Insect guide > Flies > Gad Flies / Horse Flies

Gad Flies / Horse Flies

Gad Flies / Horse Flies(Family Tabanidae.)
The insects of this important family are known as gad-flies, horse-flies or deer-flies. To this group belong the active, strongflying creatures which annoy horses to such an extent when one is driving along a wooded road, especially in pine woods, and also the smaller yellowish or greenish flies which annoy forest animals, and which bite human beings when in the woods.

The proboscis of all of the flies of this family is in the female sex adapted for piercing and sucking, the males, as in all of the bloodsucking flies, including the mosquitoes, being harmless, and the proboscis, not adapted for piercing the skin of mammals. The bites do not appear to be as painful as those of mosquitoes or of black flies, and apparently no poison is injected, but any one of these flies may be responsible for the transfer of diseases. The adults are great water drinkers, and are usually most abundant in the vicinity of inland ponds and streams. This has suggested to Porchinsky, the Russian entomologist, the desirability of coating such ponds with kerosene, and his experiments resulted in the destruction of great numbers of Tabanids.

The larvae of the Tabanidae live in the earth or in water and are carnivorous, feeding upon soft-bodied insects and water snails. The spindle-shaped brown or black eggs are deposited in summer in groups attached to the leaves or stems of herbage.

The gad-flies vary greatly in color and size, and the smaller ones of the genus Chrysops, sometimes called "deer flies", are frequently quite small and colored with yellow or green. The larger ones vary through gray and brown to black. The largest gad-fly in the U.S. is Tabanus americanus Foerst., which is an inch and a quarter long, and has a wing-spread of two and one-half inches. It inhabits the Southern States. About 1,500 species are known, and perhaps 200 occur in the U.S.

Typical Life History of a Gad-Fly
(Tabanus atratusFab.)
This is one of the common large black horse-flies which has a very wide distribution in the United States. Its larvae have been studied by Walsh, Riley and Hart. Hart has found the egg masses in July on the dry bark of a stick projecting from the water. From these eggs larvae hatched August 4th. Larvae of this species were found commonly in water, among vegetation and in the sand of the sandy shores of the Illinois River.

Pupae may be found in the early summer, and the adults appear from May to July, living all through the summer. The species is apparently single-brooded, that is to say, has but one generation annually, and hibernates in the larval stage, the adults mainly emerging in July after a short pupal period, the eggs being laid without delay and producing larvae a week later. The egg is about 2.5 mm. long, with a diameter of .4 mm., dark-brown, sub-cylindrical, more or less tapering at the end. As they are laid in masses they point obliquely upwards, and are stuck in four or five tiers, one above the other, and all gummed together in a firm mass. The larva is shining and of a transparent, whitish color, with a greenish tinge, marked with conspicuous dark-brown or gold- green irregular bands. The egg is parasitized by the little Hymenopterous insect known as Phanurus tabanivorus Ashm.