Adult flower flies feed on nectar from flowers and from aphid "honeydew" (see Aphididae).
The larvae of different species feed on different kinds of food. Some feed on decaying, damp plant material, on fungi or on green plants, some on the bulbs of plants in the lily family, some in dung. Many are aquatic and live in shallow freshwater (sometimes in water that seems foul and polluted), some in water-filled treeholes. Some species are scavengers in the nests of ants or wasps. Some of the most amazing are predators on slow-moving, soft-skinned insects like Aphididae. These predators have no eyes and no legs, but they still hunt and eat these little insects.
Adult flower flies rely on their high-speed flight and their similarity to stinging insects to avoid or discourage many predators. Larvae hide in muck and mud, and some live only in small treeholes where there are not very many predators. The species that live in nests of ants and wasps have adjusted their scent so they don't smell like food, and they stay out of the way of the other insects as much as they can.
Anura (eat adults)
Testudines (eat aquatic larvae)
Cyprinus carpio and Pimephales notatus (eat aquatic larvae)
These flies find each other by sight, sound, and maybe scent. They have good wide-angle vision to find each other and watch out for predators. They can continue to make vibration noise by moving structures in their thorax even when they are not moving or flapping their wings.
Barcode of Life Data Systems (BOLD) Stats Specimen Records: 20040 Specimens with Sequences: 16930 Specimens with Barcodes: 14426 Species: 2207 Species With Barcodes: 1966 Public Records: 1655 Public Species: 523 Public BINs: 302