330 PRINCIPLES OF VETERINARY SCIENCE 



The ox warble is an insect parasite of cattle similar in many 

 respects to the bot of horses. The adult parasite is known as 

 the "warble fly," and the young as "grubs," or "warbles." 

 When mature, the fly (Hypoderma lineata) is about J^ inch long 

 and yellowish-black in color, resembling a honey-bee. It hides 

 in the tall grass and bushes during the greater part of the time, 

 coming out to bother the cattle only when the weather is favor- 

 able. These flies appear during the summer months and deposit 

 their eggs upon the hair about the heels of the cattle, thus getting 

 the name "heel-fly." The eggs are yellowish-white in color and 

 possess a lip-like apparatus by which they hold onto the hair. 

 When infested cattle lick themselves the eggs are conveyed to 

 the mouth on the tongue. The saliva causes the shell to loosen 

 and the young grub escapes and pierces the wall of the gullet. 

 From the gullet the little grubs find their way by various routes 

 to the connective tissue beneath the skin covering the back. 

 Here they develop into grubs or warbles, forming small tumors. 

 In order to get sufficient air the parasites drill holes in the hide, 

 which greatly reduces the value of the hide. They reach their 

 full development during April. If the grub is removed at this 

 time it is found to be around % of an inch long and nearly J^ an 

 inch thick. Left to themselves they will work their way through 

 the opening in the hide and drop to the ground into which they 

 burrow. They pass from four to six weeks in soft, moist soil 

 and emerge as flies to lay eggs, which in turn repeat this life 

 cycle the ensuing year. It is almost impossible entirely to pre- 

 vent the appearance of the grubs, but the systematic destruction 

 of all warbles in the backs of cattle each year will prevent their 

 propagation very effectively and finally control the pests. The 

 grubs may be squeezed out through their breathing holes or 

 destroyed by applying a few drops of tincture of iodin to the 

 opening through which they get air. The wound heals rapidly 

 and the hide is not injured. 



The sheep grub is the larva of a bot-fly (CEstrus ovis) smaller 

 than either fly previously mentioned. It differs from the others 

 in that it lays its eggs in the nostrils instead of on the hairs. The 

 eggs hatch immediately after deposition and the little maggot- 

 like grubs crawl up into the nose of the sheep. The grubs at 

 once begin to work their way further and further up into the air 

 passage, often gaining access to the air cavities in the head. 



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