NORTH AMERICAN COLEOPTERA. 195 



THE EIJ.HOLPIXI OF ISOKEAL, AMERICA. 



BY GEO. H. HORN, M. D. 



The essay whicli follows has uo higher aim than to be a descriptive 

 catalogue of the species at present known. My desire has been to 

 place in the hands of students the means of separating the recog- 

 nized species with some degree of accuracy so that new forms may 

 be detected and proper data obtained of the food-plants. 



The Eumolpini are by no means an easy group to study. While 

 the genera seem fairly well defined as to facies, permanent and 

 sharply limited characters for their definition are not readily found. 

 In fact, practical experience and a certain amount of emj)irical 

 knowledge are important factors here as well as in many other places 

 in the Coleoptera. 



As an example we may take the first character used in the ana- 

 lytical table — the presence or absence of the lobe on the prothorax 

 below the eyes. In many cases the character is very well defined, 

 but specimens, and even species, occur in genera which normally 

 have the lobe, in which it is hardly possible to say to which division 

 they should be referred, and it is at this point that experience and 

 tact have weight. It has been found elsewhere that it is possible to 

 draw hard and fast lines in classification, but I am not aware that 

 anv portion of the Chrysomelidse admits of such treatment. 



The nundier of genera of Eumolpini at present known is about two 

 hundred, requiring the division of the tribe into numerous groups in 

 order that the genera may be correlated and studied. When genera 

 are numerous an exceptional case is allowable, and does not greatly 

 embarrass the student in his appreciation of the relation of the genus 

 to the grouj) in which it forms an exception by reason of some modifi- 

 cation of the key cliaracter or its absence. On the other hand, when 

 genera are few in number, as in our fauna, and the exceptional case 

 is the only genus we have, then other characters must be sought than 

 those used in the greater field. An instance of this sort is found in 

 Tymnes, our only Edusite, one of the principal group characters being 

 the presence of transverse rug?e behind the humerus, which are 

 totally wanting in Tymnes. 



Characters drawn from the claws must be used with the addition 

 of judgment and experience, as they are variable in all the genera 

 with several species. 



To those who desire to study in detail the modifications of the 



TRANS. AM. ENT. SOC. XIX. .lUI.V. 1892. 



