Thanks to all of you for solving this mystery.

I'd say the wasps controlled *this* beetle :-) I think the parasite is the bigger story here, so I've moved this to a new guide page for T. julis.

First, Tetrastichus julis is known as a gregarious parasitoid, but there are only maximum 20 larvae per chrysomelid host larva (16 max. documented, 20 max. from personal observations). Second, T. julis is an endoparasitoid and it doesn't live in the moist fecal coat on the back of its host, but inside of its host. What you can see on this picture is probably only the poop of the larva, that the larvae of this species carrie on their back in moist material. Usually the fecal materials more or less disolve inside the moist secretion and form a dark moist fecal coat, for some reason this time it just didn't dislove as usual.

Do you suppose the clear "skin" is really just the surface of a slimy mucus formed by fly maggots?

My Cereal Leaf Beetle larva has similar appearance and looks like this is normal...


I found this info http://www.nappfast.org/casestudies_files/oulema_melanopus.pdf
and it says
Larvae are eruciform, with only the head, legs and spiracles having significant chitinization.
Often described as slug-like, the yellow colored larvae cover the majority of its body with fecal matter,
which results in an obscuring of coloration.

Unfortunately, it is often only back home, on the computer, that I really see the detail of what I have taken. When I took the picture I remember that I found that the insect looked “fishy” but not to that extent...

I have been trying to hink of what kind of parasite those could be. The only thing I could think of is Microtonus larvae. Parasitic wasps in the genus Microtonus lay their eggs exclusivley in beetles I believe.

Microctonus are known to attack the adult stages of beetles. They can sometimes attack late instar larvae, but it is very uncommon. I also don't think they lay so many eggs within a single host. You should ask Tom Pucci at the Cleveland museum about that genus; I think he is in the process of revising the new world species. He seems like a very approachable person. I have no idea what is eating that grub from the inside out, but it is a really cool picture.

I think tachinid flies parasitize beetle larvae almost exclusively as well.

Tachinids lack a sclerotized ovipositor, so they are exclusivly ectoparasitoids. They attack their hosts by attaching eggs to the outside of the host's body and the larvae act in a similar manner as a leach would. You can always see an ectoparasitoid on the outside of its host. At some point, however, the tachinid larvae would consume the host to a point where the host will die. The parasites/parasitoids within this beetle are endoparasites (nematodes?) or endoparasitoids (Possibly some Hymenopteran).

I realize this comment is two years old, but I still think I'd better correct it. First of all, a few tachinids (e.g. Compsilura and Blondelia) do have ovipositors, which they use to insert eggs directly into the host. Second, as Hannah mentions, there are others that lay eggs that are ingested by the host. Third, those that lay eggs on the outside of the host have larvae that bore directly into the host from the egg. In other words, ALL tachinids (as far as I know) are endoparasitoids--please cite counterexamples if you know of any.

None of this, of course, pertains directly to the fine (and positively stomach-churning) image above.

for helping make bugguide a learning experience for me :-) Now that you mention it, I have seen images of tachinid ectoparasites but I didn't realize all were. Now I know.

It's a tachinid that lays eggs on leaves which are then ingested by the larvae. Does that still qualify it as an ectoparasite, because eggs are laid outside the prey, even though they end up inside?

... if the feeding is done on the inside of the host, regardless of HOW the parasitoid egg got there in the first place (via ingestion or oviposition).

but the point is that what we see here are not parasites at all, but rather fecal material from the larva with which it covers itself. Much information in the comments attached.