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Can anybody throw any light on this object. Collected from under Enteromorpha on the margins of a brackish lagoon about a week ago. My first assumption was a dipteran puparium, but of which species I have no idea!

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Well, possibly, but what came out of it was not a Dipteran. The hole shows that a parasite was inside, most likely a hymenopteran, who chew it's way out of the pupa!
greetings

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it is definitely a pupal case of a Syrphid, but I can't tell the genus without having a closer look at the tail or ventral crotches. A part of the tail seems to be broken off. Sorry, but I'm not an expert on Syrphid pupae, so it won't help in sending me a detailed photograph. But if you're able see an extra row of small teeth just in front of the last pair of ventral prolegs, then it must be an Eristalinus. Also if you're able to get hold on Tsalolikhin (ed.) 1999. Key to freshwater invertebrates of Russia and adjacent lands then, you'll find many drawings of Syrphid pupae on page 878-932. If not, you can borrow mine (Amsterdam).

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see pictures 7-8 to see what I mean with the extra row of spines

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Thanks very much! I have checked the rear prolegs and there is a row of small teeth just in front of them, so Eristalinus it is. The book by Tsalolikhin looks very useful - I shall definitely keep my eyes open for it!

Gerard, i don't suppose you have any info on parasites of syrphidae do you? I am quite interested to find out which species may be involved.

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by the way, E. aeneus is the most likely candidate for your habitat, for it is known that the immature stages of E. aeneus lives in rotting seaweed along the coast (in pools, saltmarshes etc.) (Smith 1989, Rotheray 1993, Dolezil & Rozkosny in Nillson 1997) and that she is able to withstand high salinity fluctuations (Barendrecht 1991).

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Well, by the size of the hole it should be something like Ichneumonidae or Braconidae, because Chalcidoidea parasitoids are usually much smaller.
I excluded dipteran parasitoids (like Tachinidae) because of the hole, which is clearly chewed. Diptera don't have mandibels.
But still it's impossible to say which Ichneumonid or Braconid, because there are lots of candidates who could parasitize a dipteran larva/ pupa!
There are some who specialize on Syrphids, but still that would just be speculation.
greetings

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Thanks very much guys. I was veering towards aeneus, and it is by far the commonest Eristalinus here. As for the parasitoid I shall just have to do some rearing next year and see what I get.