Acknowledgments

The following people must be thanked for their contributions to this key.  Ben Gunn (CSIRO Entomology), for his contribution of taxa and character images. Mick Smith (W.A. Department of Conservation & Land Management) and Gunther Theischinger (NSW Environmental Protection Authority) for the loan of many of the specimens that were used for photography.

General Information

Diptera comprises the true flies, that is those flies in which the adult has only one pair of wings, situated on the meso- (second) thoracic segment.  The "wings" of the third (meta-) thoracic segment are modified as balancers of halteres.  The name "fly" is attached to the common names of many flying insects, such as whitefly, caddisfly, alderfly and mayfly, none of which are dipteran flies.  Common true flies include the mosquitoes, midges, gnats, black flies, crane flies, horse flies, hover flies, house flies and blow flies.

The order Diptera ranks amongst the largest in the Insecta, with a very wide range of adult and larval diverse biologies demonstrated.  Many true flies have aquatic immature stages.  These include many families of medical significance in the adult stage, as human biters and vectors of disease, such as mosquitoes and black flies and many gnats, midges and horse flies.  Because of the public health significance of many flies, the immature stages are often better known than the immature stages of other insects.  Thus there are identification manuals to genus and species level for the mosquitoes and chironomid midges from many regions of the world, and the ecology of many immature stages may be quite well known.

Aquatic Diptera larvae include filter feeders, either planktonic (Chaoboridae, Culicidae), in the surface meniscus (Dixidae), attached to the substrate (Simuliidae, Blephariceridae) or free-ranging (Ephydridae).  Amongst other aquatic dipteran larvae, detritivory and saprophagy in the benthic depositional substrates is very common (many Chironomidae, Tipulidae, Psychodidae, Tabanidae).  Predation is found in some Chironomidae, Ceratopogonidae and many aquatic Empididae, Dolichopodidae and Muscidae). Specialised aquatic habitats include snail parasitism (Sciomyzidae) and thin water films (the hygropetric or madicolous zone) provides a home for some grazers (Thaumaleidae, some Psychodidae).

Identification

The Diptera are holometabolous, with usually 3-6 larval instars followed by a pupal stage (pupation or pupariation) where tissues are reorganised into the adult form. Amongst the aquatic Diptera, pupation may take place in the water (many of the "Nematoceran" families) or outside the aquatic environment in damp marginal habitats (many of the pupariating "higher" Diptera).  Diptera larvae are very diverse in shape and form, but all are united by the lack of any segmented thoracic legs.  However, there are representatives of other orders that lack thoracic jointed legs. Whereas aquatic Diptera larvae are active and slender or maggot-like, apodous non-Dipterans tend to be sluggish (such as endoparasitic wasps (Hymenoptera)) or often somewhat C-shaped, and swollen with undirected movement (Coleoptera: Curculionoidea).  The latter are included in this key.

In shape, aquatic Dipteran larvae range from slender and eel-like ceratopogonids to the tapering maggot-like muscids.  The head may be distinct, darkened and protruding anteriorly, reduced and partially retracted into the anterior thorax or reduced to no more than a remnant skeletal structure fully retained within the anterior thorax.  The body may be smooth, or with a wide range of welts and tubercles.  Legs, when present, are unsegmented prolegs which may have a crown of crochets or claws.  Movement may also be performed through contraction and relaxation of segments bearing circular bands of spines and tubercles that act as welts to anchor on the substrate.  The distribution of spiracles can be valuable in making identifications: some species are apneustic, others are metapneustic or amphipneustic.

References:

Colless, D.H. and McAlpine, D.K. (1970) Diptera. pp. 717-786.  In: CSIRO, The Insects of Australia. Melbourne University Press.

Foote, B.A. (1991) Order Diptera. pp. 690-915.  In: Stehr, F.W. (ed.) Immature Insects. Volume 2. Kendall/Hunt Publishing Company, Dubuque, Iowa.