6 NORTH AMERICAN DIPTERA. 



To the authorities of the National Museum and Mr. D. 

 \V. Coquillett, the writer is indebted for the communica- 

 tion of specimens of fourteen genera for purposes of illus- 

 tration. Finally he has to express his hearty thanks to 

 his friend and former colleague, Dr. F. H. Snow of the Uni- 

 versity of Kansas, for the free permission to make use of 

 whatever specimens were needed in the rich collections of 

 that University — collections which include nearly all the 

 types of diptera from the United States described by 

 Townsend, Snow, Adams and the writer. 



Since the preparation of the present edition was begun 

 has occurred the death of one whose name will ever be 

 honorably associated with American Dipterology, Dr. C. 

 R. Osten Sacken. The author can not forbear placing 

 on record here an earnest tribute of friendship and ad- 

 miration for the man, and unqualified appreciation of 

 his work as an entomologist. His constant encourage- 

 ment and kindly criticism during a correspondence of 

 more than twenty years have made this book, whatever 

 be its merits, possible; and the author only hopes that it 

 may be found not unworthy of association with his work. 

 1 1 is with pain, also, that the author here records the 

 death, in early years, of his friend and student the late 

 Mr. W. A. Snow, who assisted in the preparation of the 

 Ortalidse of the former edition. 



About twelve hundred genera are defined in the pres- 

 ent edition, with the exception of a few doubtful forms, 

 all those known from North and Central America and 

 the West Indies. That the definitions are wholly with- 

 out error is inconceivable. Whatever revision the book 

 may receive in the future must be left to others, and, 

 in taking final leave of it after these twenty-four years, 

 the writer will be pardoned in repeating the words of a 

 master, one whose works have served as models for this, 

 Rudolph Schiner: 



'Und so uebergebe ich denn diese meine Arbeit der Oeffentlichkeit 

 mit detn Wunsche, <lnss sie billigen Anforderungen entspreehen moege 

 nml mit dem Beifuegen, <lass ich dem Urtheile unparteiischeu [<eser 

 mit volkr Beruhigung entgegensehe, da ich bewusst bin, bei Lot'sun^ 

 meiner Aufgabe mit allem EJrnste und der groessten Gewissenhaftig- 

 keil vogegangen zu sein.' 



SAMUEL W. WIU.ISTON. 



University of Chicago, June, 1908, 



