OBSERVED IN GERMANY. 517 



passed at the height of 7 or 8 feet, catching insects as they 

 flew on. On June 1st and 2nd, straggling parties of five or 

 six were observed, always keeping the same direction-. With- 

 in a league to the east of Halle these swarms were everywhere 

 observed. To the west the whole valley was inundated by 

 the river Saale. Libellula quadrimaculata is rather scarce 

 at other times about Halle, as Lib. depressa is about Weimar. 



As far as the information which I have been able to collect 

 goes, this migration has extended from the 51st to the 52nd 

 degree of latitude, and has been observed within 27° 40' and 

 30° east of Ferro. But the instance of Calais renders it pro- 

 bable that it has extended over a great part of Europe, wher- 

 ever the same meteorological circumstances have prevailed. 



Several of the larger species of Libellula do occasionally 

 migrate, but the phenomenon is of rare occurrence, and the 

 circumstances which bring about such an uncommonly nu- 

 merous development of the perfect insect must be very peculiar. 



The last migration of dragon-flies (before that commemo- 

 rated in this article) which was observed at Weimar, took 

 place on the 28th of June, 1816. The insects in that instance 

 also belonged to the same species — Libellula depressa. They 

 were then, as recently, taken for locusts by the common peo- 

 ple, and the superstitious saw in them the harbingers of fa- 

 mine and war. 



The year 1816 was extremely wet, and 1817 equally so, 

 but it appears that the dragon-flies did not migrate that year. 

 Though such migrations must be very destructive to the spe- 

 cies, yet this cannot be the reason why the phenomenon was 

 not observed in 1817, as the Libellula require more than one 

 year to become perfect insects. The difference of the dates 

 of the 30th and 31st of May, 1839, and the 28th of June, 1816, 

 is also remarkable ; but I cannot account for it, as I am de- 

 ficient in regular observations upon the weather during the 

 spring months of 1816 : it proves, however, how greatly the 

 time of their development differs in different seasons. 



As to the great multiplication of these insects about the end 

 of May in the present year, it is by no means mysterious. — 

 From the beginning of that month to the 21st, the weather 

 had been exceedingly rainy ; rivers and lakes overflowed and 

 spread their inundation over immense areas of low grounds, 

 whereby myriads of the pupa l of the Libellula which, under 

 other circumstances, would have remained in deep water, and 

 become the prey of their many enemies, were brought into 

 shallow water, and the hot weather from May 21st to May 



1 Or larvce in the stage answering to that of the chrysalis. 



