INSECTS AFFECTING MAN AND ANIMALS 391 



development, they make more or less serious sores. When fully 

 mature their larvae drop to the ground and undergo their trans- 

 formation to pupae and finally to flies. Some entomologists 

 believe, however, that the eggs are laid on the backs of the 

 cattle and that the larvae bore through the skin to the place where 

 they are to complete their larval development. Although this is 

 one of the most common pests of cattle this question in regard 

 to its life-history has never been definitely settled. The sheep 

 bot-flies, Oestrus ovis, lay their eggs in the nostrils of sheep. 

 The larvae pass up into the frontal sinuses, where they feed on 

 the mucus and tissues, causing great suffering and loss. Many 



FIG. 179. Bot, larva of Hypoderma lineala, from cow. (Enlarged.) 



other species of animals are infested by their own particular 

 species of bots. Several instances are recorded where the ox- 

 warbles have occurred in man, always causing much suffering 

 and sometimes death. 



Horses and cattle that can rest in the shade during the hot- 

 test part of the day are not bothered as much by bot-flies as 

 are animals that do not have such protection. Cattle often 

 obtain further protection by standing in the water when it is 

 available. Horses that are thoroughly groomed and kept 

 free from the eggs of the bot-flies will not, of course, be infested 

 with the larvae. After they have gained an entrance little 

 can be done to cause the larvae to leave the alimentary canal 

 of the host until they are fully developed. The ox-warbles 



