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It is not safe, you'll need at least a hard hat :)

Seriously, it won't always work, although it is part of the Campsicnemus 'jizz'.
Hydrophorus females have similar flattened abdomens. Fortunately the latter are either much larger or have characteristic patterns of spots on wings, so telling them apart in hardly a problem. But don't worry, in practice it is easy to tell the north/mid European genera apart with little experience. The only exception perhaps being some Rhaphium females (R. micans etc.) being quite Argyra-like.