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Flies - the 'true' or two-winged flies (Order: Diptera)

A close-up photograph of a daddy Long legs (tipulid) showing the halteres.

The halteres of this tipulid fly (daddy long legs) are clearly visible as the small 'drum stick' shaped structures behind the wings. Halteres are modified hind wings and are used for balance when in flight.
Photograph by Eugene Zelenko, used under GFDL

The Diptera are familiar to everyone as just 'flies' - such as house flies and blue bottles - and this order of insects also includes daddy long legs, midges and mosquitoes.

Most flying insects - the Pterygota - have four wings, and the ancestors of the Diptera had four wings. However, the true flies have evolved so that their hind wings have become modified into balance organs, or halteres, which act rather like a gyroscope, providing a high level of fine control during flight. It is only the Diptera, Strepsiptera and a few true bugs that possess halteres. However, in the Strepsiptera it is the fore wings that evolved to form halteres rather than the hind wings in the Dipera and Hemiptera.

The muscles that operate the forewings are well developed and situated in the mid-section of the insect's thorax. Together, the relatively powerful forewings and the halteres enable these insects to perform amazing feats of flying, and combined with the claws and pads on their feet they can even fly and land easily on ceilings.

It is partly as a result of this flying prowess that the Diptera is such a successful group of insects. There are around 100,000 known species of two-winged flies in the world today, of which some 15,000 can be found in Europe, and around 5,500 species in the British Isles.

So, the next time you see a fly, don't imagine that it is 'just a fly' - it could be one of thousands of possible species!

Flies suck

The mouthparts of flies are designed to suck or pierce, rather than bite, and they are all liquid feeders. Some get their nutrition by drinking liquefied putrid matter, for example dung; or they suck up nectar from flowers; others have pointed mouthparts, as in the case of mosquitoes, which are used to pierce skin and drink blood.

Flies and man

Just as insects in general can be useful or harmful to man, this is particularly true of the true flies - because there are so many of them, they have plenty of scope to be 'helpful' or a nuisance, according to their various lifestyles:

A photograph of a hover fly in flight.

Hoverflies are welcome visitors to any garden. The adults help pollinate plants and the larvae eat aphids.

Lifecycle

True flies undergo complete metamorphosis - that is, they have egg, larva, pupa and adult stages. Their larvae can be distinguished from those of other orders by virtue of the fact that they are legless, though sometimes they may have small stumps or false legs. They move by simply wriggling about. Sometimes dipterous larvae are born well developed, rather than being laid as eggs - this is know as ovivipary as the egs stay within the female until they hatch and then the larvae are then 'laid'. In some flies (e.g. Tsetse flies) the larvae are full grown and ready to pupate when they are laid.

Diversity and classification (or not)

The habitats, lifestyles and diversity of the true flies are wide ranging. Some of the main groups are listed below, with the aim of providing a flavour of this fascinating order of insects. For more detailed information on all the dipteran families the resource list at the end of this article lists a number of useful publications and Web sites.

Firstly, the order Diptera is classified into three main suborders: the Nematocera, Cyclorrhapha and Brachycera ... but wait! This is not quite true (don't believe everything you read, even on this Web site!) because:

Recently, the Cyclorrhapha has been reclassified as an infraorder within the Brachycera, and renamed Muscomorpha. We will use the older classification here for the moment, as most reference texts available to amateurs use this (and who knows, someone might decide to change the classification back again anyway!).

One reason that the classification of this large insect order - the Diptera - is in flux is because of new techniques of cladistic analysis, which seeks to classify insects according to the way they have evolved from the common ancestor of all insects.

But let's move on...

The more 'advanced' flies (in evolutionary terms) are in the sub-orders Brachycera (which contains for example the blowfly) and the Cyclorrhapha (which includes the soldier fly, house fly etc). These have short antennae, but it isn't immediately obvious which of these two suborders a fly belongs to. The classification is related to the way their larvae split their skins when moulting.

However, a fly with a bristle on the upper side of its third antennal segment is likely to belong to the Cyclorrhapha. Brachycera larvae have mouthparts, but those of the Cyclorrhapha (such as the house fly) have virtually disappeared and have evolved into a tiny pair of hooks. These are the small carrot shaped wrigglers that we call maggots.

The three main suborders are also covered in more detail:

Capturing flies

The larger flies can be stalked and netted individually. To obtain the smaller species use a sweep net, but flies can be hard to identify. Some amateurs use a malaise trap (a malaise trap is like a tent, insects fly into it and then move up the ridge of the tent and into a collecting bottle). There are recording schemes for various families of British Diptera, organised by the Dipterists' Forum.

Resources

Books

For a general introductory overview of the Diptera:

Imes, R. Beginner's Guide to Entomology. Chancellor Press, 2000. ISBN 0-75370-357-2.
This is a useful introductory guide and as its title indicates it also covers other insect orders and provides advice on how to study them.

McGavin, G.C. Insects, spiders and other terrestrial arthropods. Dorling Kindersley, 2000. ISBN 0-7513-0772-6.
Another general guide to insects with colour photographs of many dipterous families.

The Dorling Kindersley 'Pocket Nature' guide to insects by the same author also provides a useful overview: McGavin, G.C. Insects and Spiders. Dorling Kindersley pocket Nature series, 2004. ISBN 1-4053-0596-7

Chinery, M. Field Guide to Insects of Britain and Northern Europe. 3rd Edition. Collins, 1993. ISBN 0-00-219918-1

More specific information:

Stubbs, A. and P. Chandler. A Dipterist's Handbook. Amateur Entomologists' Society, 1978. ISBN 0-900054-29-8
An invaluable guide aimed at those particularly interested in this order, this volume covers their life histories and behaviour in different major habitats, and recording and collecting techniques.

Colyer, C.N. and C.H. Hammond. Flies of the British Isles. 2nd Edition, Warne, 1968.
This is a highly readable general introduction to the Diptera and includes a key to the families, although as will be appreciated the classification it is not current. The book is out of print but may be obtained from the second hand and specialist booksellers, such as Pemberley Books.

Handbook for the Identification of British Insects, published by the Royal Entomological Society. There are around 17 volumes of the Handbook series covering orders of British Diptera. These are aimed at the serious dipterist and provide illustrated identification keys together with brief biological and status information. Further details are available via the Royal Entomological Society or the volumes may be purchased from a natural history bookseller such as the NHBS Environment Bookstore.

Related links: Flies (Order: Diptera)

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