1901.] 295 



markings, distinguishes it from other described species. The name 

 anomala (given under a misapprehension of the systematic position of 

 the insect) is certainly inappropriate. There is nothing really anoma- 

 lous about the species ; but having been once applied, the laws of 

 zoological nomenclature necessitate its retention. 



Peradeniya, Ceylon : 



October Uh, 190l. 



THE TWO METHODS OF DETERMINING DIPTERA. 

 BY C. U. OSTEN SACKEN. 



For the purpose of recognising a Dipteron (and perhaps most 

 other Insects) in one, or more descriptions, two different methods 

 may be used, one of which may be called the metliod of comparison, 

 and the other the method of vifiualization. 



The method of comparison is the ordinary one, when the specimen 

 to be determined is held up, and compared in every detail with the 

 description. In using this method, one is often bewildered by dis- 

 crepancies, especially when the description is long. The method of 

 visualization consists in examining the specimen first, and impressing 

 its principal features upon one's memory, so as to be capable of 

 visualizing it in its absence. The next step is to read the description 

 (or descriptions when there are several), and, while doing it, to build 

 up the described insect in one's imagination. Thus a mental image is 

 produced in which the species to be determined can easily be recognised, 

 even in the absence of the specimen. After having thus selected a 

 description apparently answering the specimen to be determined, the 

 description is read for a second time with the specimen in hand ; and 

 this second reading enables one to decide whether the discrepancies 

 are important, or not, and, in the latter case, to accept the identifi- 

 cation. 



The method of visualization is quicker and surer than the other, 

 and, with it, I have sometimes succeeded in deciphering Walker's 

 sometimes long, but unmeaning descriptions. In the following instance 

 the advantage of the method of visualization was unmistakably 

 proved. 



Loew had prepared a preliminary list of American Dasypogonina, 

 in which the species known to him were distributed among his new 

 genera. Some of Walker's and other unrecognisable specific de- 

 criptions were enumerated in this list under the heading Dasypogon 

 (in the widest sense). Loew challenged what he callcil my porspi- 



