ART. 1'8 REVISION OF DISONYCHA NORTH OF MEXICO BLAKE S 



wide. The species comprising the genus are for the most part among 

 the larger species of Halticinae. They are distinguished from 

 Oedionychis, nomophoeta^ and other Oedipodes by the lack of the 

 inflated hind claw joint. They are generally smaller than the 

 species assigned to Cacoscelis, a genus from Central and South 

 America, but, in some instances, difficult to distinguish, and they are 

 so closely related to Altica that in some cases they can be distin- 

 guished only by the lack of a well-marked basal impression on the 

 jirothorax, a feature that is not always very evident in certain 

 species of Altica. 



The chief diagnostic characters are (1) in the head, in which 

 there is a long extension of the carina or interantennal prominence 

 from between the antennal bases to the suture above the labrum; 



(2) in the prothorax, which lacks a well-marked transverse basal 

 impression, and the hind angles of which are obliquely truncate; 



(3) in the open precoxal cavities; and (4) in the hind legs, in which 

 the posterior tibiae are not deeply grooved, the tibiae have a short 

 apical spur, the first tarsal joint is about twice the length of the 

 following one, and the claw joint is slender and appendiculate. 



Horn stated that " Disonycha is far more homogeneous than 

 Oedionychis.^ the species not exhibiting any marked structural differ- 

 ences among themselves, consequently any attempt at tabulation is 

 more or less based on coloration, which seems to be quite constant as 

 to type but variable in degree." On the contrary, the groups now 

 included in Disonycha are not homogeneous but differ far more 

 among themselves than do the groups inside such a genus as, for 

 examjile, Trirhdbda. Unfortunately, the material now available is 

 not sufficient to justify an attempt at present to revise the whole 

 genus and possibly to divide it into generic or subgeneric groups. 

 Disonycha ranges from Canada to Patagonia. The forms found 

 north of Mexico represent less than half of the described species, and 

 presumably many more from south of the Mexican border are unde- 

 scribed. Many of the species are not represented in the United 

 States National Museum or other collections examined by me. With 

 our limited representation of the groups of the genus, it is impossible 

 to make more than a local contribution to a comprehensive phylo- 

 genetic study of the species of both continents. In a few groups, 

 such as those that occur on boreal food plants, as willow, the relation- 

 ship of the species can be outlined, but in other groups chiefly 

 confined to tropical regions, as that to which glabrata belongs, a 

 single United States species stands out as isolated and unrelated to 

 any other. The most that can be done in a partial study of this 

 kind is to place the species as far as is possible in groups that 

 ultimately may be assembled in a study of wider scope. 



