a subtlety in its habits and motions almost without a parallel even
among free insects. A man with a blood-sucking flat-bodied flying
squirrel, concealing itself among his clothing and gliding and dodging
all over his body with so much artifice and rapidity as to defeat all
efforts made to capturo it or knock it off, would be a case parallel to
that of the bird-fly on the small bird. It might be supposed that the
Firewood-gatherer, like some ants that keep domestic pets, makes a pet
of the fly; for it is a very pretty insect, barred with green, and with
rainbow reflections on its wings--and birds are believed by some
theorists to possess aesthetic tastes; but the discomfort of having such
a vampire on the body would, I imagine, be too great to allow a kindly
instinct of that nature to grow up. Moreover, I have on several
occasions seen the bird making frantic efforts to capture one of the
flies, which had incautiously flown up from the nest at the wrong
moment. Bird and fly seem to know each other wonderfully well.

Here, then, we have a parasitical insect specialized in the highest
degree, yet retaining all its pristine faculties unimpaired, its love of
liberty, and of associating in numbers together for sportive exercises,
and well able to take care of itself during its free intervals. 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. Let us then imagine some of
these orphaned colonies, unable to find birds, but through a slight
change in habits or organization able to exist in the imago state
without sucking blood until they laid their eggs; and succeeding
generations, still better able to stand the altered conditions of life
until they become practically independent (like gnats), multiplying
greatly, and disporting themselves in clouds over forests, yet still
retaining the old hunger for blood and the power to draw it, and ready