20 



SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL, I42 



entire. The six common types of ecdysial cleavage in the head cuticle 

 are shown diagrammatically on figure 8. 



The area between the arms of the cleavage line is sometimes rein- 

 forced by ridges that might easily be mistaken for the cleavage lines 

 when the latter are faintly marked. In a wood-boring buprestid beetle 

 larva, for example (fig. 7 I) an elaborate set of ridges {FR) in the 

 otherwise weak cuticle of the head braces the clypeus for support 



Fig. 8. — Diagrams of larval heads showing various positions of the ecdysial 

 splits along tlie arms of the cleavage line. 



of the mandibles. The true cleavage lines (CL) lie laterad of these 

 ridges. 



In a few insects, particularly in the Dermaptera and among the 

 Orthoptera, the cleavage line is retained on the head of the adult. 

 Usually it is a faint replica of the line on the nymphal head (fig. 6 C), 

 but in Forficula Strenger (1950) notes it forms an internal ridge, 

 which is particularly developed as a comb on the vertex. The reten- 

 tion of the cleavage line on the adult head might be explained as a 

 relict from times when the adult ancestors of the insects periodically 

 moulted and shed the cuticle, as do the adults of modern Thysanura 

 and most other arthropods. Adult moulting occurs now among the 

 winged insects only in the Ephemeroptera, and then but once at an 



