Monday, October 22, 2012

A strange moth that lives in a bag

What's hanging out here?
At first glance it could pass for a pine cone...  or a well camouflaged cocoon...
bagworm case - looks like a camouflaged cocoon
... but on closer examination it turns out to be a cocoon-like silk bag covered with bits of leaves, needles, and twigs.  This is the bag, or case, of a bagworm moth larva.
cocoon-like silk bag of bagworm
    The Evergreen Bagworm Moth (Thyridopteryx ephemeraeformis) has a very unusual life-cycle.  The bagworm moth caterpillars spend their life in these camouflaged bags, dragging their decorated homes about as they feed.  The caterpillars enlarge their cases as they grow. The female Evergreen Bagworm Moths never leave their cases.
   Earlier this summer, a young bagworm was feeding on a young aspen in our backyard.
When I first noticed the small bagworm case, it was less than half an inch long.  As the summer progressed, the caterpillar added to the bag until it was over two inches long.
 The bagworm caterpillar fastened its bag to a twig and stretched its head out of its bag to feed.
(Excuse the photo, I took the pic through a mesh bag I had placed over the bagworm in order to keep it from wandering away.)
   Here is a photo of the fully developed bagworm larva's case fastened to a twig of our aspen tree.
Since this photo was taken late in the season, the caterpillar has already pupated.  This case likely containes a female I mentioned before, the flightless female moths never leave their own bags.
   Sure enough, when I cut open the bagworm's case, it contained what looks like a pupa.

Here is another picture of the pupa.  Notice how the bagworm pupated head down.
   Now, some references say the worm-like female bagworm moth lays her eggs within her case. Since I found no eggs, I cut open the pupal skin.  There were all the eggs, as well as some tawny hairs that the moth probably rubbed off of its body to help protect the eggs. I have the "head" to the right in the picture below.
I've read some places that the bagworm moth retains the eggs in her body, while others say the eggs are laid within the pupal skin.  I can see either scenario in these photo, although if the female left the pupal case, I found no exit holes.  Whatever the case, she was mostly eggs, eh?

The male bagworm moths are another story.
After they are finished pupating, they wiggle partially out of the bottom of the bag.  Then the pupal case splits up the back and the male moth emerges.
The males leave their pupal skin hanging out of the bottom of their old home.  Incidentally, this makes a convenient marker of which bags likely contain eggs.
Evergreen Bagworm Moth case (Thyridopteryx ephemeraeformis)
The male bagworm moths then leave their larval home and fly around... visiting the flightless females which are emitting pheromones to attract the males from within their own bags.
   Here is a photo of bagworm cases.  The one in the foreground obviously contained a male bagworm moth, while the one in the background likely contained a female.
cases of the Evergreen Bagworm Moth (Thyridopteryx ephemeraeformis)He's been out on the town....
... she never got to dance at a street light.


  1. Never heard of this sort of moth before Dana. Quite amazing. Thank you.

  2. Anonymous7/13/2014

    Just found a really long one making it's rather graceful way across my 2nd story window today... Had to go looking for what it might be, hence my finding your page. :) Cheers!