392 PROCEEDn>rGS of the national museum vol. 117 



ous, with several distinct elements present, but all are derivable 

 from a common ancestral type. When the Neotropical species are 

 better known, it is probable that this second group may be further sub- 

 divided. The accompanjdng phylogenetic tree (fig. 3) for the genus is 

 thought to be a reasonably accurate portrayal of the P. "americanum'^ 

 group, whereas the placement of the other nine species comprising 

 the P. "rvjicorne" group is highly speculative, especiaDy in regard 

 to P. inguilinum and P. micans. 



In the final analysis, the male genitalia seem to hold the key to 

 the broad classification of this genus. In most cases (notably the 

 P. ''americanum" group) there are other more useful characters to 

 distinguish between species. No new characters were discovered 

 during the course of the present study, except the relative distance 

 separating the eyes when viewed from below, a function of the size 

 of the lower lobe of the eye. To arrive at an index, the specimen is 

 turned upside down so that both eyes are entirely in view and then 

 adjusted so that both eyes measure the same with an ocular microm- 

 eter. The longer axis of one eye is then compared to the distance 

 separating the two eyes. In most species, this index is quite constant 

 within narrow limits and is quite useful taxonomicaUy (e.g., in P. 

 teleops it is greater than three whereas in P. americanum it is less 

 than one). In some species it varies so greatly that it is useless, and 

 in others (e.g., P. excavatum) it varies clinally. 



The relative convexity of the prosternal process is another very 

 useful character if introduced at the proper place in a key. In a 

 number of species (e.g., P. americanum, P. neglectum, P. mexicanum, 

 P. micans, P. oregonense) this structure passes between the front 

 coxae and is horizontal all the way to its apex; in others (e.g., P. 

 subcostatum, P. picilabrum) it is just as constantly convex, with its 

 apex deflected behind the coxae and concealed in repose. In P. 

 laevipes and P. nigratum and several others, the form of the pro- 

 sternal process is variable and hence useless as a diagnostic character. 



The shape of the terminal segment of the maxillary palpi varies 

 enough between species to be used as a supplementary character. 

 Several of the earlier workers separated Platydema from other genera 

 on the basis of the terminal segment of the maxillary palpi forming 

 a nearly equilateral triangle. This might perhaps hold true for 

 certain limited faimal areas, but by no means prevails throughout the 

 genus on a worldwide basis. 



The Holarctic Region is poorly represented in species of Platydema. 

 In the Nearactic Region there are 19 species while Europe has only 5. 

 The tropical regions of both hemispheres, on the other hand, abound 

 in species. Gebien (1940) lists 40 from Africa, 77 from Asia, 54 from 

 Central America, 55 from South America, and 28 from Papua and 



