148 The Nahiralist in La Plata, 
wliicli so much building-material is used that the 
bird is called in the vernacular Lehatero, 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 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 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-suckiug 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 
