DRAGON-FLY STORMS.
One of the most curious things I have encountered in my observations on
animal life relates to a habit of the larger species of dragon-flies
inhabiting the Pampas and Patagonia. Dragon-flies are abundant
throughout the country wherever there is water. There are several
species, all more or less brilliantly coloured. The kinds that excited
my wonder, from their habits, are twice as large as the common widely
distributed insects, being three inches to four inches in length, and as
a rule they are sober-coloured, although there is one species--the
largest among them--entirely of a brilliant scarlet. This kind is,
however, exceedingly rare. All the different kinds (of the large
dragon-flies) when travelling associate together, and occasionally, in a
flight composed of countless thousands, one of these brilliant-hued
individuals will catch the eye, appearing as conspicuous among the
others as a poppy or scarlet geranium growing alone in an otherwise
flowerless field. The most common species--and in some cases the entire
flight seems to be composed of this kind only--is the Aeschna
bonariensis Raml, the prevailing colour of which is pale blue. But the
really wonderful thing about them all alike is, that they appear only
when flying before the southwest wind, called _pampero_--the wind that
blows from the interior of the pampas. The pampero is a dry, cold wind,
exceedingly violent. It bursts on the plains very suddenly, and usually
lasts only a short time, sometimes not more than ten minutes; it comes
irregularly, and at all seasons of the year, but is most frequent in the
hot season, and after exceptionally sultry weather. It is in summer and
autumn that the large dragon-flies appear; not _with_ the wind, but--and
this is the most curious part of the matter--in advance of it; and
inasmuch as these insects are not seen in the country at other times,
and frequently appear in seasons of prolonged drought, when all the
marshes and watercourses for many hundreds of miles are dry, they must
of course traverse immense distances, flying before the wind at a speed
of seventy or eighty miles an hour. On some occasions they appear almost
simultaneously with the wind, going by like a flash, and instantly
disappearing from sight. You have scarcely time to see them before the
wind strikes you. As a rule, however, they make their appearance from
five to fifteen minutes before the wind strikes; and when they are in
great numbers the air, to a height of ten or twelve feet above the
surface of the ground, is all at once seen to be full of them, rushing
past with extraordinary velocity in a north-easterly direction. In very
oppressive weather, and when the swiftly advancing pampero brings no
moving mountains of mingled cloud and dust, and is consequently not
expected, the sudden apparition of the dragon-fly is a most welcome one,
for then an immediate burst of cold wind is confidently 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 dragonfly movements are not
explicable on any current hypothesis regarding the annual migrations of
birds, the occasional migrations of butterflies, or the migrations 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 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. A race meeting was being held near
the town of El Carmen, on a high exposed piece of ground, when, shortly
before sunset, a violent pampero wind came up, laden with dense
dust-clouds. A few moments before the storm broke, the air all at once
became obscured with a prodigious cloud of dragon-flies. About a hundred
men, most of them on horseback, were congregated on the course at the
time, and the insects, instead of rushing by in their usual way, settled
on the people in such quantities that men and horses were quickly
covered with clinging masses of them. My informant said--and this agrees
with my own observation--that he was greatly impressed by the appearance
of terror shown by the insects; they clung to him as if for dear life,
so that he had the greatest difficulty in ridding himself of them.
Weissenborn, in London's _Magazine of Natural History_ (N. S. vol. iii.)
describes a great migration of dragon-flies which he witnessed in
Germany in 1839, and also mentions a similar phenomenon occurring in
1816, and extending over a large portion of Europe. But in these cases
the movement took place at the end of May, and the insects travelled due
south; their migrations were therefore similar to those of birds and
butterflies, and were probably due to the same cause. I have been unable
to find any mention of a phenomenon resembling the one with which we are
so familiar on the pampas, and which, strangely enough, has not been
recorded by any European naturalists who have travelled there.