484 



ANNUAL. REPORT SMITHSONIAN INSTITUTION, 1931 



form of one, two, or even three distinct plates. In the stipital 

 region there may be two separate lateral sclerites, though generally 

 most of the stipital area is covered by a single plate. 



When the basilabium contains two distinct plates (fig. 23 D), the 

 distal one is generally termed the mentum (Mt), and the proximal 



Bib 



Gl Pg-l 



FionEB 23. — Various types of labial structure 

 A, Macliilis, showing faint subdivision of basilabium (Bib) into lateral and 

 median areas (Cd, atn). B, soldier of Termopsis, with a single large 

 plate, the postmentum (Bmt). in the basilabial region. C, Blatta oricn- 

 talis, with reduced mental sclerites (Mt). D, Scudderia, a t.vpical 3-part 

 labium. Lettering as on Figure 24. 



one the submentum {Smt) . The stipital sclerotization is then called 

 the prementum {Pmt). Some entomologists, however, especially 

 those studying insects having a single basal plate, give the name 

 " mentum " to the stipital plate, and term the basal plate the " sub- 

 mentum." This system, as Kadic (1902) and Walker (1931, 1931a) 

 have shown, is the most logical nomenclature that can be applied 

 to the labium, since it is based on the primitive structure retained 



