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Collin's 1943 key says you can separate females of these species by the position of the front orbital bristles:
A line connecting front orbitals nearer to the front of the frons than to the front ocellus - rotundicornis Zett.

A line connecting front orbitals not nearer to the front of the frons than to the front ocellus - commixta Collin

Does this work, please? As the front of the frons is concave, did he mean the front in the mid-line or at the sides? In commixta, is the front orbital equidistant between frons and front ocellus, or nearer the ocellus?

Is there any other way of separating them? I find females more often than males.

Thanks in anticipation,
John Bratton