OVO-VIVIPAROUS FLIES. 



117 



eggs. He was so anxious to hatch those supposed 

 eggs that he carried them in his pocket by day and 

 took them to bed with him at night, (as Bonnet 

 afterwards did with the eggs of aphides,) for several 

 weeks successively ; but instead of grubs, as he had 

 expected, perfect flies were evolved exactly similar to 

 their parents. He calls them spider-flies, from their 

 resemblance to spiders ; and in some parts of France 

 the species which infests horses {Hippobosca equina) 

 is called the Spaniard or Breton : in England it is 

 too well known under the name of the forest-fly. 



Spider-flies (^Hippohoscidce, I.eaoh). 



We have the more willingly introduced this sub- 

 ject here, that another fly {Craterina Hirundinis, 

 Olfers), of the same family, has the instinct to de- 

 posit its egg-like cocoons in the warm feathery nest 

 of swallows, where they have all the necessary heat 

 which Reaumur, in his experiments, was so careful 

 to maintain. In return for the warmth which the 

 young has thus received, the perfect fly, during its 

 brief existence, lives by sucking the blood of the swal- 

 lows, as the one first mentioned sucks the blood of 

 horses, horned cattle, and, it is also said, of man. 



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