March fly. Scanning electron micrograph (SEM) of a female march fly (family Bibionidae). A member of a family of stout insects in the fly order, Diptera, that are commonly seen around flowers during spring and early summer. The larvae feed on the roots of plants and on decaying vegetation and may occasionally become plant pests. March flies show sexual dimorphism or visual difference between the sexes, the male has a big head and larger eyes than the female shown here. Magnification: x16 when printed at 10 centimetres wide.