A fly? Interesting. Not what I would have expected. Thank you for the lead...if I have extra time in the next few days, I might try to research more about flies in the family Sciaridae and find out if any have a particular affinity for Swiss chard because they pretty much ignored everything else in my garden (broccoli that bolted, poppies, carnations, leaves of other veggies, squash blossoms, etc...)
