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True flies, scientifically known as Diptera (Greek for two-winged), are a group of insects that are typically associated, in the public mind, with filth and disease. This is because most people only interact with them in their kitchens or in agricultural or medical settings as pests or as carriers of disease. And while these generalisations have some truth to them, the vast majority of flies are not detrimental to human health or food production, but rather form an integral part of numerous ecosystems worldwide (Triplehorn & Johnson 2005).

There are currently just over 160 000 species of fly known worldwide, of which approximately 20 000 have been described from Africa (Evenhuis & Pape 2021). This number is however quite conservative, and there are thousands of species of fly that remain undescribed.

Flies occur on every continent except Antarctica, and they are well adapted to performing various roles in natural ecosystems, such as pollinators, decomposers, predators, parasitoids, parasites (which includes kleptoparasites) and plant feeders. Some of these adaptations are what bring flies into close contact with humans, where the roles they play can lead them to be seen as pests of agricultural and veterinary importance, but also as insects of great medical importance (Marshall & Kirk-Spriggs 2017).

Flies are a very important group of pollinators, with bees being the most important. They occur in almost all environments and regularly pollinate flowers that might not be attractive to bees.  Examples of this are tiny and inconspicuous flowers in the understory of forests.  The most important fly pollinators are hoverflies (Fig. 1), which are excellent mimics (Gilbert 2005), and often mistaken for bees by the general public.

Flies are important decomposers in the environment. Most fly species develop in dead organic material such as carcasses, plants, dung and decomposing fungi. A good example of this is the common house fly Musca domestica (Fig. 2). These types of substrates are typically rich in organic nutrients, bacteria and other micro-organisms, on which the larvae feed for development. Adults of various types of fly species, including houseflies and blowflies, are attracted to the material for egg deposition, and it is this behaviour that causes them to transport pathogens from the trash to our food, and why sanitation in our neighbourhoods is so important. However, in natural environments they are critically important in breaking down organic material so that it can be fed back into the energy cycle.

Fig. 2: The common house fly, Musca domestica, family Muscidae. By USDAgov – https://www.flickr.com/photos/usdagov/8674435033/sizes/o/in/photostream/, Public Domain, https://commons.wikimedia.org/w/index.php?curid=25727555.

Predatory adults and larvae frequently occur among the Diptera, and can be beneficial to not only the environment but also to humans. The aforementioned pollinating hoverflies have larvae that like to feed on aphids (plant lice). Others, such as robberflies (also called assassin flies) are expert hunters and feed on a variety of insects (Fig. 3). Others, such as Bengalia (a type of blowfly) feed exclusively on social insects such as ants and termites (Marshall & Kirk-Spriggs 2017).

Fig. 3: A Robberfly, cf. Stiphrolamyra diaxantha, family Asilidae with its prey, a bee fly, family Bombyliidae. https://www.inaturalist.org/observations/68878067; Photo 111632331, (c) steveball, some rights reserved (CC BY-NC).

Some flies also have a more parasitic way of life: Some are parasitoids, where the larvae develop in a single host, eventually killing it. Flies are the second most important parasitoids after wasps, and make a great contribution to the agricultural industry by killing pest insects. A parasite in turn does not typically cause the death of its host as part of its development cycle. Probably the best known example is the mosquito, where the female takes a blood meal from its host for egg development. Horseflies and Tsetse flies also require blood for egg development. What many people don’t realise is that all three of these blood feeders also visit flowers for nutritional needs, and contribute to pollination. Some parasites, like the Tumbu or Mango fly (Cordylobia anthropophaga)(Fig. 4), lay eggs on hosts, normally small to large mammals (including humans). The specific name means ‘human eater’. However, humans are typically accidential hosts. The larvae emerge and bore into the skin, where they feed on living tissue, eventually exiting their flesh pit to pupate (Marshall & Kirk-Spriggs 2017).

Fig. 4: Mango, or tumbu fly, Cordylobia anthropophaga, family Calliphoridae. https://www.inaturalist.org/observations/42652308; Photo 67733652, (c) leonpotg, some rights reserved (CC BY-NC).

Kleptoparasites are flies that steal the prey or food insect of another insect. An excellent example is the satellite fly (Miltogramminae)(Fig. 5). They follow solitary female wasps and their paralyzed prey (which is on its way to the wasps’ nest, where the prey will be used by larvae when they emerge). The fly then sneaks in, lays its own eggs, and the emerging fly larvae then eat the young of the wasp as well as the prey (Marshall & Kirk-Spriggs 2017)

Fig. 5: Satellite fly, family Sarcophagidae, subfamily Miltogramminae . https://www.inaturalist.org/observations/71597100, Photo 116592794, (c) deonfriis, some rights reserved (CC BY-NC).

While most adult flies visiting flowering plants are beneficial pollinators, it is important to note that larvae of various fly families can be damaging to plant tissue. This can occur through feeding damage on leaves or even the roots, or through mining damage. The best known examples of damaging flies are fruit flies (Fig. 6) and leaf miners, which can cause great damage in the fruit and vegetable industry in South Africa.

Fig. 6: A Ceratitis sp. fruit fly, family Tephritidae. https://www.inaturalist.org/observations/39106772; Photo 62041065, (c) Herman Berteler, some rights reserved (CC BY-NC).

References

Gilbert, F. 2005. The evolution and imperfect mimicry in hoverlies. In: Fellowes, M., Holloway, G. Rolff, J. eds, Insect evolutionary biology. Wallingford: CABI, pp. 231–288.

Marshall S.A. & Kirk-Spriggs, A.H. 2017. 4. Natural History. In: Kirk-Spriggs, A.H. & Sinclair, B.J. eds, Manual of Afrotropical Diptera. Volume 1. Suricata 4. Pretoria: SANBI, pp. 135–152.

Evenhuis, N.L. & Pape, T. (eds). 2021. Systema Dipterorum, Version 2.10. http://diptera.org, accessed on 2021/03/22.

Triplehorn, C.A., & Johnson, N.F. 2005. Borror and DeLong’s Introduction to the Study of Insects, 7th Edition. Belmont, CA: Brooks/Cole, Thomson Learning.

Figure legends

Fig. 1: A hoverfly, Asarkina sp., family Syrphidae, visiting a flower. https://www.inaturalist.org/observations/71865958; Photo 117074917, (c) Ryan Tippett, some rights reserved (CC BY-NC).

Fig. 2: The common house fly, Musca domestica, family Muscidae. By USDAgov – https://www.flickr.com/photos/usdagov/8674435033/sizes/o/in/photostream/, Public Domain, https://commons.wikimedia.org/w/index.php?curid=25727555.

Fig. 3: A Robberfly, cf. Stiphrolamyra diaxantha, family Asilidae with its prey, a bee fly, family Bombyliidae. https://www.inaturalist.org/observations/68878067; Photo 111632331, (c) steveball, some rights reserved (CC BY-NC).

Fig. 4: Mango, or tumbu fly, Cordylobia anthropophaga, family Calliphoridae. https://www.inaturalist.org/observations/42652308; Photo 67733652, (c) leonpotg, some rights reserved (CC BY-NC).

Fig. 5: Satellite fly, family Sarcophagidae, subfamily Miltogramminae . https://www.inaturalist.org/observations/71597100, Photo 116592794, (c) deonfriis, some rights reserved (CC BY-NC).

Fig. 6: A Ceratitis sp. fruit fly, family Tephritidae. https://www.inaturalist.org/observations/39106772;  Photo 62041065, (c) Herman Berteler, some rights reserved (CC BY-NC).

Burgert Muller
Author

Burgert is a Senior Museum Scientist in the Department Terrestrial Invertebrates. His research interests are systematics, taxonomy, phylogeny and biogeography of true flies (Diptera), with special emphasis on Muscidae and Athericidae. He is also interested in Cybertaxonomy and literature mark-up, as well as Collections data quality assessment and use, which includes georeferencing, ecological niche modelling and collections information management. In 2014 Burgert obtained his Master’s degree for a thesis titled: Systematics of the shoot fly subgenus Atherigona s. str. (diptera: muscidae) of South Africa.

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