DIAPERINI NORTH OF MEXICO — TRIPLEHORN 361 



probably incorrect, but deferring its ultimate assignment to avoid 

 possible further confusion. 



Sexual dimorphism. — It is noteworthy that among species of Di- 

 aperini, it is either very easy or else vu-tually impossible to distinguish 

 between the sexes without dissection, depending upon the species 

 involved. In almost the entire genus Neomida and in quite a few 

 species of Platydema, the males are provided with conspicuous frontal 

 horns, whereas the females have either short tubercles instead of horns 

 or no cephalic armature at all. The size of these horns is subject to 

 a certain amount of variation, even among individuals of a single 

 population, but the horns of the male are m all cases quite distinct 

 from the tubercles of the female. This situation largely prevails in 

 these two genera even when considered on a worldwide basis. 



In the Indo-Malayan region, Gebien (1925) found species of Platy- 

 dema in which neither the males nor the females are provided with 

 cephalic armature. This is true of 15 of the 19 North American spe- 

 cies of Platydema. Gebien also reported two distinct species groups 

 of Platydema in which the horns of the male were asymmetrical, in 

 one, the right, and in the other, the left, horn being larger. In several 

 species (e.g., P. monoceros Gebien) there is but a single frontal horn; 

 in others (e.g., P. tricuspis IMotschoulsky) there is a rather large horn 

 on the clypeus in addition to the two frontal horns. 



An equal range of variation is to be found in Neomida, in which 

 the males of almost every species have some sort of frontal or clypeal 

 armature which is totally lacking or but feebly developed in the 

 females. 



It is not surprising that great use has been made of these struc- 

 tures. In Gebien's key to the species of Platydema of the Indo- 

 Malayan region, they have been used as primary characters so that 

 it would be impossible to determine females unless they could be 

 associated in some way with males. 



In Pentayhyllus pallidus, the males are provided with short tuber- 

 cles near the inner margins of the eyes, whereas the females are un- 

 modified in this respect. In P. californicus there is no sexual 

 dimorphism. 



The males of Diaperis maculata have the anterior margin of the 

 pronotum bituberculate at the middle and have two blunt but prom- 

 inent clypeal tubercles; these same structures are unmodified in 

 females of that species. No sexual dimorphism is evident in any 

 of the other species in the genus. 



Probably the most peculiar example of sexual dimorphism is to 

 be found in Alphitophagus bifasciatus (Say) . The male of this species 

 (pi. 6, fig. 57) has the clypeus greatly swollen and excavated lat- 

 erally for reception of the prolonged genae. In addition, there are 



