On the first properly cool days of autumn the sunlit side of a house, a fence or a tree trunk can go dark with flat oval insects, black with three thin red lines down the back and red along the edges. These are boxelder bugs, and they are there for the heat. They are cold-blooded, a wall facing the afternoon sun is the warmest surface around, and they are gathering before they go looking for somewhere to spend the winter, which is usually a crack in bark or a gap under someone's siding. They live almost entirely on box elder, the fast, weedy maple that towns planted by the thousand and that many people know as Manitoba maple. They feed on the leaves and seed pods of the female trees, so a wall covered in them is nearly always a wall near a tree carrying seeds. They don't bite, they don't eat the house, they do no real harm to the tree and they cannot breed indoors. The only true complaint against them is the smell if you crush one, which is a good argument for moving them with a broom.