" 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 like thistledown 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 compelling 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 wind spends its force. This is
particularly the case when the wind blows up at a late hour of the day;
then, on the following morning, the dragon-flies are seen clustering to
the foliage in such numbers that many trees are covered with them, a
large tree often appearing as if hung with curtains of some brown
glistening material, too thick to show the green leaves beneath.
In Patagonia, where the phenomenon of dragon-fly storms is also known,
an Englishman residing at the Rio Negro related to me the following
occurrence which he witnessed there.
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