THE INSECT HEAD — SNODGRASS 483 



The attachment of the basilabium on the lower margin of the cranium 

 by its basal angles in line with the cranial articulations of the 

 maxillae and mandibles (fig. 3 A), however, would suggest that the 

 basilabium contains the cardines of the second maxillae (fig. 22 B. 

 Gd). On the other hand, the origin of retractor muscles of the 

 labiostipites on the basilabium (figs. 22 A, 24, rst) makes it seem 

 unlikely that the labial base is formed of the cardines alone, since 

 there are never cardino-stipital muscles in the first maxillae ; but if 

 the basilabium contains a median sternal element (fig. 22 B, stn)^ 

 these muscles might be sterno-stipital muscles without objection. 

 In the Dermaptera (fig. 25 A) a pair of large ventral muscles of the 

 hypopharynx {inhv) also take their origin on the basilabium. 

 Furthermore, the opening of the salivary duct, which originates in 

 the embryo on the labial sternum between the bases of the second 

 maxillae, is situated in generalized adult insects anterior to the base 

 of the eulabium (fig. 4, SIO), and this fact suggests that the primi- 

 tive labial sternum has been constricted between the approximated 

 bases of the appendages while the salivary opening was crowded 

 forward between the latter. Finally, the basal part of the labium 

 is never divided medially, though the stipital region may be split 

 almost completely into its lateral components. 



A concrete example indicating the composite nature of the basi- 

 labium is seen in the labium of Machilis (fig. 23 A) : A pair of 

 faintly marked but distinct lines converge distally in the basilabium, 

 subdividing the latter according to the very pattern we might sup- 

 pose would result from the fusion of the cardines with a posterior 

 median part of the labial sternum (fig. 22 B). 



According to Holmgren (1909), the entire basal plate of the 

 labium in termites, which is greatly elongate in the soldier caste 

 (fig. 24 B, Bmt)^ is formed as a sclerotization of the neck membrane 

 behind the bases of the second maxillary appendages, which latter 

 move forward during the course of development. As to the fate of 

 the cardines, Holmgren is uncertain, though he suggests that they 

 perhaps are lost in the membrane between the eulabium and the basal 

 plate. In the termite soldier the posterior tentorial pits are stretched 

 out almost to the distal end of the basilabium, so that here the car- 

 dinal parts of the labium must in any case be very small, but this 

 condition does not pertain to the usual structure characteristic of 

 most insects. 



While there can be little doubt that the labium is fundamentally 

 a two-part structure, the sclerotization of its exposed posterior wall 

 is seldom so simple and has been the cause of much confusion in 

 labial nomenclature. The region of the basilabium may be entirely 

 membranous or wholly sclerotized; its sclerotization may take the 

 102992— S2 32 



