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Flower Flies

Skevington, J.H. and Young, A.D. (2016). 6.18. Flower Flies. pp.152-157. In: Working Group on General Status of NWT Species. NWT Species 2016-2020 – General Status Ranks of Wild Species in the Northwest Territories, Department of Environment and Natural Resources, Government of the Northwest Territories, Yellowknife, NT. 304 pp.


The Syrphidae are known as hover flies, flower flies or syrphids. This family is part of the Order Diptera, true flies. True to their common names, most adult syrphids are flower visitors as adults and some are among the most adept fliers in the insect world, reminiscent of miniature hummingbirds as they work flowers for their nectar rewards. Flies in general contribute nearly 40% of our pollination services and flower flies are the most important single group of Diptera pollinators. Because flower flies are often excellent mimics of bees and wasps (they are harmless but gain protection from predators who mistake them for stinging insects), they are commonly overlooked at flowers. Indeed, many books, advertisements, media articles and literature extolling the benefits of bees mistakenly illustrate flower flies. In contrast with the relatively uniform adult ecology of syrphids, larval flower fly ecology is amongst the most varied of any insect family. Larvae of one large subfamily of flower flies, the Syrphinae, are mostly predatory on aphids, scales and other insects. Many of these species are of critical importance in controlling pest numbers. A related subfamily, the Pipizinae, feed on specialized root aphids whereas the bizarre ant flies (Microdontinae) are predators and parasitoids of ants. From what we know (only 8% of ant flies have known larval life histories), these flies are typically host-specific and have evolved to mimic the chemical communication systems (pheromones and related) of their hosts. Ant fly larvae are thus able to wander around in ant nests with impunity while they feed on ant larvae and eggs. The other huge group of flower flies, the Eristalinae, includes almost every larval life history imaginable. There are predators here too, but the majority filter bacteria from their surroundings in a wide variety of ways. Some live in rot holes and are excellent indicators of the health of old growth forest ecosystems, some live in sap runs under bark, many live in ponds, rivers, bogs, and other wetlands, and some live in putrid water such as that found around farms or sewage lagoons (this includes the familiar rat-tailed maggots). Species that live in putrid water are often found in the billions and are critical in improving water quality. These species are being investigated for use on a commercial scale in water treatment facilities. Other eristaline larvae are plant feeders, with a few such as the bulb flies even achieving pest status. Some species are very specialized, and leaf feeding, stem feeding and root feeding species may cooccur on the same plant without directly competing. There are also a few specialized leaf miners, woodborers and pollen feeders. The most diverse genus of flower flies found in the NWT is the Platycheirus (sedgesitters). Many of these flies have fantastic modifications of the male legs. These speciesspecific ‘flags’ are used for sexual display. As the common name implies, many sedgesitters are wetland specialists and can often be seen sitting on sedges and possibly feeding on sedge pollen. So, syrphids are also extremely diverse. Over 6,200 species have been described worldwide and we estimate that 8,000-10,000 exist. This single family of flies thus rivals the diversity of birds on a global scale. In Canada, we have discovered over 500 species and add new species regularly as knowledge of them expands.

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