NO. 7 



THE INSECT CRANIUM SNODGRASS 



27 



commonly termed the "epicranial suture." The water beetle, Hydroph- 

 ilus, is sometimes cited as an example among the Coleoptera show- 

 ing the presence of a complete "epicranial suture" in the adult insect 

 (fig. 9 A) ; but an examination of the inner surface of the head (B) 

 reveals the fact that the supposed "frontal sutures" form a thick in- 

 ternal ridge {ER) broadly united with a strong midcranial ridge 

 (mcR) from the postocciput. The origin of the labral muscles 

 {Ibrmcl) above the transverse ridge demonstrates that the latter is the 

 epistomal ridge (ER) and that its external sulcus (A, es) delimits 

 the clypeus and not the frons. The true cleavage lines on the larval 

 head of Hydrophilus (fig. 81, CL) are quite different things. Simi- 



\A 



Fig. 9. — Hydrophilus triangularis Say, Hydrophilidae, head of adult. 



A, facial view of head. B, inner surface of facial wall of head with labral 

 muscles, showing a strong midcranial ridge (mcR) united with the epistomal 

 ridge (ER), forming the supposed "epicranial suture" on the outer surface (A). 



larly in other beetles the epistomal sulcus has often been mistaken 

 for the arms of the "epicranial suture," as in figures by Stickney 

 (1923), with the consequence that the clypeus is wrongly identified 

 as the "frons." 



In most of the Coleoptera, both larval and adult, the cibarium is a 

 preoral food pocket (fig. 13 A, Cb) as it is in Orthoptera and other 

 more generalized insects (fig. 2D). The hypopharynx, though always 

 united with the labium (fig. 13 A, F), retains its frontal muscles 

 (hphmcls). In the dytiscid larvae, however, the cibarium becomes 

 a closed chamber by the interlocking of the labrum with the labium, 

 and is drawn out between the mandibular bases into a long transverse 

 tube, the hypopharyngeal floor of which, or sitophore, takes the form 

 of a strongly sclerotized gutter. The cibarial dilator muscles (fig. 

 13 D, dlcb) are therefore distributed transversely on the distal part 



