148 The Naturalist in La Plata. 



which so much building-material is used that the 

 bird is called in the vernacular Leiiatero, 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-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 



