420 HERMANN AUGUST HAGEN. 



paper, a List of the Dragon-flies of East Prussia, was printed in the 

 Preussische Provinzialblatter for January, 1839, when he was still at 

 the University ; * from that date until incapacitated in 1890 his con- 

 tributions to science number several hundreds. 



Some of these deserve special notice. In 1842, when studying in 

 Paris, Dr. Hagen met Baron de Selys-Longchamps of Liege, and 

 the friendship then begun resulted in the production through their 

 joint studies of the well known "Revue des Odonates d'Europe" 

 (1850), "Monographic des Calopterygines" (1854), and "Mono- 

 graphic des Gomphines " (1857). The co-operation of De Selys and 

 Hagen was not limited, however, to the works published conjointly ; 

 the writings of both had the advantage of a most generous inter- 

 change of notes, specimens, and drawings for a period of more than 

 forty years. 



The study of the insects found in amber and of the fossil Neurop- 

 tera early and continuously engaged his attention, his well known 

 accuracy and thoroughness both in description and in delineation pro- 

 ducing some of the best work yet done in palaeo-entomology. The 

 absence of generalizations, apparent in all bis work, is conspicuous 

 here ; he was slow in framing hypotheses, ready and critical in testing 

 their validity. 



His " Monographie der Termiten," published in the Linngea Ento- 

 mologica for 1855, 1858, and 1860, is a masterpiece of original work, 

 supplemented by a most exhaustive and thorough resume of all previous 

 studies in this most difficult of all families of insects ; it will always 

 remain a classic. 



One of Dr. Haven's earliest and most useful contributions to 

 American entomology is the " Synopsis of North American Neu- 

 roptera," prepared at the special request of the Smithsonian Institution, 

 and published by the Institution in 1861. With a keen insight into the 

 systematic relations of forms, and quick to perceive specific differ- 

 ences, he possessed the rare power of stating these differences clearly 

 and concisely. 



Probably his best known work, certainly the one more frequently 

 consulted by entomologists and zoologists than any other, is the 

 " Bibliotheca Entomologica : die Litteratur tuber das ganze Gebiet der 

 Entomologie bis zum Jahre 1862," published in two volumes in 1862 

 and 1863. Modelled somewhat after the earlier work of Percheron, 

 it is to day one of the most complete and accurately prepared of scien- 



* Two earlier genealogical studies were printed in 1834. 



