Cocoons, big and small

Argyresthia thuiella? cocoon

I’ve had this cocoon sitting around since mid-March, a loose end without a blog companion to parade the web with. I also didn’t know what it was, although I imagined a Google search would turn something up quickly enough. However, recently I had two things happen. The first was I finally received my copy of Stephen Marshall’s book Insects (the first copy that was sent was lost by Canada Post, something I’ve never personally had happen before; the seller was kind enough to courier the second parcel overnight – I didn’t specifically need it overnight, but I thought it was a nice gesture). In flipping through it recently I came across practically the exact photo of the little cocoon I’d taken. The second was that I got some partnering photos to post it with. They’ll come next.

This first one, above, is the itsy-bitsy cocoon of a cedar leaf miner. It’s one or the other of a couple moth species from the genus Argyresthia that occur around here, but likely Argyresthia thuiella. This little moth is tiny. You can tell just by looking at its cocoon that it’s going to turn into a small moth. It has a wingspan (not length) of about 8mm as an adult. I happened across it while looking for bagworm moth cases (I didn’t find any), and just by chance spotted a little dash of white on the underside of a cedar branch.

You can notice the dead brown sections of cedar “leaf” nearby. These are areas that have been mined by the larva of the moth. On a deciduous leaf you’d see little trails, but the structure of the evergreen leaf hides it. Larvae overwinter inside the mined tunnels, then come out to pupate in late March or April, with adults emerging in May to June and hanging around for a couple months. They lay eggs mid-summer, and the larvae, once they hatch, spend the rest of the fall munching on cedar leaves. They overwinter in the tunnel and the cycle begins again. I gather that at the peak of their flight season, approaching a cedar where they’re gathering and laying eggs can result in momentary clouds of moths as they take off at the disturbance and swirl before landing again.

Polyphemus moth cocoon

This second photo I encountered while tracking the Yellow-bellied Sapsucker at TTPBRS last week. It’s the cocoon of a silk moth, I’m pretty sure a Polyphemus Moth. I know that the Polyphemus occurs down there because several years ago, in the first fall season I was volunteering there, we found one of the caterpillars dangling in a mist net, either having fallen off a branch above, or dropped by a bird when the bird that was carrying it flew into the net (there was no bird in the net, so if that was the case, the bird had escaped by the time we checked it). We took the caterpillar home with us, curious about what it was. Shortly after bringing it home it spun itself a cocoon, which sat for some time on the top of a dresser. When it finally emerged, it had turned into a beautiful, big, rich brown moth with gorgeous big eyespots on its hindwings. In sharp contrast to the previous moth, this one has a wingspan of nearly 6 inches.

The name Polyphemus comes from the mythical cyclops with the same name, mentioned in The Odyssey by Homer, and presumably refers to the moth’s giant eyespots. It’s the most common and widespread of the silk moths found pretty much across the continent north into southern Canada. There’s a neat series of photos of a newly-emerged adult moth at When a moth or butterfly first leaves its cocoon its wings are small and crumpled. The moth then has to pump haemolymph (the same blood-substitute body fluid that the jumping spider uses to jump) into its wings to extend them before they dry. If they dry before they’re fully filled out, or if the moth is in cramped quarters without room to extend them, the wings will be deformed and the moth most likely unable to fly.

Mystery cocoon

This last one… I don’t know what it is. When poking around the sides of the building at TTPBRS (the same building where I found the jumping spider), I found dozens of these little coils of sand grains stuck to the walls. They weren’t especially clustered, although they did seem to mostly be on the south and west sides of the building (the sunny sides). They’re only 7-8mm in diameter. I thought they were the neatest little things, and whatever made them had to be fairly common. The closest thing I could find was the cocoons of antlions, which make spherical balls of sand, but they’re found actually in the sand, not stuck to a wall, and they’re round, not coiled. I posted an ID request to, and will add an edit if I figure it out.

Edit: I have an answer! The folks at have come through: it’s the cocoon of the Snailcase Bagworm, Apterona helix. It belongs to the same family, Psychidae, as the bagworm moth I posted about previously. It was accidentally introduced to North America from Europe in the 1940s, and is now found in many states and provinces on both sides of the continent. The coolest thing about this species is there’re no males – the females reproduce parthenogenically (unfertilized eggs). Also cool, the adults are wingless, and the moths spend their entire lives within their case, only crawling out once they’ve laid their eggs, at which point they die.


Author: Seabrooke

Author of Peterson Field Guide to Moths. #WriteOnCon Mastermind. Writer of action/thriller SF/F YA. Story junkie. Nature nut. Tea addict. Mother. Finding happiness in the little things. Twitter: @SeabrookeN / @SeabrookeLeckie

11 thoughts on “Cocoons, big and small”

  1. What an intriguing variety of pupal living quarters! I’ve never seen those twirly sand castles either — hope you get an answer soon!

  2. Thanks for the update! And I thought I was introverted. “Snailcase Bagworm” sounds like a great blogger handle, though, doesn’t it?

  3. my daughter has 3 big cocoons over her drawer chest close to a window, they look just like your second picture, light brown covered with dried leaves, and I am wondering what kind of insect she is going to get there…, they look kind of dried are they dead???, how long is going to take til they get out?? thanks so much for your answer

  4. Lavenderbay: It really would make a good handle! Or a character from Harry Potter…

    Aureli: If they are the same as the one in my post, the cocoons will likely produce a silk moth when they emerge. It could either be a Polyphemus or a Cecropia; you’ll have to wait and see! They’re probably not dead, but the leaves will dry out over the winter naturally. I can’t say how long it will take before they emerge, they should probably come out with the warmer weather.

  5. Thanks for posting the identity of the Snailcase Bagworm. I’d been trying to find out what they were, but I think my ID request at BugGuide got lost in the shuffle (this looks like a busy time of year for them).

  6. Glad it helped, Tim! I found out their ID through BugGuide, too, and it took a little while before I got a response. It’s true it’s a busy time of year over there (pretty much anytime from spring thaw to winter snow would be!). Still, it’s a great resource to have available!

  7. I am not a cocoon connoisseur. I am likely not on subject or cue with the current discussion. However, I wondered if anyone could help me identify the brown dirt based looking thing growing (I assume it grew – I never noticed it until today) from the metal or attached to the metal in b/w my porch screen and glass sliding door (at the top). I call it a cocoon or nest. But, when I google these terms in the pictures format, I don’t see anything like it. I am grossed out and very concerned about what may be imminent or transpire. Can I knock it down or is a swarm of something going to infiltrate me and my son’s home? Any advice, ideas, or the like? Most kind regards, DJL

  8. I may add – it looks clay colored, like a hardened (although I haven’t touched it to see if it is indeed, hard) clay mass of dirt with something inside – by something inside I mean maybe I see imprints of what would look like the body of a white caterpillar the width of my finger (I don’t have skinny fingers) pressing against the edges of the dirt (but, again just a description to give you an idea of what I see – I imagine they are not caterpillars)

    Again, most kind regards,


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