On the 30th and 31st of May, 1839, immense cloud-like swarms of Dragon-flies passed in rapid succession over the German town of Weimar and its neighborhood. They were the _Libellula depressa_, a species which, in general, is rather scarce in that part of Germany. The general direction of this migration was from south by west to north by east. The insects were in a vigorous state, and some of the flocks flew as high as 150 feet above the level of the River Ilm. At Gottingen on June the 1st, at Eisenach on May the 30th and 31st of the same year, swarms of the same species were seen flying from east to west; and at Calais, June 14th, similar clouds, though of a different species, were noticed on their way toward the Netherlands. At Halle, also, on May 30th, a short time before a thunder-storm, swarms of the Dragon-fly, _L. quadrimaculata_, were seen by Dr. Buhle, flying very rapidly from south to north. The _L. quadrimaculata_ is not generally found in the neighborhood of Halle. This wonderful migration, for it is a phenomenon of rare occurrence, extended from the 51st to the 52d degree of latitude, and was observed within 27° 40′ and 30° east of Ferro. But the instance of Calais renders it probable that it extended over a great part of Europe. Another migration of Dragon-flies was observed at Weimar on the 28th of June, 1816. The insects, in this instance, belonged also to the _L. depressa_. They were taken then, as were they also in 1839, for locusts by the common people, and looked upon as the harbingers of famine and war. In these migrations they followed the direction of the rivers, with the currents. They did not, however, always keep close by them, since they must spread over wide districts in order to subsist. To account for the great multiplication of these insects, in the year 1839, is by no means difficult. From the beginning to the 21st of May (in the latter part of which month, it will be remembered, they appeared), the weather had been exceedingly rainy; rivers and lakes overflowed their banks and inundated immense areas of low grounds, whereby myriads of the _larvæ_ and _pupæ_ (which live entirely in water) of the _Libellulæ_, which, under other circumstances, would have remained in deep water, and become the prey of their many enemies, fish, etc., were brought into shallow water, and hot weather following, from May 21st to May 29th, converted these shallows and swamps into true hotbeds for them. Their development into perfect insects was thus rendered rapid, so that, somewhat earlier than usual, they appeared, and in far greater, their undiminished, numbers; and, being very voracious in their appetite, as well in the imago as the pupa state, they were obliged to migrate immediately to satisfy it.[474]