456 



ANNUAL EEPORT SMITHSONIAN INSTITUTION, 1931 



OSc 



it as a possibility. The paragnatha are said to be innervated by 

 branches of the mandibular nerves. 



If the superlinguae are true appendages, the gnathal region of 

 the insect head must contain four segments instead of three, as it 

 does in some Crustacea. Then, if the paragnatha are not true 

 appendages, the insect superlinguae must represent the first maxillae 

 (maxillulae) of Crustacea, the maxillae the second maxillae (maxil- 

 lae), and the labial ap- 

 pendages the first pair of 

 crustacean maxillipeds. 



The dispute as to the 

 nature of the superlinguae, 

 the number of segments in 

 the gnathal region of the 

 insect head, and the ho- 

 mology between the mouth 

 parts of insects and crus- 

 taceans is now over 30 

 years of age and appears 

 to be destined to a long 

 life. The best advice that 

 can be offered to the stu- 

 dent is that there is much 

 to be said on both sides, 

 which has already been 

 said, and that some con- 

 clusive evidence would be 

 more convincing. The 

 subject of the segmenta- 

 tion of the arthropod 

 head and the homologies 

 of the head appendages 

 between insects and crus- 

 taceans has been reviewed 

 recently by Denis (1928), Henriksen (1929), Imms (1931), and 

 Tuxen (1931). 



III. STRUCTURE OF THE MATURE HEAD CAPSULE 



Wliatever may be the exact facts concerning the evolutionary 

 history of the cephalic region of insects, the component elements of 

 the mature cranial capsule are so closely consolidated in modern 

 insects that entomologists can find little evidence of the lines that 

 formerly demarked the constituent parts or segments. Only one 

 constant suture of the insect head appears to have an intersegmental 



FiGURio 8.- — Diagram of the principal sutures and 

 areas of an insect's cranium 



a', posterior articulation of mandible ; a", articu- 

 lation of maxilla ; a"', articulation of labium ; 

 Ants, antennal socket ; as, antennal suture ; 

 A8c, antennal sclerite ; at, anterior tentorial 

 pit ; c, anterior articulation of mandible ; Clp, 

 clypcus ; cs, coronal suture ; E, compound eye ; 

 es, epistomal suture ; Fr, frons ; fs, frontal 

 suture ; fts, hypostomal suture ; list, hypostoma ; 

 Lm, labrum ; Oo, occipital arch ; occ, occipital 

 condyle ; ocs, occipital suture ; os, ocular suture ; 

 OSc, ocular sclerite ; Plst, pleurostoma ; Poc, 

 postocciput ; pos, postoccipltal suture ; Prtl, 

 parietal ; ps, pleurostomal suture ; pt, posterior 

 tentorial pit ; sgs, subgeual suture. 



