As a rule flies are of small or moderate size, and many, such as certain blood-sucking midges of the genus Ceratopogon, are even minute; as extremes of size may be mentioned a common British midge, Ceratopogon varius, the female of which measures only 14 millimetre, and the gigantic Mydaidae of Central and South America as well as certain Australian robber-flies, which have a body 1-11n.
In bodily form Diptera present two main types, either, as in the case of the more primitive and generalized families, they are gnator midge-like in shape, with slender bodies and long, delicate legs, or else they exhibit a more or less distinct resemblance to the common house-fly, having compact and stoutly built bodies and legs of moderate length.
One of the worst pests of pear trees is the pear midge, known as Diplosis pyrivora or Cecidomyia nigra, the females of which lay their eggs in the flowerbuds before they open.
The best-known dipterous pests are the Hessian fly (Cecidosnyia destructor), the pear midge (Diplosis pyrivora), the fruit flies (Tephritis Tyroni of Queensland and Halterophora capitata or the Mediterranean fruit fly), the onion fly (Phorbia cepetorum), and numerous corn pests, such as the gout fly (Chloropstaeniopus) and the frit fly (Oscinis frit).
Metchnikoff observed (1866) in the development of the parthenogenetic eggs produced by the precocious larva of the gall-midge Cecidomyia that a large " polar-cell " appeared at one extremity during the primitive cellsegmentation.
Ritter (1890) have since observed a similar early origin for the germ-cells in the midge Chironomus and in the Aphidae.
Wagner made his classical observations on the production of larvae from unfertilized eggs developed in the precociouslyformed ovaries of a larval gall-midge (Cecidomyid), and subsequent observers have confirmed his results by studies on insects of the same family and of the related Chironomidae.
P. Marchal has (1904) described this power in two small parasitic Hymenoptera - a Chalcid (Encyrtus) which lays eggs in the developing eggs of the small moth Hyponomeuta, and a Proctotrypid (Polygnotus) which infests a gall-midge (Cecidomyid) larva.
Von Kolliker described the formation of the blastoderm in the egg of the midge Chironomus.
It lives in the grub of a gall-midge and it ultimately becomes changed into the usual white and fleshy hymenopterous larva.