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Mollusk-like Larva - Microdon

Mollusk-like Larva - Microdon
Meadow border at Yuba Pass, Sierra County, California, USA
July 11, 2007
Strange larva uncovered by peeling a piece of loose bark off a downed log, revealing an ant nest (of Camponotus?). After the ants grabbed their larva and scurried for cover this guy was left in view. (Taken during an SFSU field class with John Hafernik, who informed us this was larva of Microdon sp.. We replaced the piece of bark before leaving.)

I'm curious about the black blotches visible through the semi-transparent "carapace".

Microdon larva
Beautiful photo.
I added a very old image, which I think includes a pupa:

I believe you're right!
Looking at your image...I think the two light immatures on the left are larvae, and the dark one on the right is a puparium.

I've recently been searching for literature and images on Microdon (and have added various references to the info page). So far, the best images I've found for clarifying the distinction between Microdon larvae vs. puparia are on page 8 of this PDF of a paper on a Brazilian species.

Your puparium looks smooth on the anterior dorsal fifth and reticulate beyond, similar to the figure for M. cothurnatus at the bottom of this page (though it seems to lack the medial longitudinal invagination illustrated there). Camponotus spp. are listed as hosts for M. cothurnatus, and the species occurs in Washington. Just circumstantial speculation on my part, but perhaps your image is of M. cothurnatus or a closely related species?

Microdon are certainly fascinating...I hope that over time we continue to accumulate more detailed info and species placements on BugGuide.

I don't have the citation, but I learned in a class I took about larvae that Microdon larvae adopt cuticular hydrocarbons that make them 'smell' like part of the nest, and they move veeery slowly, eating ant larvae, so the ants 'think' it's just part of the nest and don't realize it's quietly devouring their brood. Also, Microdon larvae were originally described as molluscs by taxonomists in the early 1800s until someone reared one out.

Thanks for your interesting remarks Keith...
I hunted down some links to citations expanding on your remarks:
  • For cuticular hydrocarbons and acceptance in the ant colony see here.
  • For more fun details on original confusion in taking Microdon larva for mollusks (and more), see here.

thanks for finding the citations
they were fun to read. I didn't realize that Microdon was described as a mollusc 4+ separate times.

The first one I found
had me completely stumped. I reared one, proving it was a fly and not a displaced marine organism ;-)

They *are* bizarre.
If I hadn't been told, I'm sure I would have been fairly flabbergast to a find a fly metamorphosed from a mini-mussel mimic. (Dare I ask what the pupae look like?)

You're an inspiration...someday I may try to rear larva. I imagine it must be a suspenseful, surprising and generally amazing experience...provided one has the know-how and finesse to successfully pull it off :-)

I never saw the pupa.
It was inside its puparium that, if I recall correctly, was itself encased in the old larval skin. That makes sense per Kieth's comment above. It wouldn't do to have the ants discover an exposed fly pupa in their midst.

I think I get it now.
Your comment prompted me to read up a bit. Earlier I didn't realize that many dipterans (e.g. Microdon) complete their metamorphosis inside a "puparium" that's simply the hardened cuticle of the last larval instar. So, as you experienced, the pupa itself won't be visible unless you dissect the puparium, which may or may not reveal some further microdonic weirdity :-)

Also, you're right about the fly's vulnerability once they exit the puparium. I read that if they emerge in the middle of the active nest and draw the attention of the ants they're quickly attacked and destroyed. Apparently the larva normally avoid this by slithering toward the entrance of the nest shortly before pupating. Then after the pupa has matured and is ready to exit the puparium, it does so in the cool of night...when the ants have moved in towards the center of the nest and/or are relatively inactive.

Thanks, Aaron.
Good further info on how the flies pull off the caper and get away with it.

Perhaps they are developing gonads. In some caterpillars testes and ovaries are visible through the cuticle...
That is just a guess though.
This is a really cool photo and shows the "Limuloid" (Horseshoe crab like) body form typical of inquilines in Hymenoptera nests.

Maybe, though they seem rather asymmetrical for that. I was wondering if they might be partially digested ant remnants.

Didn't know that body form was typical of inquilines of Hymenoptera...that's pretty interesting. I wonder why? And are there inquilines of other orders (besides Diptera) that have similar larva?

Glad you like the photo...I find the "micro-bead reticulation" pattern intriguingly beautiful:-)

Perhaps they are not gonads...
With regard to limuloid inquilines, see Wilson's "Insect Societies" (1971). Don't quite remember what chapter, but it is on nest associates and parasites. I think there are some scarab and Tenebrionid larvae that also have the same body form.
The limuloid body form, I believe, allows the insect to hold fast and be protected should aggression from the workers threaten it.

Thanks Sean!
Interesting info (and a fascinating reference, I'm sure). Come to think of it, some of the myrmecophilous Lycaenid larva appear kinda half-way between a generic caterpillar shape and a "mollusk" shape (at least to my imaginative eye). Maybe given a few tablespoons of geologic time they'll look a lot more like Microdon. (Convergent Evolution---a bane for those who dream of a tidy taxonomy:-)

Thanks again for your of the great things about BugGuide is learning and sharing all this wondrous stuff :-)

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