We’re not sure of the exact identification of this species …but have you ever seen such a sight! We believe these are a species of millipede in the order Julida and may be congregating because the increased moisture we’ve had in MT has made their hiding place temporarily unsuitable (just a guess). As with most of the millipedes that invade our homes and other buildings, this is likely an introduced / non-native species. In any case, our photographer, James, seems to have taken this gathering in stride and shares our fascination. Photo taken on 9/22/19 in Missoula, MT.