For it must be borne in mind that I do not assert that
these "occasional" or "accidental" parasites, as some one calls them,
explaining nothing, 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. Perhaps 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 circumstance 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Spence, with apologies to those who regard the _Introduction_ as out of
date, only because I am not aware that we have any later observations on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.
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