Flies

Many people view flies as pests, and there are definitely some pesky species, but the world would be a far different place without them. Few people realize the beneficial activities provided by flies, including pest control, as food for valued species such as birds and fish, as decomposers and soil conditioners, as water quality indicators, and as pollinators of many plants.
Most people are aware that bees are vital for the pollination of flowers, but many don’t realize that flies are second in importance to bees as pollinating insects.
Flies have been documented to be primary pollinators for many plant species, both wild and cultivated.
Flies live nearly everywhere on Earth and they are abundant in most terrestrial habitats.
There are over 160,000 species of flies.
Some flies have specialized relationships with specific flowers, while other flies are generalists, feeding from a wide variety of flowers.
In arctic and alpine environments, under conditions of reduced bee activity, flies are often the main pollinators of open, bowl-shaped flowers, with readily accessible pollen and nectar.
More than 100 cultivated crops are regularly visited by flies and depend largely on fly pollination for abundant fruit set and seed production. A large number of wild food plants, numerous medicinal plants and cultivated garden plants are aided from fly pollination as well.
If you love chocolate, then you love flies. Flies are essential for pollinating flowers of the cocoa tree.
Some of the other fruits and vegetables flies help pollinate include: pears, apples, strawberries, cherries, plums, apricots, peaches, rowanberries, raspberries, blackberries, roses, mangoes, fennel, coriander, caraway, kitchen onions, parsley, carrots, and many more.
Compared to bees, which must provision a nest with floral food, adult flies have low energy requirements. Although this makes flies less devoted to the task of moving quickly between flowers, it also frees them to bask in flowers and remain active at low temperatures.
Some flies, such as syrphids, can be easily mistaken for bees and wasps. However, the pollinating flies can be distinguished by looking at how many wings they have. The flies have only one pair of wings while bees and wasps have two pairs of wings
Some flowers have adapted putrid smelling blossoms to attract certain fly pollinators looking for rotting meat. Some flowers have even adapted to look like rotting meat, usually having a brownish red color.

Sources Cited:

USDA Forest Service: Fly Pollination

The Diptera Site: Flies –Pollinators on two wings