What insects are on this caterpillar?
April 27, 2010
I saw this caterpillar holding onto a cedar beam of the arbor above my deck. I’m curious if the insects piled up on this caterpillar are parasites or progeny. Could they be a symbiotic species??
Don
Austin, Texas, USA

Underwing Caterpillar with Parasitic Fly Larvae

Dear Don,
This double mystery is one of the most unusual submissions we have ever received, but we have a couple of guesses and a theory.  The caterpillar looks like an Underwing Caterpillar in the genus Catocala, and they are well represented on BugGuide.  If not an Underwing Caterpillar, perhaps a related species like a Black Witch Caterpillar, also pictured on Bugguide. The hitch-hikers are definitely not progeny, and they are not acting like parasites, though parasites might be a possibility.  The passengers look like fly larvae to us, possibly Syrphid Fly Larvae, though the behavior is most unusual.  Might we fathom a crazy guess and suppose that the fly larvae are taking advantage of the mobility of the caterpillar to transport the larvae to a food source?  This behavior is known as phoresy, and it is common in the world of arthropods.  We would really love a professional opinion on this phenomenon.  We will contact Eric Eaton and our friends at the Natural History Museum of Los Angeles County for assistance.

Underwing Caterpillar with Parasitic Fly Larvae

Awesome.  I thank you very much for your obvious passion.
My brother and I have been inquisitive about nature since we hatched.  He and I both marvel at the macro world that most don’t take the time to uncover.  Now that film and processing is so cheap (digital photography ) we try never to waste a photo op in this world that gives us back aches to expose.
Now that I have discovered your site, I will take advantage of your expertise, in situations whose mysteries evade my browsing abilities.
Heartfelt thanks for your help,
Don Soderberg
South Mountain Reptiles

Eric Eaton provides a partial identification
Hi, Daniel:
Ok, I’m not sure of the identity of the caterpillar, but the other larvae are erupting from inside of it.  They are most likely larvae of a braconid wasp (family Braconidae).  That synchronous emergence, from one exit hole in the host, is not uncommon.  They will spin cocoons in a mass, too.
Eric
P.S.  Might I have permission to blog about this, using those two images?

Hi Eric,
What’s That Bug? would grant you permission to use anything since we know it will be for educational purposes.  We hope Don agrees.

No problem using the pix.
Wish I’d stuck them into a container that would have been suitable for all this to unfold.  I’m sure I would have gotten the temps and humidity wrong, so . . . oh well.

Brian Brown thinks they look like Fly Larvae
April 29, 2010
They look like fly larvae to me. I asked Mike Sharkey, a braconid expert, to look at this, and he said “They do not look like bracs or any Hym to me. With the sharp posteriors they look like dips to me. Very interesting.”
They don’t look like syrphid larvae; more like phorids, the group I work on. Many are parasitoids. Did Don collect any specimens or try to rear these out?
Brian Brown
LACM Entomology

Thanks Brian,
We will write back to Don to see if he kept specimens.