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I used the T&H key
Males solivaga: "...light side spots of the abdomen extended, clearly visible from above."
"dorsal hairs of tergites 3 and 4 fine, as long as 1/7 - 1/5 of the marginal bristles"
Males nigripes: "...abdomen usually appears dark when seen from above."
"middle dorsal hairs of tergites 3 and 4 as long as 1/4 - 2/5 of the marginal bristles"

The other characters are not clearly visible but may be present, I thought.