X PREFACE. 



boundary, from those which are known to belong south of that 

 line, A species, belonging to two groups simultaneously is placed 

 in the earlier group; within each group the species are arranged 

 alphabetically. — This change was rendered necessary by several 

 considerations of expediency. In the first place, the work of 

 criticism is much more advanced for the diptera of the United 

 States and especially of the northern and middle States, than 

 for those of Mexico, Central-America and the West-Indies: the 

 reason is, that the bulk of the available collections came from 

 the former regions. It was found expedient, therefore, to separate 

 the uncritical and merely compilatory portions of the lists from 

 those, that are more carefully sifted. At the same time, this 

 arrangement offers another advantage in the better survey it 

 affords of the geographical distribution of the diptera. Any one, 

 running over the Catalogue , will now be able at a glance to 

 form an idea of the character of the fauna of the temperate 

 regions of North America, as distinguished from the tropical 

 and subtropical faunae. Finally, this arrangement will be found 

 very convenient in putting the Catalogue to the principal use for 

 which it was intended, that of identifying species of diptera with 

 the existing descriptions. As the Western , and especially the 

 Californian fauna, is very different from the fauna of the Atlantic 

 States , I have formed a third , intermediate group of those 

 species in each genus, that are peculiar to that fauna. Whether 

 this distribution in two or three groups should be maintained 

 in the future editions of the Catalogue, is a question which Avill 

 have to be decided then, as it has been decided now, on con- 

 siderations of practical expediency. 



Many species living in the lower and warmer regions of 

 Mexico, also occur in Texas, and in the southern States in 

 general. On the other hand mexican species from the higher 

 altitudes, (from Mexico, Puebla etc.) extend quite far north, along 

 the high plateau of North America and in the Rocky Mountains. 

 Thus Dejcania corimlenta Wied. and JDcjcanki nitilioidcs 

 Jaennicke, both first described from Mexico, were found by me 

 in the Rocky Mountains. It is only recently, since I examined 

 the mexican species in the collections in Darmstadt and in Turin, 



