Tl PREFACE. 



years I have been occupied with the study of Diptera, and have 

 been obliged to spend many hours in identifying (how often fruit- 

 lessly!) the published descriptions. What renders the identi- 

 fication of a very great number of the existing descriptions so 

 very difficult, is the inexactness of the system used. For however 

 natural the axiom may appear, that a new species is only to be 

 located in the genus to which it really belongs, it is so little re- 

 spected by most dipterological writers, that a long list could be 

 made out of the instances in which they have sinned against it; in- 

 deed the number of cases, in which a new species has been placed 

 in a wrong family, is not small. It is not even always sufficient 

 to place it in the right genus, for as soon as this genus is at all 

 numerous in species, or the species are difficult to distinguish, the 

 peculiar group of the genus to which the new species belongs 

 should be pointed out, and if among the species already well 

 known there are any very similar to those described, they ought 

 of necessity to be specially mentioned. Consequently only those 

 entomologists will publish new Diptera with success, who are 

 Completely acquainted with the system of this order of insects, 

 whereas he who lias a defective knowledge of it, far from advancing 

 science, lays impediments in its way. The first task, then, for 

 those who intend to come forth with satisfactory papers on the 

 rield of Dipterology, will of course be to acquire a most complete 

 and sure knowledge of the system. 



As an introduction to the following essays of a more mono- 

 graphic character, will be found a short sketch of the terminology 

 of Diptera, as well as one of the dipterological system. The latter 

 afforded me an opportunity of giving an outline of the Xorth Ame- 

 rican dipterological fauna, as far as known to me at present. An 

 elaborate classification, equally detailed in all its parts, would 

 require not only a larger amount of materials than I have at my 

 disposal, but also, in order to be intelligible, a considerable num- 

 ber of plates. I am compelled, therefore, to give up such an un- 

 dertaking for the present, I hope, however, to be able to execute 

 it at some future time. Although I trust that my short sketch 

 will prove of some help to the student, by furnishing him occasion- 

 ally a useful hint, or guiding him aright in general, it will be readily 

 understood that in the prosecution of the study he will require more 

 detailed information. I will, therefore, briefly indicate the works 

 in which he may find it : Meigen's Zweifliigelige Insecten (7 vols. 



