The island has many small ponds; the biggest one by far is Sachem Pond but it's fairly saline and also not particularly close to me. I don't know how far they typically disperse from their breeding grounds but there is a small, swampy pond a bit (less than 1/2 mile) to my north. There aren't any ponds within a mile to the south and east and west are ocean. What size frashwater ponds or lakes are the typical habitat?

I saw one at the edge of a large lake, large enough to have a pile of rotting debris washed up on the shore.

I just looked back through my photos and realized that I'm pretty sure this was the fly I found to be quite numerous on the north shore of Sachem Pond right at the water's edge on sand, foam, and algae (though there wasn't much of a pile of debris). I didn't take any pictures of flies from the lights until 90 minutes later. I took pictures of collected specimens in order of the location I found them (Sachem Pond then lights then Clay Head the next morning). The source of the error is probably that this was the last Sachem Pond speciemen I photographed. I'm glad I caught my error.