10 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I42 



by entomologists. Chaudonneret (1956), however, has revived this 

 segment and its supposed homologue, the "paragnathal segment" in 

 Crustacea. Accepting it as a real segment, he points out, explains 

 the appendagelike nature of the paragnaths and superlinguae ; but he 

 admits the idea is only a hypothesis. 



In the mature insect head (fig. 3 B) the cephalic components have 

 been so completely united that, with the possible exception of a groove 

 {pos) around the occipital foramen, no trace is left of the interseg- 

 mental lines. It is suggested by Strenger (1942) that the oblitera- 

 tion of the segmental limits is an adaptation to the need of a uniform 

 cranial surface for muscle attachments, which have spread from one 

 segmental area to another. The secondary development of ridge- 

 forming grooves in the head cuticle is a device for strengthening the 

 cranial walls. 



That the groove around the occipital foramen, known as the postoc- 

 cipital sulcus (figs. 3, pos), is a true intersegmental line is indicated 

 by several structural features. First, it sets off behind it a narrow post- 

 occipital flange on which the membranous neck is attached. Second, 

 it forms a strong internal ridge that gives attachment to the muscles 

 from the thorax that move the head, and this ridge appears to cor- 

 respond with the intersegmental ridges of the segmental body plates 

 on which are attached the intersegmental dorsal muscles of the trunk. 

 Third, in a head of generalized type of structure, the maxillae are 

 attached on the lower cranial margins before the postoccipital sulcus 

 (fig. 3 A, iMx), and the labium (Lb) is suspended from the postoc- 

 cipital flange behind the sulcus. The postoccipital sulcus, therefore, 

 appears to be the persisting intersegmental groove between the maxil- 

 lary and labial segments of the head. The labial segment in Symphyla, 

 according to Tiegs (1940), is the last body segment to be added to 

 the head in embryonic development, and the groove before it is the 

 only intersegmental line that remains on the adult head. The postoc- 

 cipital sulcus is well said by Strenger (1952) to owe its origin to the 

 union of segments, its retention in the adult to its functional im- 

 portance. 



Chaudonneret (1950), in his study of Thermobia, admits that the 

 lower lateral parts of the postoccipital sulcus mark the intersegmen- 

 tal line between the maxillary and labial segments. The dorsal part, 

 however, he contends must be the line between the labial segment and 

 the prothorax, because the prothoracic muscles are attached on its 

 internal ridge. This interpretation creates a rather complicated situa- 

 tion, but otherwise it must be assumed that the intersegmental groove 

 between the labial segment and the prothorax has been lost some- 



