190 BULLETIN UNITED STATES GEOLOGICAL SURVEY. 



Lieut. W. L. Carpenter. Here and there I have introduced descriptions 

 of some remarkable species from the Atlantic States. 



In treating of the Californian fauna (or flora) it must be borne in mind 

 that what is called Sierra Nevada is not only a mountain range, but 

 a whole country — a high plateau from 6,000 to 8,000 feet above sea- 

 level, forming a long and comparatively broad belt of land, with its 

 lakes, rivers, forests, and plains — an upper story of California, partaking 

 of some of its products, but on the whole entirely different. For the 

 better undestanding of the facts bearing on the geographical distribu- 

 tion of insects, I will state here, once for all, that my collections about 

 Summit, Sierra Nevada, and Webber Lake were formed at an altitude 

 from 7,000 to 8,000 feet above sea-level ; that the altitude of Lake Tahoe 

 is 6,200 to 6,300 feet, and that of Yosemite Valley about 4,000 feet. My 

 collecting grounds in Southern California, as well as in Marin and So- 

 noma Counties, were all at comparatively low levels, except the Gey- 

 sers, Sonoma County, which are about 3,000 feet above sea level. 



It is not my intention to describe all the western Diptera which I pos- 

 sess or can get hold of. Always keeping the higher aims of science in 

 view, my effort will be to contribute toward those aims. The detailed de- 

 scription of special entomological faunas must of necessity be left to 

 local students. An outsider, a transient collector and describer, has to 

 keep their interest in view, and to try to pave the way for them rather than 

 to block up their progress by an indiscriminate and aimless publication 

 of new species. 



In prefixing diagnoses to some of my descriptions, my aim was to en- 

 able the reader at a single glance to get hold of the principal features 

 of the described species, and thus to save his time in the work of iden- 

 tification. Such a diagnosis, in order to be useful, must be short, even 

 at the risk of being applicable to more than one species. Wherever the 

 species in a genus are more numerous, I prefer to give an analytical 

 table. The attempt of some authors to draw diagnoses which are tanta- 

 mount to definitions of the species is very difficult to carry out, espec- 

 ially in the larger genera ; such diagnoses finally become as long as the 

 descriptions themselves, and therefore practically useless. 



In quoting species described in North American publications or in Dr. 

 Loew's "Centuries", I will simply refer to them without repeating the 

 descriptions, as it is to be expected ttjat a dipterologist is in possession 

 of the works thus quoted. In some cases I will reproduce or translate 

 descriptions which are less easily accessible. 



All the type specimens of these my papers I intend to deposit, for 

 future reference, in the Museum of Comparative Zoology in Cambridge, 

 Mass., where my former dipterological collections are also to be found ; 

 the few exceptions will be mentioned in their place. 



I owe a special tribute of gratitude to Mr. Henry Edwards, of San 

 Francisco, for his manifold assistance, as well as for the contribution of 

 valuable specimens. 



