XX PREFACE. 



unavailable descriptions, the number of described diptera of 

 North America, north of Mexico, will hardly reach 2500. But 

 the undescribed materials, accumulated in the collections, if 

 worked up, would largely increase, perhaps double, that number. 

 Considering the little attention hitherto paid to the order of 

 diptera, these figures seem to prove that the number of existing 

 species of diptera in North America will easily reach and per- 

 haps exceed the number of Coleoptera. 



THE FUTURE OF AMERICAN DiPTEROLOGT. Of all orders of insects, 

 the diptera offer probably the most difficulties to the describer. 

 The reason lies in the minuteness of the characters, on which generic 

 and specifics distinction are based. In consequence of this diffi- 

 culty, there is and was more blundering in this order of insects 

 than in any other, and the mischief done by the incompetent 

 is greater here, than in any other order. By incompetent I do 

 not merely mean those, who know little or nothing about 

 diptera; I mean even dipterologists who attempt to write about 

 a family of diptera before having made a special study of it. 

 And in this respect, every one of us, in the course of his career, 

 is often tempted to do some work, which he is incompetent to 

 perform, and every one of us has, some time or other, actually 

 done such work. 



In order to preserve, as much as possible, American dip- 

 terology against the evils of incompetence, I attempted, several 

 years ago , to draw up some recommendations as to the best 

 course to pursue in that study (in A. S. Packard's Record 

 of American entomology for 1868). As these recommendations 

 have lost nothing of their appropriateness, I may be allowed 

 to reproduce them here. 



,,If I am asked now what the desiderata for the future 

 of this branch of science in America are, I would answer: 



!. Continue the publication of North American diptera in 

 monographs." 



n 2. Avoid as much as possible the publication of detached 

 species, either singly, or in numbers." 



