148 The Naturalist in La Plata. 
which so much building-material is used that the 
bird is called in the vernacular Lefiatero, or Fire- 
wood-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 con- 
cealment. 
The parasitical habit tends imevitably 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 quick- 
ness 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. 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 capture 
