198 STUDIES IN THE GENUS THANAOS 



p. 79 gives a generic diagnosis and a list of the species. Dr. W. J. 

 Holland, in the Butterfly Book, gives some good figures of some of 

 the species and says, "The genus Thanaos is one of the most difficult 

 genera to work out in the present state of our knowledge of the 

 subject. The species are not only obscurely marked but they vary 

 in the most extraordinary manner. Except by a microscopic 

 examination of the genital armature, which can be carried on only 

 when the student possesses considerable anatomical knowledge 

 and an abundance of material, there is no way of reaching a satis- 

 factory determination in many cases." 



I have kept all the specimens of this genus collected or received 

 in exchange and have also had access to the other collections that 

 have become the property of the Academy of Natural Sciences of 

 Philadelphia. This aggregation makes what I think is the largest 

 and most complete collection of the genus extant. All the refer- 

 ences to the literature of the subject have not been given in the 

 following pages, but only those considered important or useful. 

 This also applies to figures and illustrations, the best and most 

 exact being chosen. The main object of the study is to aid students 

 in the determination of these difficult butterflies. 



The older authors in their descriptions attached great importance 

 to the size and position of the vitreous spots on the primaries. As 

 used they had very little value for specific differentiation. A care- 

 ful study of the spots from large series of a given species would 

 doubtless be of value in an effort to obtain the limits of individual 

 variation. The sexes in Thanaos are very easily determined by 

 the presence or absence of the costal fold of the primaries, as it is 

 a male secondary sexual character. 



Before we can have an absolutely stable specific nomenclature it 

 will be necessary to absolutely fix the types. We do not know what 

 some of the older species represent. The* description oi juvenalis, 

 for instance, will not enable us to say whether it may not be 

 horatius that was described by Fabricius. Indeed the description 

 agrees better with horatius than it does with juvenalis. An exami- 

 nation of the genitalia of the type of tristis Boisduval would make 

 an addition to our knowledge. Pacuvius may be pacuvius of 

 authors, or may be an allied species described later. 



It has always been supposed that the black-fringed species are 

 all related to each other and that the white-fringed species form a 



