310 Mr, Clark's Obfervattons on the Genus Oe/trus. 



The obje&ions to this idea are — that the anus is rather clofed than 

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

 crufhed in attempting to pafs the fphintler of a horfe's reclum; 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 300, muit be depofited in one horfe, as it is impof- 

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

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

 deflroy its winrs. 



I mention thefe objections, not as merely relating to this fpecies, 

 but that it may not be credited of the GE. 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 infe£l for this purpofe is the lips of the 

 horfe, which is very diftrefling to the animal from the exceflive 

 titillation it occafion>; for he immediately after rubs his mouth 

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

 if two are (landing together they often rub themfelves againfl each 

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

 -moves his head backwards and forwards 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 rinding this mode of defence in- 

 fufBcient, the enraged animal endeavours to avoid it by galloping 

 away to a diftant part of the field. If it ilill continues to follow 

 and teafe him, his lad refource is in the water, where the Oejirin 

 never is obferved to follow him. 



The 



