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Hi Paul, thanks a lot for your comment. Sounds very plausible. In the species list of Naturpark Nordeifel I found Leptothorax acervorum, but that doesn't mean that there aren't any other small ant species in our area. On checking Xenos vesparum again I realized that they have no legs, so they're definitely out. Looks like the small ants were feeding on the dead wasp when it was blown into the water.