These are tiny flies that are very common in Iowa.  There are at least three species that can be found here--T. geminatus, T. marginatus, and T. politus.  Differences are subtle and won't be discussed here.

The adults of these species can often be found on flowers.  The larva are free-living and eat aphids.

These flies can be challenging to photograph but getting a good photograph makes all the bad ones worth it.