AGENTS OF DISEASE 285 



familiarly known as Gad-flies. The disease is 

 transmitted in the same way as Ngana, by the 

 insect first sucking the blood of an infected 

 animal, and then passing to a healthy one. The 

 Gad-flies are among the handsomest and most 

 powerful of the Diptera, they have large strong 

 wings, broad and stout bodies, and most formid- 

 able lancets and sucking proboscis. The females 

 are particularly objectionable, exercising their 

 biting habits to the fullest degree, gorging them- 

 selves upon the blood of the horses and cattle 

 upon which they settle. The loud buzzing noise 

 which they produce, and the sharp painful stab 

 of their piercing organ, terrifies the animals to 

 such a degree that they will rush madly about 

 trying to escape these bloodthirsty flies. The 

 Gad-fly is also responsible for the distribution of 

 other cattle diseases, and is frequently the trans- 

 mitting agent of that dread disease anthrax to 

 man. A disease of all too frequent occurrence in 

 South Africa among cattle, and called gall-sick- 

 ness (gal-zickte), is transmitted by a curious 

 creeping fly similar to our forest fly, called 

 Hippobosca rufipes. 



Various species of Ticks (Ixodidce) play an 

 important and sinister part as the transmitting 

 agents of disease to man, and to cattle, horses, 

 sheep, poultry, dogs, and other animals. They 

 are divided into two sub-families, called re- 

 spectively the Ixodince and the Argasince. 

 Though popularly looked upon as insects, their 

 proper place is in the class Arachnida, which 

 comprises the Scorpions and Spiders, the Mites 

 and Ticks, the King Crabs, and several other 



