VIII PREFACE. 



enumerated in the new Catalogue, are represented in a collection. 

 The collection of diptera of the Museum of Comparative Zoology 

 in Cambridge, Mass., contains what may be called the typical 

 specimens of this Catalogue, that is the types of the descriptions 

 published by Mr. Loew and by myself, as well as the species 

 identified by him or by me, from earlier descriptions. That 

 collection thus contains a little over 2000 named and described 

 ;pecies of diptera from North America*), north of Mexico, besides 

 a considerable accumulation of unnamed and undescribed materials. 

 In that collection the americau dipterologist now possesses an 

 advantage not shared by his European colleagues, and that is, of 

 having very nearly all the typical specimens, necessary for his 

 work, collected in the same spot. It is highly desirable that this 

 advantage should, as far as possible, be maintained, and that 

 describers of new species should deposit their types in the same 

 Museum, which offers the best guarantees of their permanent 

 preservation. Sixty years ago , Wiedemann (in the first chapter 

 of his Magazin fiir Zoologie"), foreseeing the future difficulties of 

 dipterology, suggested the formation of a central, or as he called 

 it, normal Museum, in some European city, to contain types of 

 all the described species ; no new species were to be published, 

 without previous comparison in that Museum. May the Museum 

 in Cambridge realize that idea for America ! 



LITERATURE. The literary references, which I give in the 

 notes , are not meant to be a complete index of dipterological 

 literature, but merely a guide to beginners, who might be easily 

 deterred by the preliminary work to be gone through , before 

 attempting the study of any family. Those who intend to go 

 deeper into the subject will have to form a more complete index 

 for themselves, by looking over the yearly entomological 

 Records**;, as well as the works in the libraries. That the 

 majority of the papers quoted by me are those of Dr. Loew, 

 arises from the fact that for the last 30 years he was the prin- 



*) These species are marked with a star in the Catalogue. 

 **} A yearly Record on the progress of entomology is published 

 in Germany since 1838, iii Wiegraanii's Archiv fur Zoologie. This 



