50 IOWA STUDIES IN NATURAL HISTORY 



er margin rounded to wavy ; costal fold present except in gesta. 

 Hind tibiae of male with tufts in a few species. Fig. 18. 



A key to the species of Thanaos based on superficial characters 

 is of comparatively little use, for the species are closely related 

 and there are few which do not intergrade with others. The fol- 

 lowing key is based on fairly typical specimens, but in a long 

 series I have found many which could not be definitely placed 

 by it, so I have thought it w^se to speak a word of caution re- 

 garding its use. The genitalia of the males offer the ultimate 

 means of determination, and since they can usually be examined 

 fairly well by brushing away the scales from the tip of the 

 abdomen and using a hand lens or binocular, their use in class- 

 ification of the species should be practiced. 



Key to the species 



1. Fringes of hind wings white 13 



Fringes never white 2 



2. Primaries without distinct hyaline spots, sometimes with one or two 



clouded spots next to costa 3 



Primaries with at least a subapical row of hyaline spots (or with very 

 dark wings, terentkts $) 5 



3. Expanse under thirty mm ; $ with tibial tuft ; apex of primaries rather 



sharply angled icelus 



Expanse usually over thirty mm ; no tuft in male ; apex of primaries 

 more obtuse and outer margin more rounded 4 



4. Gray powdering heavier toward apex of primaries; distribution gen- 

 eral brizo 



Primaries usually evenly powdered with gray scales; inner part of 

 median band usually obsolete or broken; Southwestern and Calif ornian 



species bwrgessi, lactistra 



Powdering scant or absent; dark marks of primaries united to form 

 broad bands; under surface without distinct spots; $ with tuft but 

 no fold ; southwestern species gesta 



5. Under surface of secondaries with two pale subapical spots, or at least 



a trace of them 6 



No subapical spots 7 



6. Gray vestiture mostly of fine hairs properties 



Gray vestiture scaly juvenalis 



7. Hyaline spots large, at least a trace of one in end of cell; dark mark- 

 ings of primaries contrasting (9)» or spots' lacking; primaries with 



little or no gray vestiture ( $ ) horatius 



Spots small, color very dark, or gray powdering conspicuous 8 



8. Very dark, sometimes with a brownish patch at end of cell; macula- 



tion obscure; $ with tibial tuft terentius 



Tuft absent; marks more or less contrasting 9 



