Square-mesh cocoon mystery solved!

Well, that didn’t take long!  Jaap Vogel sent his photos to Dr. David Britton of the Australian Museum, who recognized the cocoons as belonging to Cyana meyricki, or at least something very closely related*.  This species is one of the lichen moths, which under the latest classification are tribe Lithosiini within subfamily Arctiinae (tiger moths), which in turn is in the family Erebidae (no common name that I know of).  The adult is thought to be a mimic of net-winged beetles (Lycidae), which are distasteful to predators.

An adult specimen of Cyana meyricki. ©David Britton, Australian Museum

When I contacted Dr. Britton for permission to use the above photo, he passed along an article devoted to these cocoons**.  It is well illustrated with photos of the cocoon, pupa, adult, and the presumed caterpillar.  In it, Geoff Monteith describes some interesting details of the cocoon, which measures about 3 cm by 2 cm:

High power magnification reveals that almost every cross-intersection of setae [hairs] is bound together by a thread of silk.  It also shows that individual setae are too short to reach from the substrate to the upper midline of the cage, so the vertical side bars of the cage actually consist of two more setae laid precisely end to end to reach the full length of the cage.  The completed cage is remarkably strong and rigid, and is under some tension due to the setae being curved and held in place by the silken hitches.  If partly depressed by a finger tip, the cage will spring back into shape when released.

Inside the cage, the pupa hangs in space, perfectly in the centre and thus equidistant on all sides from the presumed probings of predators and parasites.   The pupa is enclosed in a loose harness of several silken loops, like a hammock, and from this several suspension lines lead to the sides of the cage. . . Mysteriously, the shed skin of the last larval instar is always outside the cage.

We have not seen the adult moth actually emerge, but in those that have hatched for us in the lab the moth magically appears outside the apparently intact cage leaving the empty pupal shell still swinging in its hammock inside.  But close examination shows that the overlapping setae at the two pointed ends of the cage are not silked together like the rest of the cage.  This means that a creature can push them apart from inside, and they close behind it as it emerges.

He goes on to suggest that the pupa pushes out the skin of the caterpillar through the rear one-way exit as it molts.  A live caterpillar has never been found and raised, so it’s still a mystery how exactly it puts this structure together.  Being a lichen moth, the caterpillar is presumed to feed on lichens and algae on dead wood as its relatives do.

If you’re wondering how that tachinid fly got inside the Singapore cocoon, many tachinids lay their eggs on caterpillars rather than on pupae, and the larvae wait until their host has completed its development before completing their own.  Other tachinids lay tiny eggs on foliage and hope that a foraging caterpillar will swallow them.  So this fancy cocoon is of little use in preventing attacks from tachinids, and it ends up instead protecting the parasitoid–which has cleverly waited to kill the caterpillar until the cocoon has been completed–from its own would-be parasitoids.

* Added 4/30/2012: I should point out that there are many species of Cyana, all of which may make similar cocoons.  At least five occur in Singapore, according to this paper, so the parasitized cocoon in my first post could have belonged to any of those.

** Monteith, Geoff. 2008. The mystery of the arctiid moth, Cyana meyricki Rothschild – An Insect Houdini. Entomological Society of Queensland 36(9):237-243.

About Charley Eiseman

I am a freelance naturalist, endlessly fascinated by the interconnections of all the living and nonliving things around me. I am the lead author of Tracks & Sign of Insects and Other Invertebrates (Stackpole Books, 2010), and continue to collect photographs and information on this subject. These days I am especially drawn to galls, leaf mines, and other plant-insect interactions.
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12 Responses to Square-mesh cocoon mystery solved!

  1. Seabrooke says:

    How neat! I love little mysteries like that. The cocoon is quite a work of art. Out of curiosity, what would be the advantage of building an open-mesh cocoon like this, rather than something denser? It’s obviously not just that the caterpillar spends less time on this, because if every intersection is secured with silk, it’s a very deliberate and carefully-executed construction. Turns into quite a beautiful moth, too.

    • Good question! These open-mesh cocoons seem to have arisen independently in a number of different moth families, not to mention beetles (and spongillaflies, but those have a denser cocoon within the open-mesh one). I don’t have a good answer at the moment… will keep pondering…

  2. Paula says:

    Do they live in humid places where mold is an issue? Ease of exit? It’s a teeny tensegrity structure!

    • I think both of those probably have something to do with it, although among the North American giant silk moths, the ones that make gauzy, open-mesh cocoons live in the southwestern deserts, and it’s thought that that design helps with air circulation so the pupa doesn’t roast in the sun. Not really a similar structure, but it’s interesting to think that an open-mesh design could be an adaptation to both humid and arid environments.

  3. Paula Peng says:

    Awesome – so kind of like a tiny shade-structure. I guess bugs are way ahead with that kind of technology, thinking of how silk is great both as a warm under-layer for cold weather and as a lightweight garment for hot weather.

    What I meant to imply about tensegrity is that it seems like if the thing accidentally detached, it would bounce, rather than shatter/break.

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  5. I have another little piece to add to the mystery. I posted this story – with a link to your blog – to the Iowa insect listserve. Today Linda Scarth wrote that she saw a photo of a similar cocoon on page 400 of Thomas Eisner’s book: For Love of Insects. It’s made by Urodus parvula – a moth that lives in the southern U.S. The bugguide photos of this moth are all adults – it would be fun if someone could rear one and see it make a cocoon.

    • Yes, I had written to Thomas Eisner to ask if he would be willing to contribute a photo of an Urodus parvula cocoon to my book, but he declined. I’m traveling into that species’ range in a few weeks; maybe I’ll luck out and find one. A few years ago someone from Brazil posted a photo of a similar cocoon to BugGuide, and I was able to convince folks to keep it at the Urodidae family level until someone finds the North American version. You can see it here: http://bugguide.net/node/view/188271. The only other member of this family in North America is Wockia asperipunctella. Seabrooke Leckie recently found its cocoon and was able to identify it using the description in my book: http://bugguide.net/node/view/484477. Note that these moths are unrelated to Cyana meyricki and their cocoons are only superficially similar–they are made entirely of silk, without incorporating caterpillar hairs.

      • Thanks – interesting to read about all these insects that have figured out how to make mesh. I’ll keep an eye out for Wockia – you never know – it might live in Wisconsin too.

  6. Jaap Vogel says:

    After discovering a few cages here, on Tamborine Mountain, Southeast Queensland, Australia, I found out more about this moth and its caterpillar and pupa. I set up a little project, and in between we have identified a few hotpots: locations with at least a dozen of cages. Locals are involved after publicity in regional newspapers, and keep a close watch. We hope to – sooner or later – see a caterpillar of Cyana Meyricki build a cage. That woul be a world first. I will keep you posted.

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