132 The Naturalist in La Plata. 
for then an immediate burst of cold wind is con- 
fidently looked for. In the expressive vernacular 
of the gauchos the large dragon-fly is called hijo del 
pampero—son of the south-west wind. 
It is clear that these great and frequent dragon- 
fly movements are not explicable on any current 
hypothesis regarding the annual migrations of birds, 
the occasional migrations of butterflies, or the mi- 
erations of some mammals, like the reindeer and 
buffalo of Arctic America, which, according to Rae 
and other observers, perform long journeys north 
and south at regular seasons, ‘‘from a sense of 
polarity.’ Neither this hypothetical sense in 
animals, nor “ historical memory” will account for 
the dragon-fly storms, as the phenomenon of the 
pampas might be called, since the insects do not 
pass and repass between ‘‘ breeding and subsistence 
areas,” but all journey in a north-easterly direction ; 
and of the countless millions flying lke thistle- 
down before the great pampero wind, not one solitary 
traveller ever returns. 
The cause of the flight is probably dynamical, 
affecting the insects with a sudden panic, and com- 
pelling them to rush away before the approaching 
tempest. The mystery is that they should fly 
from the wind before it reaches them, and yet travel 
in the same direction with it. When they pass 
over the level, treeless country, not one insect lags 
behind, or permits the wind to overtake it; but, 
on arriving at a wood or large plantation they 
swarm into it, as if seeking shelter from some 
swift-pursuing enemy, and on such occasions they 
sometimes remain clinging to the trees while the 
