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Updated: May 7, 2025
There is in La Plata a small very common 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.
The connection between the flea and the mammal it feeds on is even less close than that which exists between the Ornithomyia and bird.
The flea, without the subtlety and highly-specialized organs of the Ornithomyia, or the stick-fast powers and leathery body of the Ixodes, can only escape its vigilant enemies by making itself invisible; hence every variation, i.e. increase in jumping-power and diminished bulk, tending towards this result, has been taken advantage of by natural selection.
And probably when thrown on the world, as when nests are blown down, or the birds get killed, or change their quarters, as they often do, it is able to exist for some time without avian blood. It might be said that if such a result were possible it would have occurred, but that we find no insect like the Ornithomyia existing independently.
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.
Entomologists regard the flea as a degraded fly. Certainly it is very much more degraded than the bird-borne Ornithomyia, with its subtle motions and instinct, its power of flight and social pastimes.
The parasitical habit tends inevitably to degrade the species acquiring it, dulling its senses and faculties, especially those of sight and locomotion; but the Ornithomyia seems an exception, its dependent life having had a contrary effect; the extreme sensitiveness, keenness of sight, and quickness of the bird having reacted on the insect, giving it a subtlety in its habits and motions almost without a parallel even among free insects.
The last is certainly the most teachable; and if fleas were generally domesticated and made pets of, probably there would be as many stories about their marvellous intelligence and fidelity to man as we now hear about our over-praised "friend" the dog. With regard to size, the flea probably started on its downward course as a comparatively large insect, probably larger than the Ornithomyia.
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