92 ECONOMIC ZOOLOGY 



The widely prevalent and dangerous disease of horses and cattle in 

 the orient, surra, is probably conveyed by flies, as was noted before. 

 Numerous other illustrations of the injury wrought by various species 

 of flies might be given. 



Anyone will recognize the difference between the house-fly and the 

 blue bottle fly or the horse-fly or, perhaps, even between it and the simi- 

 lar stable-fly. Fig. 69 is an excellent representation of the house-fly, 

 and a comparison of Figs. 70 and 71 will show- one of the means of dis- 

 tinguishing it from the similar stable-fly; it will be noticed that the 

 lowest or most posterior of the three long veins in the wing of the house- 

 fly bends upward or cephalad at almost a right angle while the corre- 



FIG. 72. Last three segments of the leg of the house-fly, showing claws, pulvillse 

 and hairs. (After Doane, Insects and Disease.) 



sponding vein in the stable-fly's wing has only a wide curve toward 

 its distal end. 



What is the reason for the present crusade against the house-fly? 

 It is not merely because it is such a filthy little animal, as will be pres- 

 ently noted, but because it is one of the worst carriers of pathogenic 

 bacteria that we have; this is particularly true of the germ of typhoid 

 fever, but it is thought that several other diseases are spread in the 

 same way, such as tuberculosis, cholera, dysentery, smallpox, diph- 

 theria, etc. It has been suggested that we call the house-fly the typhoid 

 fly to remind people of its dangerous habits. The fly is an ideal in- 

 strument for carrying bacteria, which it does in two ways; it may eat 

 them with its food and then deposit them on our food or dishes in its 

 excrement as "fly specks," or it may carry them mechanically entan- 



