1875.] 



SENATE— No. 75. 



15 



nearly one thousand (the number of undetermined species 

 cannot be estimated with any degree of probability). 



For the sake of comparison, I will state that the total num- 

 ber of the described species of diptera from North America, 

 north of Mexico^ is about 2,500 ; the number of those from 

 Mexico, Central America and the West Indies is about 800. 



It is important, at this place, to define exactly the degree of 

 reliance which can be placed in the determinations of the col- 

 lection. In this respect the thousand named species of the 

 Museum collection can be referred to the following four 

 classes : — 



1. In the course of my correspondence with Dr. Loew, 

 for many years, a considerable number of specimens were 

 returned by him to me with labels in his handwriting. Most 

 of such specimens are, therefore, if not the original types of 

 his descriptions, at least may be called author's types. All 

 labels of this description are preserved in the collection. 



2. A still more considerable class of specimens are num- 

 bered duplicates, which I kept, while sending to Dr. Loew 

 the identical species, labelled with the same numbers. After 

 describing such species, Mr. Loew used to send me the name, 

 with the number attached. Species thus named are almost 

 as good as authors' types, the rare but not impossible cases 

 excepted, where I may have been mistaken as to the identity 

 of the specimens sent with those retained under the same 

 number. This class of species, — there are some four hundred 

 of them, — generally represented by a number of well-preserved 

 specimens, are a very valuable, perhaps the most valuable, 

 portion of the collection (especially in the difficult families 

 of EmpidGe^ Tachydromidce^ etc.). 



3. The original types of my own descriptions, embracing 

 the whole family Tipididoe hrevipalpi and a small number of 

 other species (about 160 types in all). 



4. Species identified by me from the existing descriptions. 

 The credit which these identifications deserve depends, of 

 course, on the merit of the descriptions and the difficulties 

 inherent to the identified subject. 



It results from the foregoing statement that, for a consider- 

 able majority of the named species, the standard of trust- 

 worthiness of the identification is a very high one. 



