482 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 31 



The labium in its simplest generalized form (fig. 22 A) is divided 

 by a transverse suture, or line of flexibility {Ihs) , into two principal 

 parts, one proximal, the other distal. The distal part has been 

 termed the eulabium {Elb) by Crampton (1928). In a functional 

 sense, at least, it is the " true " labium, i. e., the under lip of the 

 insect, though it is perhaps not the entire second maxillae as Cramp- 

 ton implied. The proximal, basal part of the labium may be called 

 the hasilahium (Bib). The basilabium is generally not a free part 

 of the labium, since it is usually implanted by most of its extent on 

 the posterior ventral wall of the head, anterior to the neck. Its 

 lateral basal angles («"'), however, are attached to the ventral 

 margin of the cranium (fig. 3 A) just behind the posterior tentorial 

 pits (pt) in line with the articulations of the maxillae {Mx) and the 

 mandibles {Md). The body of the eulabium (fig. 22 A, Elh) hangs 

 as a free flap from the distal edge of the basilabium. It bears all 

 the appendicular parts of the labium, including a pair of lateral 

 palpi (Pip), and four terminal lobes {Gl, ^9^)- The eulabium is 

 sometimes deeply divided between the median pair of terminal lobes 

 (fig. 23) , but the cleft never extends into the basilabium. 



The identities of the parts of the labium, as compared Avith the 

 parts of a pair of united maxillae, can be established only by a study 

 of the labial musculature. The principal internal muscles of the 

 labium (fig. 22 A) consist of levators and depressors of the palpi 

 ((9, Q), flexors of the terminal lobes {fgl, fpgl)-, and retractors of 

 the eulabium {rst). The muscles of the palpi and the terminal lobes 

 take their origins within the body of the eulabium ; the retractors 

 of the eulabium {rst) arise medially in the basilabium, and are 

 inserted on the proximal margin of the eulabium. 



A comparison of the labial musculature (fig. 22 A) with that 

 of a maxilla (B) shows at once that the body of the eulabium is 

 the stipital region {St) of the united second maxillae. The body 

 of the eulabium, therefore, is the pars stipitalis lahii, and is correctly 

 named stipites lahialis, or lahiostipites. Lateral lobes of the labial 

 stipites supporting the palpi (A, Pig) are equivalent to the palpifers 

 of the maxillae (B, Plf)^ but are sometimes distinguished as pal- 

 pigers {Pig). The terminal lobes of the labium are called glossae 

 (A, Gl) and paraglossae {Pgl) ; they clearly correspond with the 

 laciniae and galeae of a pair of maxillae (B, Zc, Ga) . Sometimes the 

 two labial lobes on each side are united, sometimes the two median 

 lobes are fused, or again all four may be unified in a single flap. 

 The terminal lobes collectively constitute the ligula (fig. 24, Lig). 



The morphology of the basilabium is much more difficult to deter- 

 mine than that of the eulabium. Some writers believe that the sus- 

 pensory plate of the eulabium is the sternum of the labial segment. 



