312 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 117 



of tubercules apparently tend to become most pronounced in the tribe 

 Rhysodesmini. Several species of Rhysodesmus are tuberculate; 

 one, R. toltecus, is characterized by several transverse rows of meta- 

 tergal raised areas. 



Sexual dimorphism. — In most xystodesmids the females tend to 

 be slightly larger than males from the same locaUty. Usually the 

 antennae are relatively smaller in females and the sterna relatively 

 broader than in males of the same size. In some species, particularly 

 in the Rhysodesmini, the size difference between the sexes becomes 

 pronounced and the extreme condition occurs in Boraria deturkiana 

 in which the female may attain nearly twice the bulk of the male. 

 Presumably, pronounced sexual size dimorphism is accountable as an 

 evolutionary speciahzation and, in this case, affords a useful criterion 

 in judging affinities. Taking the case of B. deturkiana, the com- 

 bination of coxal spination, strongly depressed paranota, and difference 

 in size between male and female sets the species in apposition to 

 B. stricta and B. media and perhaps warrants retention of the name 

 Howellaria as a subgeneric designation. 



Epicranial suture. — In all polydesmoids there is a distinct median 

 suture on the vertex of the head capsule that is reflected internally 

 as a median septum upon which the mandibular adductor muscles 

 originate. In earUer literature the suture was referred to as the 

 vertigial sulcus, but I think that it is essentially homologous to the 

 epicranial sutm-e of insects and should be so designated. In most 

 species the suture becomes indistinct in the interantennal isthmus, 

 but in a few it is ventrally bifid, and the two lateral branches extend 

 nearly or quite to the antennal sockets. This is the case in Boraria 

 media and B. deturkiana, and, along with the nearly identical gonopods, 

 attests a close relationship between these two species. In B. stricta, 

 at least in the adult condition, the two ventral branches are obscure 

 or invisible although they are distinct in young or recently moulted 

 specimens. 



Probably only specific value can be assigned to the ventrally 

 bifid condition since B. media and B. stricta are undoubtedly congeneric 

 and quite similar in nearly all other characters. We could consider 

 this feature, perhaps, a retention of a juvenile character, but, since it 

 occurs so prominently in B. deturkiana, a species that obviously is 

 speciahzed in other details, an equally good case could be made for 

 interpreting the bifid epicranial suture as some kind of specialized 

 development. 



Gonopods. — I have remarked previously (1960) the tendency among 

 rhysodesmines toward conservatism in gonopod form even though the 



