Small in size, boxelder bugs measure only .5 inches long on average with a width of less than .2 inches. Mature boxelder bugs are mostly black, but can be identified by the three red lines that run across their thorax and the additional thin red lines on their wings.
These insects lay their eggs during the summer months. When first produced, the eggs of the boxelder bug are yellow in color, but eventually become a solid shade of red. This red matches the appearance of the hatched nymphs, or immature boxelder bugs, which are flightless before metamorphosis. Most humans come into contact with these insects during the fall months, when the adult bugs and the nymphs begin seeking shelter for the winter.