This may look like a fly (and have “fly” in its name), but it is actually in the order Hymenoptera (wasps, bees, and ants). And the saw part? Females of most species have a sawlike ovipositor that they use to cut a slit into plant tissue before laying each egg. This species is widespread across North America, primarily in northern and montane regions. Their larvae are caterpillar-like in appearance and are fond of leaves of alders (Alnus), ash (Fraxinus), poplars (Populus), willows (Salix), and cherries (Prunus).
Photo by Doug Emlen on 5/16/20 in Missoula, MT