TUK FLY 



81 



fly is so dangerous that horses and dogs cannot penetrate the 

 region infested by it. Even herds of cattle may Ije Idlled by 

 tliis fly. Consequently travellers have been hindered in 

 penetrating into this country, and the opening up of tropical 

 Africa to agriculture and commerce has been much interfered 

 with. The injurious effect of the bite of this fly is due to a 



Fig. S4. — Glossina inorisfa/is, the tsetse-fly. 

 From the "Standard Natural History." 



Fig. S4 f. — Syrphus 

 From Packard. 



parasite introduced by it into the body of the victim, and caus- 

 ing the disease known as " sleeping sickness." 



Another destructive family includes the bot-flies, which 

 have a general resemblance to hone3'-bees or buml^lebees. 

 Their larvse are parasitic in mammals. Of this family the 

 bot-fly of the horse is the most generally known. These flies 

 hover about the legs of horses and lay their eggs upon the 

 horse's hair. The larvte, irritating the horse's skin, are licked 

 by the horse into its mouth and swallowed. In the stomach or 

 intestine of the horse they get abundant food, and there they 

 develop. The larvse of another species, " the ox-warlile " 

 or " grubs," are taken into the mouth of cattle, and live for 

 a time in the gullet. Then they bore their way through the 



