146 The Naturalist in La Plata, 
vegetable juices, or, like the ephemerz, 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 /nétroduction, 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out of date, only because Iam not aware that we 
have any later observations on the subject. 
There is in La Plata a small very common 
