THE LABIUM 



35 



of the parts involved during the later phases of development 

 remains to a large extent unexplored. Evidence afforded by the 

 musculature is unquestionably important, but, for reasons already 

 stated, too exclusive reliance upon this one criterion is undesirable. 

 It is more especially on this account that Borner's contention that 

 the cardo is homologous with the thoracic subcoxa remains an 

 open question. Furthermore, additional evidence is necessary, 

 perhaps more especially from the 

 developmental aspect, before 

 Crampton's view that the palpifer is 

 a reduced segment bearing the galea 

 as an endite can be regarded as 

 untenable. 



(h) The Labium (Second Maxillce). 

 Evidence, afforded by development 

 and by comparative morphology, 

 provides proof that the labium is 

 formed by the fusion of the second 

 maxillae. The correspondence of the 

 free distal parts of each half of the 

 labium with similar parts of the first 

 maxillse is undisputed. Consequently 

 it is agreed that the palpi are homo- 

 logous in the two cases, and that the 

 paraglossae and glossae are the respec- 

 tive counterparts of the galea and 

 lacinia of the first maxillae. In so 

 far as the basal labial sclerites are 

 concerned, the homologies are prob- 

 lematical, and it has been too 

 readily assumed that the mentum and submentum are formed 

 by the simple fusion of the stipites and cardines respectively. 



A comparative study of primitive types points to the conclusion 

 that the labium is composed primarily of a distal region or 

 prementum which is demarcated by means of the labial suture 

 from a proximal or basal region — the postmentum (Fig. 21). The 

 prementum may be defined as the region bearing the ligula and 



2—2 



pm< 



Fig. 20. hahium of Mastotermes. 

 pm, postmentum. For ex- 

 planation of other lettering, 

 see Fig. 21. 



