Bodenheimer (Animal Life in Palestine, 1935) has this to say:
The large majority of the Gall-gnats (Cecidomyiidae) develop in plants on which they cause excrescences or galls to form. Most of these tiny gnats have not yet been caught, but are determined from the galls on the host plants, for which their larvae are responsible. Some of the most common species are as follows: Asphondylia conglomerata, Stefaniella trinacriae on Atriplex; Perrisia oleae on Olea; Perrisia viciae on Vicia faba; Arnoldia cerris , A. szepligetii on Quercus; Asphondylia scrophulariae on Scrophularia; Amblardiella tamaricum on Tamarix.
Other species develop under the bark of trees. Beneath the violet discoloration of the twigs of olive trees we discover small groups of the orange colored larvae of Clinodiplosis oleisuga. Other species have predacious larvae. The pink larvae in the ovisacs of Mealybugs are very common. They destroy these ovisacs very efficiently in the autumn.

From Wikipedia:
Cecidomyiidae (sometimes misspelled Cecidomyidae) is the correct name for the insects known as gall midges or gall gnats. They are flies (Order Diptera). As the name implies, the larvae of most gall midges feed within plant tissue, creating abnormal plant growths called galls.
These are very fragile small insects usually only 2-3 mm. in length and many are less than 1 mm long. They are characterised by hairy wings, unusual in the Order Diptera, and have long antennae. Worldwide there are 3000+ species but since 1,100 are from well-studied North America it is clear this is an underestimate. Many are economically significant especially the important insect pest of wheat, the Hessian fly, the galls causing severe damage to the crop. Other important pests include the lentil flower midge (Contarinia lentis), the lucerne flower midge (C. medicaginis) and the alfalfa sprout midge (Dasineura ignorata) on the Leguminosae; the Swede midge (Contarinia nasturtii) and the brassica pod midge (Dasineura brassicae) on the Cruciferae; the pear midge (Contarinia pyrivora) and the raspberry cane midge (Resseliella theobaldi) on fruit crops.
Curiously a large number of species are natural enemies of other crop pests. The larvae of these species are predaceous, and some are even reported as parasitoids. The most common prey are aphids and spider mites, followed by scale insects, then other small prey such as whiteflies and thrips and many eat the eggs of other insects or mites. Because the tiny larva are incapable of moving considerable distances, there usually has to be a substantial population of prey present before the adults will lay eggs and Cecidiomyiidae are most frequently be seen during pest outbreaks. One species Aphidoletes aphidomyza is an important component of biological control programs for greenhouse crops and is widely sold in the United States of America.
Cecidomyiidae are also known for the strange phenomenon of paedogenesis in which the larval stage reproduces without maturing first. Even stranger in some species the daughter larvae produced within a mother larva consume the mother and in others the reproduction occurs in the egg or pupa.