All About Hoverflies

Colin V Duke, BSc (Hons)

Images by Leon Truscott, David Iliff, Chris Webster, David Skingsley

© All Rights Reserved 2006
 

What Are Hoverflies?

Hoverflies (known in America as Flower Flies) belong to a large family of small to large flies. They are true flies or Diptera, with only one pair of wings in the Family Syrphidae. (Wasps and bees have two pairs.)

Many hoverflies have spots, bands or stripes of yellow or brown against a dark-coloured background, sometimes with dense hair covering the body surface (emulating furry bumblebees). Their fast flight, ability to hover and, in some species, their size are astonishing characteristics. Some hoverflies are among the largest flies of Central Europe. Many species are very colorful. It is not always that easy to identify hoverflies. Some thick-headed flies and beeflies are similar and their dark coloration makes it hard to identify them correctly at a glance. Beeflies tend to be longer, hairy, have snouts and are a study in themselves!

Hovering is a speciality although other flies can also hover—the head of the insect remains absolutely still whilst in flight. They may be seen "Nectaring" on many wild and garden flowers where they are amongst the most frequent of visitors. In Holland and Belgium alone over 300 species exist! In Britain about 270 species are known at present, but significant species and numbers can migrate like butterflies with a powerful flight such as Scaeva pyrastri. The Marmalade Fly Episyrphus balteatus is one of the most common hoverflies to be seen in the garden. The distinctive double stripes on the abdomen make it almost unmistakable.

Many are seen in the summer season in numbers while mixing with butterflies, bees, bumblebees and other flower dependent insects. Male hoverflies tend to emerge and mature first, earlier in the season to ensure reproduction is successful. Many species are useful to the gardener since their larvae eat pest aphids on garden plants and crops. The degree to which they contribute to pollination is also ironically poorly investigated but no doubt are important for carrots, onions and fruit trees.

This group is a useful indicator for evaluating site ecology, being active during the day, with a varied range of larval habitat specialisations.

 

Table 1 General Classification of the Family: Syrphidae

Sub-Family: Syriphinae

Genera

Tribe Syrphini e.g. Syrphus, Epistrophe, Scaeva

Tribe Bacchini e.g. Baccha, Melanostoma, Platycheirus

Sub-Family: Eristalinae

Genera

Tribe Cheilosiini e.g. Ferdinandea, Rhingia, Cheilosia

Tribe Chrysogasterini e.g. Chrysogaster, Neoascia

Tribe Volucellini e.g. Volucella

Tribe Sericomyiini e.g. Sericomyia

Tribe Xylotini e.g. Syrittia, Xylota

Tribe Eumerini e.g. Merodon

Tribe Eristalinus e.g. Eristalis, Helophilus, Myathropa

Sub-Family: Microdontinae

Genera

Rare in UK

What is the purpose of the bright bodies and patterns?

Many of the of hoverflies have ornate body patterns, often of black and yellow, to mimic wasps and bees but are harmless. Hoverfly mimicry includes warning coloration of yellow and black, a narrow waist like a wasp and even the ability to mimic the stinging action of a wasp, by pushing the tip of the abdomen into your fingers if they are caught and held.

The superficial resemblances (a) to honeybees (for example the genus Eristalis spp), to (b) bumblebees (in the genera Pocota or Volucella) and to (c) wasps (in the genus Chrysotoxum) is often striking.

 

Fig 1 The drone fly Eristalis tenax (left) & honeybee - Apis melifera (right) feeding on everlasting daisy.

IMAGE WITH THANKS TO YVONNE © 2007

Fig 2. The common wasp (Paravespula vulgaris), left, compared to the hoverfly, right, (Chrysotoxum cautum - female).

IMAGES © COLIN DUKE 2007 © DAVID ILIFF 2007

Fig 3. The bumblebee (Bombus), below left, compared to the hoverfly, below right (Volucella bombylans).

IMAGES © COLIN DUKE 2007 © LEON TRUSCOTT 2007

Comments

(a) In the Eristalis there are more subtle varieties as they attempt to emulate the various forms which the honeybee takes, e.g. Merodon equestris var naricisus, var equestris. There is some evidence to suggest that colour form depends on the temperature exposure that larvae experience, with lighter forms appearing mid-summer and darker forms earlier in the year.

(b) Volucella bombylans var plumata imitates the earth or common garden bumblebee. The hoverfly larva of this species go one step further and actually live in the nests of bumblebees, eating the rubbish produced, being both detritivores and larval predators and possibly the bees' larvae as well. (The exception is V. inflata which live in aqueous insect messes.)

(c) Members of the genus Chrysotoxum admirably display features such as wasp-like stripings, dark wings to resemble social wasps. The antennae are long and wasp-like, typically not seen in other flies. Fig 2. Another good but less common example is Doros profuges, a large hoverfly, the adult of which is a spectacular wasp-mimic.

Some species wave their front legs in front of their face to mimic the jointed antennae of the potter wasps. It is thought that this mimicry protects hoverflies from falling prey to birds and other insectivores which avoid eating true wasps because of their sting. Hoverflies do not sting and are harmless.

This kind of mimicry is know as Batesian mimicry and describes a palatable, unprotected species (the mimic - a hoverfly ) that closely resembles an unpalatable or protected species (the model - the bee or wasp ). Birds know not to attack a bee as they will be stung. Gilbert (2004) Ref 11 notes that hoverflies also mimic in behaviour the patterns and habits of their Hymenopteran models.

Clearly hoverflies not only mimic the host but have also developed elaborate evolutionary mechanisms to ensure that the larvae survive.

What is the easy way to confirm if a fly is a hoverfly?

Much information can be revealed by the wings—two in the case of hoverflies as opposed to 4 in the mimic—the bees.

Despite the rather random appearance of venation in hoverflies, a logical appraisal of characteristics can be applied based on veins that radiate outward—radial, median and anal. The distinctive presence of the two cross veins in the hoverflies are described below.

As for many Diptera looking at venation characteristics takes identification one stage further. To confirm an insect is a hoverfly one of their most characteristic features is the presence of a longitudinal false vein in the wing, if an examination of wing veins is necessary. In hoverflies a greater part of the edge of the wing is without veins. The vein running all the way to the edge in most flies (radial), only reaches the last transverse vein, not the edge in a hoverfly's upper outer cross vein (A) and lower outer cross vein (see Fig. 4). Another feature of hoverflies is the so-called 'floating vein' (B). This vein just ends nowhere. Usually veins end either at the edge of the wing or in another vein. Both these features being present means you are actually looking at a hoverfly.

Generally the dorsal thorax does not have coarse bristles.

Looking at the behaviour of the fly in the air also confirms that it is a hoverfly, either by its characteristic controlled hovering or its rapid start-stop darting.

How do I begin to identify the many species?

Becoming familiar with hoverfly anatomy will soon yield subtle features used to distinguish the many similar flies down to genera as well as to species level. Using keys will readily assist. It is beyond the scope of this article to list or describe the identification features in detail but the presence and shape, size and colour of bands and/or bars, the presence or absence of hairs on anatomical parts and banding on the eyes are important characteristics. Also whether for example the antenna is long / short / elongated or the arista is plumose (feathered) and noting the colour of hairs on the scutellum. The colour and shape of the face (flat, convex etc) is also important, as are the angle of which the eyes are set to the frons and the relative positioning of the ocelli to the front or rear on the head. Considerable attention is given to the "venation" or patterns of veins on the wings.

Gilbert Ref: 4 is an excellent introduction to basic hoverfly identification. As expertise progresses the definitive works of Stubbs Ref: 1 is invaluable in narrowing down each species using the many keys available. The identification of hoverfly larvae, in itself a study, can be explored using an excellent guide on The Hoverfly Larva by Rotheray Ref: 5. The amateur naturalist may make very valuable contributions by reporting on, for example, distributions valuable to the Hover Fly Recording Scheme Ref 5

Glossary

Abdomen - The last of 3 major components making up an insect containing digestive organs etc.

Alula - A membranous flap close to the squama.

Antenna - Composed of 3 segments with a hair like projection - the arista arising out of the third or final segment.

Arista - A bristle-like structure arising out of the 3 segmented parts making up the antenna.

Frons - The space behind the antenna between the eyes when viewed dorsally.

Haltere - Balance organs located on the mid-thorax which act as a "gyroscope" to control flight.

Humerii - The raised corners to the front of the dorsal thorax.

Ocelli - An arrangement of single-celled eyes usually in 3, on the top of head in a triangulated vertex.

Occiput - The margin immediately behind the compound eyes.

Postalar Cali - Elongated swellings at the posterior corners of the thorax.

Pre Genital Segment - The end segment located just below the 4th Tergite containing the genitalia.

Plumose - Feather-like.

Scutellum - Plate like structure between the abdomen and the thorax when viewed from above.

Spiracle - (Anterior, posterior) breathing pores located on the side of the thorax toward the head and abdomen respectively.

Squama - where the hind margin of the wing meets the thorax there is a membranous flange known as squama.

Sternites - The ventral part of the abdomen which is segmented into 4 sternites.

Stigma - A small cell portion arising on the outer costal margin on the forewing which may not be transparent or may, in fact, be coloured.

Tergites - The dorsal part of the abdomen which is segmented into 4 tergites.

Thorax - The portion between the head and abdomen. The second of 3 major components of an insect, containing respiratory organs etc.

Where do they complete their life cycle?

Like other flies, hoverflies go through all stages of insect life: egg-larva-pupa-imago. The larvae of hoverflies are remarkably diverse for just one family of flies.

Some have adapted to aquatic life in extremely dirty water (including stagnant), eating all kinds of decaying materials. In order to breathe they developed a long pipe at the rear end of the body, which they stick into the air. Examples include the rat-tailed maggots (about 40 species).

Other larvae hunt for plant lice or aphids. Over one third of hoverflies have larvae that eat aphids (over 110 species).

Some live in decaying wood, or sap runs on live trees (33 species).

Some are a pest in agriculture, eating live plant tissue such as roots, stems and flower bulbs from within or as leaf miners (about 30 species).

Like the Large Blue butterfly, The larva of the hoverfly, Doros profuges, is believed to live within nests of the ant Lasius fuliginosus. Little is known but one assumption is larva must either feed upon root aphids that have been herded by the ants or gain some other benefit from living within their nest.

There is much to learn and contribute about the life cycles of this interesting group with many species very poorly documented or understood.

What do they eat?

Adults

Adults feed mainly on nectar and pollen. Beside nectar, hoverflies feed on honeydew produced by aphids. Hoverflies are one of the few kinds of insects that can digest pollen, which is a protein rich source for the eggs. The surface coating of pollen is resistant to most insect digestive juices. The yellow patterning can reflect the amount and type of pollen which the insects have eaten, they are often seen hovering or nectaring at flowers, while the larvae (maggots) eat a wide range of foods.

Larvae

In some species, the larvae are saprotrophs, eating decaying plant and animal matter in the soil or in ponds and streams. For example the rat-tailed maggot, larva of the drone fly Eristalis tenax is found in polluted pools and sewage. They obtain air by extending their snorkel like tail breathing tubes to reach the water surface, breaking it with feathery hairs which emerge from the tube. Adults are so-called because of their resemblance to the drones of bees.

Larvae may feed externally on plants or they may be internal feeders, attacking the bulbs; for example the narcissus fly, Merodon equestris, also known as the great bulb fly . In other species, the larvae are insectivores and prey on aphids, thrips, and other plant-sucking insects.

Fig 8: Hoverfly larvae grazing on aphids.

Sexing hoverflies

Like many other flies, males and females often look alike, having the same coloring, size etc. Exceptions are found especially among the drone flies, where females differ from the males. However it's always easy to tell males and females apart. Like all other flies the males have bigger eyes which come closer together at the top of the head. Females have much smaller eyes, placed farther apart. Tiny eyes or ocelli are composed of single cells and are found at the top of the head in a triangle between the large compound eyes—perhaps this is why it is sometimes easier to get "underneath a hoverfly" when it hovers. (The nature / physics of eyes in the male also influences flight behaviour, with males more able to judge distances and predate females).

A more obtrusive method of identifying the sex of the species is to look at the underside of the abdomen, males have curbed asymmetrical genitalia. The abdomen of the female is more pointed with inconspicuous genitalia.

Hoverfly habitats

Hoverflies indulge in a wide range of habitats, many of which are in decline. The favoured habitat for the adult Doros profuges appears to be the transition zone between woodland or scrub and calcareous grassland but clearly this hoverfly is dependent on its host the ant and factors which affect it such as the ants. Woodland and forest management also play crucial roles with much dead and rotting wood being removed.

The golden hoverfly Callicera spinolae is a large, colourful, hairy hoverfly that is also at risk, adults can be found feeding on ivy flowers in autumn. It is found mainly in East Anglia. The larvae live in rotten holes in trees. Golden hoverflies are saproxylic, i.e. they are dependent on decaying wood.

Other species include aspen hoverfly, Hammerschmidtia ferruginea which lives in open aspen woodland in the Highlands of Scotland and depend on decaying soily layers in aspen and the pine hoverfly, Blera fallax.

Aquatic habitats such as bog give rise to distinctive fauna. The bog hoverfly Eristalis cryptarum, a bee mimic, also may be in decline due to the disappearance of boggy habitat. Bog hoverflies have been seen nectaring on marsh plants such as the flowers of bogbean, marsh marigold and cuckoo flower.

Hoverfly enemies

Hoverflies generally have no major enemies aside perhaps from opportunistic spiders and birds. Based on observations of a tame spotted flycatcher, Davies (1977) notes that hoverfly mimicry does not fool it—the ability for this bird to distinguish between bees and the less conspicuous black and yellow Syrphinae, rubbing off stings in bees but readily eating the hoverflies with no such precaution. However, no species is generally without an enemy somewhere in the food chain. Species of solitary wasps (Ectemnius cavifrons) specialise in taking hoverflies, social wasps will also take hoverflies in summers when numbers are high. The hoverfly parasitoid wasp, Diplazon laetatorius, an ichneumon wasp, in the field, can attack, and eventually kill, over half of all aphid-eating hoverfly larvae. Failing behavioural responses, the hoverfly larvae may develop an immune response deterring up to 1/5th of such attacks, for example the marmalade fly, Episyrphus balteatus, due to host immunity. The parasitoid egg is surrounded by specialised blood cells which release poisonous compounds that kill the invader. Nevertheless many hoverflies may readily fall prey to an even more insidious fate such as the insect "eating" fungal infection.

What is the definitive reference on Hoverflies?

1. Stubbs, A.E., British Hoverflies. An Illustrated Identification Guide. 469 pages, 12 col plates, b/w illus. Pub. British Entomological and Natural History Society [ISBN: 1899935053]. 276 species are described and their identification is made easy by the extensive keys, which incorporate over 640 line drawings. The 12 colour plates by Steven Falk show 263 specimens depicting 190 different species. Good information on the families.

2. Ball, S.G. & Morris, R.K.A.,. Provisional Atlas of British Hoverflies (Diptera, Syrphidae). Pub. Biological Records Centre, Huntingdon. [ISBN 1 870393 54 6]. 2000 maps of distribution and analysis of flight periods are published in this BRC atlas.

3. The Hoverfly Recording Scheme provides a good contact point for those with a shared interest in studying and recording hoverflies.

4. Gilbert, Francis, Hoverflies, Cambridge University Press, No 5 Naturalists’ Handbooks series. ISBN 0 85546 255 8

5.Rotheray, Graham, Colour Guide to Hoverfly Larvae, pub. Derek Whiteley, Sheffield, 1993.

6. Ball, Stuart and Morris, Roger, Britain's Hoverflies: An introduction to the hoverflies of Britain.  Pub. Princeton University Press, 2013.

7. van Veen, M.P., Hoverflies of Northwest Europe in English, published in Utrecht by KNNV Publishing in 2004.

8 . UK Hoverfly Discussion Group UK Hoverflies.

9. A forum for dealing with Diptera Dipterist Forum.

10. Gilbert (2004) Does the abundance of hoverfly (Syrphidae) mimics depend on the numbers of their Hymenopteran models?, Evolution) ISSN 0014-3820 2004, Vol. 58, No2, pp. 367 - 375.

11. Chandler, Peter (1998), Handbooks for the Identification of British Insects: Volume 12 - Checklists of Insects of the British Isles (New Series) Part 1. Diptera. Pub. Royal Entomological Society

12. Facebook group "UK Hoverflies"

(Note: current refs. 6 and 12 are replacements in July 2015 for the now failing original reference links.)

Acknowledgements

The author gratefully acknowledges the input of Leon Truscott, David Illif, Chris Webster, Dave Skingsley whose input would not have made this article as useful as it is and for the many folk on discussion groups who have shared knowledge, contributed and made suggestions. The work is an evolving project and the author welcomes definitive imaging for the Photo Matrix. Email Colin Duke.

The works of Stubbs Ref 1 and Gilbert Ref 5 are praised and recommended.

Micscape Editor acknowledgement: The editor would like to thank the author and photographers for their time, skill and patient photography which has been required to compile this resource. The article is mirrored with the kind permission of the author from his original webpage www.ukwildlife.bravehost.com/article//hovers/hover2.html but the author's page was no longer available as of July 2015.

July 28th 2015: The mirrored version of this article hosted here on Microscopy-UK has been edited. With thanks to readers for pointing out errors, in particular to Mike Kerry for his help in making text corrections including some misspelt scientific names.

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