Bibionidae (march flies and lovebugs) is a family of flies (Diptera). Approximately 650-700 species are known worldwide. Bibionid larvae grow up in grassy areas and are herbivores and scavengers feeding on dead vegetation or living plant roots. Some species are found in compost. Adults of Plecia and some species of Bibio do not eat, but subsist solely on the food taken in during the larval stage. Adult stage bibionids are quite short-lived, and some species of Plecia (lovebugs) spend much of their adult lifetime copulating. The slow-flying male and female attach themselves at the rear of the abdomen and remain that way at all times, even in flight. Adults swarm after synchronous emergence, sometimes in enormous numbers. Bibionids have the most extensive fossil record of any Diptera family. Fossil bibionids are known questionably from the Jurassic, while some forms from the early part of the Upper Cretaceous look quite similar to modern species. Bibionid flies are very abundant among insect fossils from the Tertiary period, and a large number of species have been described, although often based on highly fragmentary material.