PREFACE. 



THE impulse to write on North American Diptera was given 

 to me by Baron Osten-Sacken, who, first by sending me rich 

 collections of such Diptera and finally by intrusting me with the 

 greatest part of his own Diptera collected in North America, has 

 enabled me to undertake this task, and, I hope, with some success. 

 If my observations had been written in German, and published in 

 any of our German Transactions, I should have had good reason 

 to fear that the results would not become sufficiently known in 

 North America, and would at all events be longer in obtaining 

 access there. I resolved, therefore, to give them in English, and 

 the Smithsonian Institution in Washington has added to the many 

 proofs it has already given of an energetic furtherance of any 

 studies relative to the natural history of North America, the 

 liberal resolution to print my paper on North American Diptera 

 at its own expense. If these papers, according to my intention, 

 contribute to the increase of the study of this interesting order of 

 insects, the principal thanks are due to the Institution and to 

 Baron Osten-Sacken. As for the English text, I use, it is true, 

 the assistance and advice of a friend who is well acquainted with 

 the English language ; should, nevertheless, some roughness occur, 

 I beg that it may be attributed to some supplementary alterations 

 of my own, which circumstances prevented me from submitting 

 to him. I hope that shortcomings of this kind will be kindly over- 

 looked, provided my descriptions be not deficient in precision and 

 clearness. As I do not wish to remain the sole describer of 

 N. A. Diptera, but hope soon to have many fellow-laborers, I 

 take the liberty of pointing out briefly what, in my opinion, 

 should be chiefly borne in mind in making and publishing such 

 descriptions. Without any disposition to find fault with others, I 

 believe I may be permitted some remarks, since for the last twenty 



