42 SMITHSONIAN MISCELLANEOUS COLLECTIONS \0L. 8 1 



The points of origin of the lahral muscles serve to identify the frontal 

 sclerite, or the true frontal region when the frontal sutures are lack- 

 ing. Frequently there is only one pair of lahral muscles (fig. 50 E, 

 G), and when the labrum is immovable on the clypeus, both pairs 

 are lacking. The labro-frontal muscles are to be regarded as median 

 muscles of the prostomium. On the posterior surface of the labrum 

 there is often a median lobe, the cpipharynx (fig. 19 EpJiy), that fits 

 between the bases of the closed mandibles, and obstructs the entrance 

 to the mouth (Mth) when the labrum is closed upon the hypopharynx. 



THE STOM ODEUM 



In the embryonic development of arthropods, the endodermal part 

 of the alimentary canal, which becomes the true stomach, is formed 

 within the body and has at first no opening to the exterior. The an- 

 terior and the posterior ectodermal parts, or stomodeiim and procto- 

 deum, of the definitive alimentary tube are ingrowths of the ectoderm 

 at the two extremities of the blastopore. Their inner ends abut 

 against the ends of the endodermal sac, and their final union with 

 the latter takes place by an absorption of the adjacent walls. In some 

 insects the proctodeum does not open into the ventriculus until the 

 end of the larval stage. 



If the ontogenetic development of the alimentary canal is to be 

 translated literally into phylogenetic evolution, we should have to 

 believe that the arthropod stomach was once a closed sac, and that the 

 stomodeum and proctodeum are secondary means of communication 

 with it. But, if insects have had a continuous line of free-living 

 ancestors, this seems unlikely, and it is more probable that, in their 

 actual history, the stomodeum and the proctodeum have been formed 

 as open invagination of the primitive circumoral and circumanal 

 regions, and that the discontinuous development of the three parts of 

 the alimentary canal in ontogeny is an adaptation to embryonic or 

 larval conditions. 



It has been proposed by Janet (1899, 1911) that the stomodeum 

 consists of the walls of three primitive segments that once formed 

 the true anterior end of the body, but which have been inverted, as the 

 primitive mouth, now the orifice from the stomodeum into the stomach, 

 was retracted. This theory would give a plausible explanation of the 

 presence of the stomodeal ganglia, but it must assume that these 

 ganglia have been formed from paired ventral rudiments which have 

 migrated dorsally and fused on the upper surface of the stomodeum. 

 The known origin of these ganglia from the epithelium of the dorsal 



