TABANIDS AND DISEASE 487 



each end, and willi a number of spines or warts on the body. 

 They are voracious feeders and prey upon various soft-bodied 

 animals which they find in the water or mud in which they hve, 

 and are not averse to the practice of cannibahsm if food is scarce. 

 The larva; grow rapidly dui-ing the remainder of the summer, but 

 remain inactive and with little or no growth during the winter. 

 In the spring they complete their development and creep out to 

 drier ground to pupate. The pupa (Fig. 224E) often resembles 

 the chrysalis of a butterfly in form. The adults of the species 

 of temperate climates emerge after two or three weeks, but King 

 states that tabanids in the Sudan exist as pupse only six to eight 

 days. The whole life history of species of temperate climates 

 therefore occupies about a year, but it is shorter in tropical species, 

 in which there are probably several broods a year. 



The adult flies are strictly diurnal, and are often active in the 

 clear sunlight of a summer day, though many forest-dwelling 

 forms, e.g., the deerflies, Chrysops, prefer shade. They do not 

 go in swarms as do many other biting insects but are usually 

 sohtary in habit. On account of their powerful wings they are 

 sometimes found at considerable distances from their breeding 

 places. As remarked before, only the females are blood-suckers; 

 the males, and very probably the females to some extent also, 

 feed on plant juices, the dew of leaves which hold a little organic 

 matter in solution, excretions of insects, etc. Gadflies collect 

 near pools and skim over the surface of the water, the under side 

 of the body often touching the water. Portchinsky, in Russia, 

 has devised a means of trapping the flies, based on this habit 

 (see p 489). 



Tabanids and Disease. — Although tabanids are not known 

 to serve as the intermediate hosts of any disease-causing pro- 

 tozoans, they have been shown to be efficient as mechanical 

 disseminators of various disease germs, being especially dangerous 

 in this respect on account of their intermittent feeding. It is 

 quite common for them, having been disturbed while feeding on 

 one animal, to continue their meal on another. 



Surra, an important disease of horses in southeastern Asia and 

 Madagascar, caused by a trypanosome, is transmitted in this 

 manner, and also El debab, a trypanosome disease of camels. 

 Other trypanosome diseases of animals, normally transmitted 

 by tsetse flies, can be transmitted experimentally by tabanids, 



