TIic Naturalist in La Plata.
vegetable juices, or, like the ephemerae, 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 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more than the authors imagined; and I refer to Kirby and Spence, with apologies to those who regard the Introduction as out of date, only because I am not aware that we have any later observations on the subject.
There is in La Plata a small very common
Mosquitoes and Parasite Problems. 147
Dendrocolaptine bird--Anumbius acuticaudatus--much infested by an Ornithomyia, a pretty, pale insect, half the size of a house-fly, and elegantly striped with green. It is a very large parasite for so small a bird, yet so cunning and alert is it, and
Firewood-gatherer and Bird-fly.
so swiftly is it able to swim through the plumage, that the bird is unable to rid itself of so undesirable a companion. The bird lives with its mate all the year round, much of the time with its grown-up young, in its nest--a large structure, in L 2