There was a lot of these floating in a puddle in the snow on my lawn. There is a low place there that holds water until the ground thaws enough for the water to percolate down. I suspect these came up because their habitat was flooded. It was a fairly warm day, but I found very few anywhere else, either on my lawn or in my woods. I have a few pictures of individual specimens that show the underside, but I didn't attach them here because they are probably not specimens of any one of these. If it would help with identification, I could post them. Are all these the same species, with the smaller green ones immatures, or is there more than one species here?