232 TABANIDiE AND DISEASE [CH. 



f Second segment of abdomen unusually large, spurs of tibiae small 

 15 "S Pronopes Loew (Cape Colony). 



L Abdomen and tibiae not as in Pronopes . . . . , . . . =16 



jg /Face concave in the middle Rhinomyza Wied. (Cape Colony and Java). 

 \ Face not concave .. ,. .. .. ., .. ..=17 



Wings with first posterior cell open SiLvius Meigen (subgenus Silvius). 

 Wings with first posterior cell closed 



Silvius Meigen (subgenus Esenbeckia Rond., Brazil). 

 Third segment of antennae with an acute spine 

 V. Gastroxides Saunders (India). 



17 





TahanidcB and disease. It must be admitted that the 

 evidence in support of the view that Tabanidae act as disease 

 carriers is rather unsatisfactory. Their voracious blood-sucking 

 habits and conspicuous size have given the flies a bad reputa- 

 tion in many parts of the world, and natives frequently assign 

 various ill-effects to their bites. 



Most of these accusations, however, have not yet been 

 justified, although it seems probable that certain trypano- 

 somiases of animals, especially Surra, may be carried by 

 Tabanidae. 



There are many difficulties in the way of conducting experi- 

 ments with these flies. They are always very impatient of 

 captivity and spend most of their time attempting to escape. 

 As a result it is difficult to make them feed on any particular 

 animal, and the flies soon die. Up to the present time hardly 

 a single experiment has been recorded in which the possibility 

 of a cyclical mode of transmission of trypanosomes by Tabanidae 

 has been investigated, for in no case hav^e the flies lived a 

 sufficient length of time to decide the point ^. Possibly the 

 best way of testing whether Tabanids act as the true inver- 

 tebrate hosts for any disease, would be to capture large numbers 

 of wild flies and feed them on susceptible animals. In this 

 manner, the presence of any naturally infected insects might 

 be detected. The occurrence of flagellates in the alimentary 

 canal of a large proportion of Tabanids is certainly rather 

 suspicious, for in many cases it is difficult to distinguish 



^ Mitzmain has recently conducted experiments on the transmission of 

 Surra by Tabanus striatus and has definitely excluded the possibility of a 

 cyclical development, but has succeeded in obtaining direct transmission. 



