...but how do you know it's a female? Was it dissected, or perhaps the ovipositor is protruding in the voucher? Only a few Acylomus have external dimorphism (in the hind tibia) while eastern NA Stilbus are (frustratingly) difficult to sex without dissection.

If the specimen is still available, have a look whether the prosternal process has a row of several long, stiff setae or whether it has just a few small setae, primarily at the outer corners. Former is Stilbus, latter is Acylomus (WARNING: character only applies to NE US species!).

I tend to dissect these things, in the hope that future ID will be faster (at least for males) if this step is conducted when the specimen is mounted.

The prosternal process does have the row of stiff setae, so Stilbus it is. Thanks Matt!