I didn't know what it was at the time, but research indicates that I stumbled across a tree squirrel bot fly hobbling around on my deck. Seems to have identical markings - head, flanks and abdomen - as I see online.

Does this tell me anything about the squirrel population in my yard (we have lots of squirrels)? (If I'd found a larva, I'd be looking for a dead body). Is it odd to find an adult out-and-about like that?

I did find two larvae last year that came out of two mice I trapped in my kitchen. Don't know if that's related.

Here is a link that might be useful: