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I see that some of the flies in my box have become very greasy. First I thought maybe it had something to do with stomach content or maybe pollen or other external ‘pollutants’.
But I also have reared specimens that have turned out this way so in those cases it can’t be stomach content and probably not any other pollutant... I usually do not pin flies while eating chicken wings 
Could it be that the specimens have not dried enough before I have moved them to the box? I think the box is pretty air-tight (entosphinx trype).

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Grease does seem to be a problem with certain types of fly. I don't have many issues with tachinids but every pantophthalmid I take out of alcohol starts looking great and then in 1 week they look like I have dipped them in cooking oil My assumption has always been that it is the body fats leaking through the cuticle ... perhaps accentuated by their stay in alcohol?

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Asilidae also get 'greasy' quite often (but pantophthalmids are the worst!). My cure is to submerge the specimen in acetone for several days. The easiest way to do this is to use a wide-mouth jar with some poly foam tightly forced into the bottom of the jar (to act as a substrate for the insect pin). Place enough acetone in the jar to cover the pinned fly. Upon removal, the specimen will quickly dry and, with luck, will be grease-free. Hairy specimens may need to be 'blow-dried' while drying, to 'fluff' them -- to prevent matted hair (most flies do not need this precaution). Some inks and laser-print labels will run in acetone, others not; use caution. Some plastic (nylon?) -headed pins react (loose their heads!) when fully submerged in acetone for days; use caution (try to keep the pin head above the fluid level).

Many other organic solvents, etc. will serve as degreasing agents but acetone seems the best (safest and yet still works...).

Eric

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Chris, I don't think alcohol is a factor in the grease problem. My moths have never been dipped in alcohol and suffer the same greasiness. I have nothing else to offer though as I've never been able to work out why some are greasy and most are not.
Cheers, Graeme

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My experience with acetone is that the specimens get very brittle after the treatment. This is especially a problem with smaller, more delicate specimens, so it can get almost impossoble to pin and mount them without breaking of heads and legs. So, make sure you have already pinned the specimens before soaking them in acetone! (This is what Eric's reply also seems to imply, but I thought I'd just give an extra warning...)
As a alternative you may also use amyl acetate. I use this for treating specimens that have been on alcohol: it avoids the specimens from shrivelling up, and as an extra bonus it degreases them. Amyl acetate is smelly stuff (not a bad smell I think, though), so be sure to ventilate very well when you use it.

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Thanks for the acetone tip.

It is strange that out of 10 rared Triarthria setipennis 2 have become very greasy (they did not feed). Maybe I have punctured something inside the fly that made grease leak out. I have other greasy specimens too, but like I said I thought it could be stomach content or something.

Sadly I will porbably never experience the "grease capabilities" of Pantophthalmids

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It's more likely to be the fat-body (accumulated when it was a larva) dissolving and seeping out through the cuticle/spiracles etc.

I have used ethyl-acetate in the past but it isn't easy to buy over here ... I might give amyl-acetate a try next then

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Menno is quite correct re. acetone: it is for brittle specimens. I use it only on long-dead, very dry, pinned specimens. (Asilidae usually don't show greasing until weeks or months after pinning. I've always assumed it has to do with their high-protein diets??)
Alcohol is not a factor with asilids, as I treat fresh alcoholic specimens in different ways (the best method being the difficult "HMDS" treatment: <http://cdfa.ca.gov/phpps/ppd/entomology/HMDS.html>. Even 100% alcohol fails to harden robber flies sufficiently to allow them to dry into 'attractive' (= realistic/ similar to original state) specimens -- they need additional exposure to other highly volatile solvents to dry them further once they have been in alcohol -- like ethyl acetate, maybe acetone, etc., none being nearly as effective as HMDS.
My recommended use for acetone is strictly as a degreaser for dried, pinned specimens.
Chris, Hyperbolizer: I suggest an experiment... take a greasy pantophthalmid, etc. (preferably one with a brass-headed pin if possible), remove the labels from the pinned specimen, then float it upside down (pinned onto something that floats) for several days in acetone (this will suffice as a "quick & dirty' acetone jar). Chances are quite good that it will emerge as a relatively fresh-looking, grease-free specimen. (Note that this technique does not always work -- presumably because of additional 'chemical factors' in the history of the specimen; usually works about 90% for asilids.)

Eric

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Even simpler and possibly a better system than floating a fly in acetone is the following.
Use a shallow polyethylens food container with a tight-fitting polyethylens lid. Drill or bore a hole in the lid and insert a wine cork. Pin the fly to this cork and have just enough acetone
in the bottom so that the fly is just immersed (upside down) in the acetone. Very easy to retrieve the fly.