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TO Mr, CtARK'i Ohfervat'ions en the Genus Oejlruu 



The obje£tions to this idea are — that the anus is rather clofcd than 

 opened by any irritation externally applied. — The fly would be 

 crulhed in attempting to pafs the fphincler of a horfe's re£lum; and 

 having no means of holding while depofiting its eggs, it would be 

 quickly evacuated with the dung. — The whole of the ova, to the 

 amount of 2 or 3Q0, muft be depofited in one horfe, as it is impof- 

 fible, if the fly furvivcd, that it could undergo this punifhment a 

 fecond time, for the heat and moifture of the reSlum would at leafl 

 deftroy its wings. 



I mention thefe obje£lions, not as merely relating to this fpecies, 

 but that it may not be credited of the Oil. nafalis, or indeed of any 

 of them, that they really enter the body of the animal to obtain for 

 their young a fituation there. ^ 



I have not feen any writer who has defcribed the mode in which 

 this fly depofits its ova; which having had repeated opportunities of 

 feeing, I can fpeak of with certainty. 



The part chofen by this infeft for this purpofe is the hps of the 

 horfe, which is very diflireflTnig to the animal from the excefllve 

 titillation it occafions ; for he immediately after rubs his mouth 

 againft the ground, his fore-legs, or fometimes againft a tree ; or 

 if two are ftanding together they often rub themfelves againft each 

 other. At the fight of this fly the horfe appears much agitated, and 

 moves his head backwards and foi^wards in the air, to baulk its 

 touch, and prevent its darting on the lips ; but the fly, watching 

 for a favourable opportunity, continues to repeat the operation 

 from time to time ; till at length finding this mode of defence in- 

 fufficient, the enraged animal endeavours to avoid it by galloping 

 away to a diflant part of the field. If it ftill continues to follow 

 and teafe him, his laft refource is in the water, where the Oejlrus 

 never is obferved to follow him. 



The 



