In animals galls occur mostly on or under the skin of living mammals and birds, and are produced by Acaridea, and by dipterous insects of the genus Oestrus.
The dipterous gall-formers include the gall-midges, or gallgnats (Cecidomyidae), minute slender-bodied insects, with bodies usually covered with long hairs, and the wings folded over the back.
A strong north-west wind, at such times, is of incalculable value to the farmer."8 Other gall-making dipterous flies are members of the family Trypetidae, which disfigure the seed-heads of plants, and of the family Mycetophilidae, such as the species Sciara tilicola, 9 Low, the cause of the oblong or rounded green and red galls of the young shoots and leaves of the lime.
Hartig, Die Familien der Blattwespen and Holzwespen (Berlin, 1860); Walsh, " On the Insects, Coleopterous, Hymenopterous and Dipterous, inhabiting the Galls of certain species of Willow," Proc. Ent.
Typical dipterous insects (flies) closely resemble in general form aculeate Hymenoptera belonging to the families of bees and wasps.
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).
The dipterous garden pests, such as the onion fly, carrot fly and celery fly, can best be kept in check by the use of paraffin emulsions and the treatment of the soil with gas-lime after the crop is lifted.
Dipterous insects are also very numerous in species, especially in those of sanguinary habits, such as the mosquito, ilium, maroim, carapana, borochudo, &c. In some places these insects constitute a veritable plague, and the infested regions are practically uninhabitable.