346 VETERINARY HYGIENE 



Oestridce. They are true flies, but like the rest of the Ostridcz are 

 unable to feed, as their mouth parts are rudimentary. The three 

 species of importance in this country are H. bovis, H. lineata and 

 H. silenus, and of the three H. bovis is the most common. H. diana 

 attacks deer in Great Britain. H. silenus attacks horses. H. boms 

 is on the wing from July to September in this country and H. 

 lineata usually appears a month earlier. The life-cycle is very 

 complex, and all stages of it are not yet known. Hadwen* gives 

 it as follows : The eggs which are laid on the skin, take about a 

 week to hatch and the issuing larvae bore through into the subcutis. 

 A skin lesion results due to bacterial invasion and partly to anaphy- 

 lactic reactions ; H. lineata produces the more severe lesions. The 

 larvae next appear in the oesophagus, but how they get there is not 

 known ; they pass down the submucosa of the oesophagus and then 

 wander to the neural canal, passing from the gullet under the pleura 

 either up the crura of the diaphragm or up the posterior border 

 of the ribs and enter the canal by the posterior foramen. From 

 there they descend the canal under the dura mater, to emerge again 

 through the foramen and ultimately reach the back where they 

 mature and form the swellings known as warbles. 



Larvae are never found in muscular tissue, but confine their 

 wanderings to connective tissue. Hadwen gives the average pupal 

 period for H . boms as thirty-two and a half days and a little less for 

 H. lineata. Motef found that H. boms formed 83-5 per cent, of the 

 total warble flies collected in the State of Ohio, and MacdougallJ 

 records the examination of 190 maggots by Steven taken from the 

 hides of beasts slaughtered in Scotland, and of this number 148 

 proved to be H. boms larvae and the remainder H. lineata. 



When the mature larva emerges from the skin, it drops to the 

 ground and there pupates for about six weeks. The complete life- 

 cycle occupies nearly a year. 



The flies terrify cattle and cause them to stampede, and accidents 

 sometimes result. The continual unrest which is sometimes caused 

 by these flies is known to diminish the production of milk, and 

 the damage done to hides is enormous. 



PREVENTIVE MEASURES. If a careful examination were to be 

 made periodically of cattle known to be harbouring warbles and 

 these destroyed, the warble fly would, in time, be eradicated. 



It is useless to press out mature grubs and let them drop to 



*Journ. Amer. Vet. Med. Assoc., 1917, LI., p. 541, through Vet. Rev., 

 1917, I., p. 389. 



t Ohio Journ. Sci., XVII., p. 169, through Vet. Rev., 1917, I., p. 390. 

 $ Trans. High. Agric. Soc., 1915, XXVIL, p. 230. 



