and mayflies also leave behind these ghostly but tangible shadows of their former selves.

The average person rarely sees an animated version of a cicada nymph because the mature nymphs emerge from the soil at night, climbing the nearest vertical surface, and then splitting the exoskeleton down the middle of the back to allow the escape of the wet, soft adult under the cover of darkness. The fresh, pale adults are extremely vulnerable, but their principle predators are diurnal, so they avoid instant death by coming out at night. Cicadas are large insects, so they can be quite conspicuous under the best of cryptic circumstances.

Most people in eastern North America see the shells of “annual” or “dog day” cicadas in the genus Tibicen. They then hear the loud “songs” of the adult male cicadas, not often seeing the insect that makes such a racket. Despite the name, annual cicadas still take a long time to grow up. They live a subterranean existence as nymphs, sucking the sap from tree roots for at least five to seven years. The generations are staggered, though, so some adults emerge every summer. This is in contrast to the synchronous broods (populations) of the periodical cicads or “17-year locusts” that emerge en masse in the late spring or early summer every 13 or 17 years, depending on the latitude of the population.

I like to get questions about things like cicada exuviae. It shows that people are observant and curious, two qualities I really admire in my own species

18 comments:

  1. I remember as a youth in South Africa, walking through a forested area near the town of East London, the cicada 'hulls' crackling underfoot.

    Just a bit eerie...

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  2. It is funny we don't see more of these shells, considering their size and the relative abundance of cicadas in the summer. Last year I saw one for the first time - and also got the opportunity to watch an adult emerge. Strangely, it was at midday... http://themarvelousinnature.wordpress.com/2008/08/09/arrival-of-the-dog-days/

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  3. This sounds so odd. I'm surprised cicada exuviae still surprise people. Mind you, I grew up in the south where summer wasn't summer without all the discarded exoskeletons and the days full of cicada song. Maybe my POV is a tad skewed. But as you say, it's at least good that people are seeing new things and wanting to know what they are.

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  4. Hi, can you help identify a bug I posted a photo of it on my blog. I live in Wyoming near Yellowstone and this large grasshopper like bug (doesn't jump fast or high though) started appearing last month. I see them on the trails a lot. Please see the photo on my blog today and help me i.d. it. Thanks.

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  5. I identified Human Footprint's mystery insect as a female "Mormon cricket," Anabrus simplex; I think that there may literally be fewer cicadas than in years past, at least in urban areas where sprawl has literally paved them over. Further, before the internet, people mostly just wondered about such things because they did not have ready access to "experts." The WWW now provides those opportunities. I find my questions from men are often 'nostalgic,' inquiring about insects and arachnids seen decades earlier.

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  6. Thanks Eric for visiting and identifying. Seems like they might even be good eating as I read that Native Americans as well as coyotes ate/eat them. I think I'll be satisfied with just knowing their i.d.

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  7. Great article! I just posted a photo of a cicada shell on Instagram.

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  8. What are the white stringy pieces left in the Shell after the cicada hatches ?

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    1. I believe those are the tracheal tubes (breathing system), but I cannot be positive. I am not an insect physiologist. Excellent question, though!

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  9. its weird, i have at leaset two of these on each of my oak tree trunks. If this is rare i am curently looking at 8 of them. What does this mean??

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    1. No, cicadas are not rare at all. Very abundant, in fact! I simply find that people who have not seen them before are perplexed by them (the shed exoskeletons especially).

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  10. I haved lived here for 15 years and have never seen this before. I have a mason jar full...my family thinks im crazy...lol@!

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  11. A nymph is in my house! It was on it's back wiggling around so i scooped it up into a jar and am hoping to watch it molt! It is freaky looking, my 5 year old daughter called it dinobug!

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  12. i actually had a cicada crawl into my bed! it was so cool

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  13. My bradford pear is 20 this year. First time noticed yellow leaves all around base of tree. Lots of them. Investigate and saw the shells around the trunk. Can they harm the tree. Doubt it but wonder...dottie

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    1. You'd think they certainly would do damage to the tree by feeding on the roots....but they rarely if ever do. Females *can* damage trees in the act of ovipositing (laying eggs). They can break or girdle a twig causing "flagging."

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