146 The Naturalist in La Plata. 



vegetable juices, or, like the ephemeras, on nothing 

 at all. For it must be borne in mind that I do not 

 assert that these "occasional" or "accidental" 

 parasites, as some one calls them, explaining no- 

 thing, do not feed on such juices. I do not know 

 what they feed on. I only know that the joyful 

 alacrity with which gnats and stinging flies of all 

 kinds abandon the leaves, supposed to afford them 

 pasture, to attack a warm-blooded animal, serves to 

 show how strong the impulse is, and how ineradicable 

 the instinct, which must have had an origin. Per- 

 haps the habits of the bird-fly I have mentioned 

 will serve to show how, in some cases, the free life 

 of some blood-sucking flies and other insects might 

 have originated. 



Kirby and Spence, in their Introduction, mention 

 that one or two species of Ornithomyia have been 

 observed flying about and alighting on men ; and in 

 one case the fly extracted blood and was caught, the 

 species being thus placed beyond doubt. This cir- 

 cumstance led the authors to believe that the insect, 

 when the bird it is parasitical on dies, takes to 

 flight and migrates from body to body, occasionally 

 tasting blood until, coming to the right body to 

 wit, that of a bird, or of a particular species of bird 

 it once more establishes itself permanently in the 

 plumage. I fancy that the insect sometimes leads 

 a freer life and ranges much more than the authors 

 imagined ; and I refer to Kirby and Spence, with 

 apologies to those who regard the Introduction as 

 out of date, only because I am not aware that we 

 have any later observations on the subject. 



There is in La Plata a small very common 



