Bibionid flies (ke-bae) have many common names in the USA, the most common being “march flies” or “love bugs” (although these are different species to the one below). The larvae are in huge numbers in the soil/leaf-litter of the forest floor and are very important for decomposition and the recycling of nutrients.
University of Florida article on love bugs here.
Wikipedia article on love bugs here.
Bibio omani on Gifu University insect site here.
It was getting dark when I took these photos, so they are pretty poor quality.
Diptera: Bibionidae (= ke-bae)
Bibio omani (I think)
Location: Sweet chestnut grove at the edge of Fureiai-no-mori in Munakata (map)