by Axel Ssymank, Bonn & Carol Kearns, Santa Clara

see http://www.bfn.de/0502_skripten.html

1. Diptera as pollinators

Diptera, the true flies, are an important, but neglected group of pollinators. Diptera can be distinguished from other insects by their two membranous front wings and the highly reduced halteres that represent the remnants of the second pair of wings. They are an ancient group, and were probably among the first pollinators of early flowering plants.

Many people think of flies as pests, and certainly there are many pest species. Fewer 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. 

At least seventy-one of the 150 (Evenhuis et al. 2008) Diptera families include flies that feed at flowers as adults. More than 550 species of flowering plants are regularly visited by Diptera (Larson et al. 2001) that are potential pollinations. Diptera have been documented to be primary pollinators for many plant species, both wild and cultivated. 

Flies live almost everywhere in terrestrial ecosystems and they are abundant in most habitats. With over 160,000 species, flies form an extremely large and diverse group, varying in mouth parts, tongue length, size and degree of pilosity. The diversity of flower-visiting flies is reflected in their effectiveness as pollinators. Some flies, such as long-tongued tabanids of South Africa, have specialized relationships with flowers, while other flies are generalists, feeding from a wide variety of flowers. In some habitats, such as the forest under-story where shrubs may produce small, inconspicuous, dioecious flowers, flies seem to be particularly important pollinators. 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.

2. Why do flies visit flowers?

Flies visit flowers for a number of reasons. The most important is for food in the form of nectar and sometimes pollen. Nectar, a sugary solution, provides energy. Pollen is rich in proteins, which is required by some adult flies before they can reproduce. 

Other flies visit flowers to lay eggs, and the larvae feed on the flower heads or the developing fruits and seeds. Plants with carrion flowers deceive flies into visiting and effecting pollination by providing a scent and appearance that mimics the carcasses where these types of flies normally lay their eggs.
In cold, arctic and alpine habitats, some flowers attract flies by providing a warm shelter. Flies bask in the warmth, which can be more than 5 degrees C warmer than the ambient temperature (Luzar and Gottsberger 2001). This keeps their flight muscles warm, and allows them to fly at temperatures that would thwart most bees. Their movement between flowers results in pollination.
Flowers can also serve as rendezvous sites for mating. Large numbers of flies will congregate at a particular type of flower, and the byproduct of their behavior can be pollination.

3. Cultivated plants pollinated by flies

More than 100 cultivated crops are regularly visited by flies and depend largely on fly pollination for abundant fruit set and see production (Ssymank at al. 2008). In addition a large number of wild relatives of food plants, numerous medicinal plants and cultivated garden plants benefit from fly pollination. Klein et al. (2007) reviewed the literature for crop pollination and concluded that 87 out of 115 leading global food crops are dependent on animal pollination. They present a table of pollinators for those crops where this information is known. For thirty crop species flies are listed as pollinators and visitors (with 14 cases referring to flower flies, Syrphidae). This result certainly underestimates
pollination are much more dispersed and often published in smaller journals with less complete indexing. From just my own non-systematic field data (Ssymank) we could add at least 12 crop species which are visited or partly pollinated by flower flies, such as Fagopyron esculentum (18), Mangifera indica (6), Prunus spinosa (35), and Sambucus nigra (24; number of fly species known to visit in brackets).