The Naturalist in La Plata

Ch. 9: 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 n***o 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.