Hover Flies (lat. Syrphidae)

Description

The size of hoverflies varies depending on the species. For example Paragus tibialis is 3–5 mm (1⁄8–13⁄64 in) long, while Criorhina nigriventris is 13.6–20.6 mm (17⁄32–13⁄16 in) long. Some, such as members of the genus Baccha, are small, elongated, and slender, while others, such as members of Criorhina, are large, hairy, and yellow and black. As members of the Diptera, all hoverflies have a single functional pair of wings, with the hind wings reduced to balancing organs. Many species are brightly colored, with spots, stripes, and bands of yellow or brown covering their bodies. Due to this coloration, they are often mistaken both by insect-eating birds and by humans for wasps or bees; they exhibit Batesian mimicry. Despite this, hoverflies are harmless to humans. Drone flies, Eristalis tenax, are an example of a species of hoverfly who exhibit Batesian mimicry. With a few exceptions, Hoverflies are distinguished from other flies by having a spurious vein, located parallel to their fourth longitudinal wing vein. Adults feed mainly on nectar and pollen. Many species also hover around flowers, lending to their common name. Bee flies of the family Bombyliidae often mimic Hymenoptera and hover around flowers, as well, rendering some bombyliid species hard to tell apart from Syrphidae at first glance. Hoverflies can, nevertheless, be distinguished in the field by anatomical features such as:

The legs and mouthparts of hoverflies are usually not particularly long and thin (some bombyliids have a long and needle-like proboscis, many have legs that are noticeably longer and thinner than in similar-sized syrphids) Their facial cuticle often has prominent bulges and/or beak- to knob-like projections (most bee flies have an evenly curved or sloping face). The wings are often clear or have smooth gradients of tinting, and their veins merge posteriorly into a “false edge” that runs parallel to the wing’s true rear edge and extends along half or more of the wing length (bombyliid wings lack a “false rear edge” and often have large dark areas with sharp boundaries, or complex patterns of spots). Their abdomens and thoraces often have glossy cuticular body surfaces, abdominal colors are usually mainly due to cuticular pigments (bee flies are usually very hairy, their abdominal colors are almost always due to pigmentation of hairs and not the underlying cuticle).

Evolution

The oldest known fossils of crown group Syrphidae are from the Eocene aged Florissant Formation, Green River Formation and Baltic amber. The genus Prosyrphus, found in Late Cretaceous (Cenomanian) Burmese amber, appears to represent a stem group to the family.

Pollination

Hoverflies are important pollinators of flowering plants in many ecosystems worldwide. Syrphid flies are frequent flower visitors to a wide range of wild plants, as well as agricultural crops, and are often considered the second-most important group of pollinators after wild bees. Relatively little research into fly pollinators has been conducted, compared to bees. Bees are thought to be able to carry a greater volume of pollen on their bodies, but flies may be able to compensate for this by making a greater number of flower visits. Like many pollinator groups, syrphid flies range from species that take a generalist approach to foraging by visiting a wide range of plant species through those that specialize in a narrow range of plants. Although hoverflies are often considered mainly nonselective pollinators, some hoverflies species are highly selective and carry pollen from one plant species. Cheilosia albitarsis is thought to only visit Ranunculus repens. Specific flower preferences differ among species, but syrphid fly species have repeatedly been shown to prefer white- and yellow-coloured flowers. Nonvisual flower cues such as olfactory cues also help these flies to find flowers, especially those that are not yellow. Many syrphid fly species have short, unspecialized mouth parts and tend to feed on flowers that are more open as the nectar and pollen can be easily accessed. Also, a number of interactions occur between orchids and hoverflies. The orchid species Epipactis veratrifolia mimics alarm pheromones of aphids which attracts pollinating hoverflies. Another plant, the slipper orchid in southwest China, also achieves pollination by deceit by exploiting the innate yellow color preference of syrphids.

Identification guides

Skevington, J.H., et al., 2019. Field Guide to the Flower Flies of Northeastern North America. Princeton University Press ISBN 9780691189406. This book “covers all 413 known syrphid species that occur in or north of Virginia, Kentucky, and Missouri, west to include Iowa, Minnesota, Ontario, and Nunavut, and east to the Atlantic Ocean, including Greenland.” Stubbs, A.E. and Falk, S.J. (2002) British Hoverflies An Illustrated Identification Guide. Pub. 1983 with 469 pages, 12 col plates, b/w illus. British Entomological and Natural History Society ISBN 1-899935-05-3. 276 species are described with extensive keys to aid identification. It displays 190 species on colour plates. 2nd edition, pub. 2002, includes new British species and name changes. It includes European species likely to appear in Britain. Additional black and white plates illustrate the male genitalia of the difficult genera Cheilosia and Sphaerophoria. van Veen, M.P. (2004) Hoverflies of Northwest Europe: Identification Keys to the Syrphidae. KNNV Publishing, Utrecht ISBN 9050111998. Miranda G.F.G., Young A.D., Locke M.M., Marshall S.A., Skevington J.H., Thompson F.C. (2013) Key to the Genera of Nearctic Syrphidae. doi:10.3752/cjai.2013.23 Bot, S. and Van de Meutter, F. (2023) Hoverflies of Britain and North-west Europe: A photographic guide (Bloomsbury Naturalist). ISBN 978-1-3994-0245-3.

External links

Hoverfly – index to scholarly articles All About Hoverflies A website about Dutch hoverflies Hoverfly Recording Scheme – UK Dipterists Forum world Syrphidae checklist, including relationship with other taxa. Diptera.info Picture Gallery