[This family of gad or horse flies contains a great number of genera, all of which may bite animals and man more or less severely. The female alone is blood-sucking, the males feed upon the juices of flowers. The females deposit their spindle-shaped white, black, or brown eggs on leaves, stems of plants that either overhang or stand in water, and amongst rushes; they are at first white, but become brown or black. The eggs are laid in rounded, flattened or conical masses composed of layers one upon the other. The larvæ are carnivorous, feeding upon snails, worms, other larvæ, etc., and have a distinct head; they are cylindrical, composed of eleven segments, the last with a vertical breathing pore, or the last two segments may form a breathing tube. The majority taper to a point at each end, in colour shining white or dull grey to yellowish, many of the larger specimens mottled or banded with dark brown or black. The first seven abdominal segments are encircled near the anterior margin with a ring of fleshy protuberances consisting of a transverse dorsal ridge which may be divided by a depression into two. The young larvæ burrow into any soft vegetable substance; they live both in the water and under damp soil surrounding water, also in damp earth generally. The larvæ are not only carnivorous, but they are cannibals, frequently devouring their own species. They may take more than a year to mature.
[The pupæ are found close to the surface of mud and earth, and are mostly dull yellowish to brown in colour, with rows of spines on the distal third of each abdominal segment; the thorax bears a pair of ear-shaped spiracular structures, and there are also six denticles at the apex of the abdomen.
[A habit common to the adults of most of the Tabanidæ of considerable economic importance is that of the adults coming to water to drink. Portschinsky[421] has found that by applying kerosene to the pool they frequent the adults are killed, and Hine[422] that the same oil kills the larvæ that fall into the water from eggs laid on plants above.
[Tabanidæ are not only of importance as purely biting insects, for they may often convey pathogenic organisms from one animal to another, such as the bacillus of anthrax, which they are known to carry, and possibly also trypanosomes in regard to man. Chrysops also acts as a host of Filaria loa in South Nigeria (Leiper, Brit. Med. Journ., January, 1912, pp. 39–40). Two species are incriminated, viz., C. silacea and C. dimidiata. With animals these flies play a more important part, for MM. Sergent, in Algeria, have proved that species of Tabanus are able to transmit three forms of animal trypanosomes by biting a healthy animal as long as twenty-two hours after having bitten an unhealthy one. In India they have also been shown to transmit the parasite of “surra” in dogs and rabbits by Rogers. Other observers have since corroborated these results, and Mitzmain, who has recently performed valuable work in this connection, states that T. striatus is undoubtedly the carrier of this disease in the Philippine Islands. Certain members of the genus Hæmatopota have also been shown to be capable of the direct transmission of Trypanosoma evansi. Martoglio (Ann. d’Ig. sper., 1913, xxiii, N.S., No. 3, pp. 363–366) states that the trypanosome disease of dromedaries known as salaf is transmitted by Tabanidæ, especially Pangonia (P. magretti and P. beckeri) in Italian Somaliland. It is quite likely that these flies play a much greater part in the spread of such diseases than is imagined at the present time.
Fig. 415.—The ox gad fly (Tabanus bovinus, Linn.).
[The Tabanidæ are divided into two groups or subfamilies: (1) The Pangoninæ, and (2) the Tabaninæ; the former have spurs on the hind tibiæ and usually ocelli; the latter have neither tibial spurs nor ocelli.
[The Pangoninæ contain two main genera, Pangonia and Chrysops. In the former the proboscis is much elongated, and the third antennal segment is composed of eight rings, and is never angulated or ungulated at the base. The proboscis is often very long.