390 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 117 



species were available for study, so it is not known to what extent 

 they would conform to the present treatment. 



The relatively slender antennae, entire epipleura, well-developed 

 prosternal process and elongate basal segment of the hindtarsi should 

 serve to separate species of Platydema from other genera of Diaperini 

 in most instances. Even these characters are subject to a certain 

 amount of variation, so that it is necessary to consider them in 

 combination when dealing with many of the species in order to 

 determine to which genus they belong. As was mentioned pre- 

 viously, Platydema and Neomida are for the most part distinct 

 enough. Certain species of the latter overlap the former genus in 

 regard to a number of characters so that it becomes difficult to estab- 

 lish clearly defined generic limits. Platydema is much more homo- 

 geneous and, at least in regard to the above mentioned characters, 

 is relatively stable. Reasons for the retention of the two genera 

 as distinct despite this apparent interdigitation at the fringes are 

 summarized under Neomida. 



Cosmonota Blanchard, a small neotropical genus containing nine 

 species, is even more difficult to separate from Platydema. As La- 

 cordaire (1859) points out, these insects are extremely closely re- 

 lated to Platydema, and are distinguished more by their general form 

 and system of coloration than by any precise characters. 



It is not surprising that various workers have attempted to di- 

 vide this unwieldy genus. There are a number of variable charac- 

 ters which would seemingly aft'ord good generic or at least subgeneric 

 criteria. Again, much the same situation prevails here as was en- 

 countered in the generic characters discussed under Neomida. Di- 

 vision of the genus based on any single arbitrarily selected character 

 would be vastly different from the system derived by the utilization 

 of another character. For example, if we used the armed condition 

 of the head (frontal horns in males, tubercles in females) as a generic 

 or subgeneric character, we would place P. excavatum, P. teleops, 

 P. cyanescens, and P. erythrocerum in one group. A study of the 

 genitalia of the males of these species (pi. 4, figs. 22, 23, 24, pi. 5, 

 figs. 43, 44) indicates almost conclusively that the armed condition of 

 the head has originated at least twice in the evolution of the genus 

 Platydema, and that, whereas the other three-homed species are quite 

 closely related, P. erythrocerum is widely divergent from them and 

 should properly be placed near P. flavipes and P. nigratum. 



Surface lustre has been suggested as a useful criterion for dividing 

 the genus, not without considerable merit. Motschoulsky (1873) 

 proposed the name Neomida (a most unfortunate choice) for all of 

 the dull lustreless species and reserved Platydema for the shiny ones. 



Chevrolat (1878) suggested a similar system in which the dull 



