Family Bibionidae

Bodenheimer (Animal Life in Palestine, 1935) has this to say:
Among the large fly-like Bibionids, Bibio hortulans abounds in February on the borders of wheat fields, on the coast. B. marci appears only one month later, on the coast and two months later in the hills. The large-eyed males of B. hortulans are black, its females brown; both sexes of B. marci are black. Their larvae feed mainly on roots of grasses and live together in small groups.

From Wikipedia:
Bibionidae , march flies and love bugs, 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(Hardy, 1981). Adults of Plecia do not eat, but subsist solely on the food taken in during the larval stage. Upon reaching maturity bibionids spend much of their lives and in the case of Plecia 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.