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. CHAPTER X. MOSQUITOES AND PARASITE PROBLEMS. There cannot be a doubt that some animals possess an instinctive knowledge of their enemies--or, at all events, of some of their enemies--though I do not believe that this faculty is so common as many naturalists imagine. The most striking example I am acquainted with is seen in gnats or mosquitoes, and in the minute South American sandflies (Simulia), when a dragon-fly appears in a place where they are holding their aerial pastimes. The sudden appearance of a ghost among human revellers could not produce a greater panic. I have spoken in the last chapter of periodical storms or waves of dragon-flies in the Plata region, and mentioned incidentally that the appearance of these insects is most welcome in oppressively hot weather, since they are known to come just in advance of a rush of cool wind. In La Plata we also look for the dragon-fly, and rejoice at its coming, for another reason. We know that the presence of this noble insect will cause the clouds of stinging gnats and flies, which make life a burden, to vanish like smoke.