( cxxxviii ) 



another in general aspect, can be distinguished at a glance, as 

 we have just seen, by the character of their scent-scales ; and 

 this serves to illustrate the usefulness of these structures as 

 aids in specific diagnosis. I have found them, for instance, of 

 great service in distinguishing between the extreme dry-season 

 forms of some tropical species of Pierines. These may some- 

 times resemble each other so closely in general aspect as to 

 baffle even the experienced naturalist. Another interesting 

 case is the following — -A rather worn and battered Pierine 

 was brought from Mexico, and pronounced by competent 

 authorities to be probably a Ganoris. The occurrence of a 

 member of this genus in such a locality would be, to say the 

 least, remarkable ; and I thought it desirable to submit the 

 specimen to a careful examination. The microscopic inspec- 

 tion of a few scales showed me at once that the butterfly was 

 neither a Ganoris nor a Sjpichloe, but a Leptophohia. A similar 

 test proves the very curious South American form Mathania 

 agasicles to be practically a HesjMrocharis. 



In some instances, as in the two Pereutes lately mentioned, 

 the examination of a single scent-scale from each would be 

 sufiicient to separate the species ; but this would not always 

 be the case. Just as in the use of the ordinary methods of 

 diagnosis one may sometimes doubt about a single specimen, 

 but find one's difliculties removed if a series is available, po 

 with the scent-scales. And in measuring, or otherwise estimat- 

 ing, the average character of the scent -scales in a given pre- 

 paration, we have the advantage of knowing that they all come 

 from a single individual ; whereas we cannot always be certain 

 that a supposed series of a species is really homogeneous. It 

 is of course a limitation to this method of diagnosis that the 

 scent-scales belong only to the male sex. This is a limitation 

 shared to a very great extent by diagnosis from the genitalia ; 

 but it is largely counterbalanced in the case of the scent-scales 

 by the ease with which the examination can be made, by the 

 facts that no special mode of preparation is necessary, and that 

 the requisite material can be obtained without in the least 

 interfering with the natural appearance of the specimen ; also 

 by the further fact that few specimens are so badly preserved 

 as not to afford ample scope for this kind of investigation. 



