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 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 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 which so much building-material is
used that the bird is called in the vernacular Lenatero, or
Firewood-gatherer. On warm bright days without wind, during the absence
of the birds, I have frequently seen a company of from half a dozen to a
dozen or fifteen of the parasitical fly wheeling about in the air above
the nest, hovering and gambolling together, just like house-flies in a
room in summer; but always on the appearance of the birds, returning
from their feeding-ground, they would instantly drop down and disappear
into the nest. How curious this instinct seems! The fly regards the
bird, which affords it the warmth and food essential to life, as its
only deadly enemy; and with an inherited wisdom, like that of the
mosquito with regard to the dragon-fly, or of the horse-fly with regard
to the Monedula wasp, vanishes like smoke from its presence, and only
approaches the bird secretly from a place of concealment.

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