Moved from Small-headed Flies.

Gary, you're definitely correct regarding family Acroceridae, and genus Eulonchus. Getting beyond there (at least for now) is often not possible...at least in a formal sense...because there are a number of undescribed species in the genus Eulonchus (see "Remarks" on the info page).

But in this instance, due to the abundant diagnostic characters visible in your very clear and complete images AND the particular species involved...I think a reasonable ID of E. sapphirinus can be made here. In particular, the entirely yellow legs (beyond the coxae); the proboscis length and shape (see Fig. 19 here); and agreement with other details in the description here from Cole(1)(1919)...all point very clearly to that species. This is one of the well-defined species, and I'm presuming the undescribed species must be distinct from the currently clearly defined species like E. saphirrinus. (If I'm wrong here, I trust Dennis Haines, Chris Borkent, Martin Hauser, or others will correct me...but I'm confident enough to move your post for now.)