Cuterebra atrox. The face markings, especially the little white half moon under the eye, the black mesopleural spot, combined with being found in Aug and in the southwest all point to C. atrox. I have not seen this species in the wild. So a great find. Very little known about this species, even the host mammal is totally unknown, but the looks suggest a mouse or rat bot.
Many museum specimens were collected near streams, so this species may use streams as leks (ie singles bars to find mates). But most specimens are singletons, so not sure a lek has ever been found for sure. Very little biology known about this one. I would be interested in learning more about the habitat you saw it in, time of day or any other information you might remember. As I say, we don't know much about this species.
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