INSECT CUTICLE — RICHARDS AND PIPA 257 



crossed-grid structure necessitates postulating the existence of such a 

 bond, and hence of a chitin-protein entity during cuticle development.^ 



Fraenkel and Rudall (1947) have listed the X-ray diffraction char- 

 acteristics of normal dry cuticle. These include sharpness of the b 

 reflections, diffuseness of both the a and c reflections, and the presence 

 of a 33 A spacing perpendicular to the cuticle surface, the latter being 

 increased by 50 percent or more on wetting with water. The a and b 

 axes are parallel to the cuticle surface, and the side chains of the 

 chitin molecules are perpendicular to the cuticle surface. On a basis 

 of similarity of the X-ray diffraction repeat distance in extracted ar- 

 thropodin with the length of a chitobiose unit, they suggested a parallel 

 arrangement of chitin and arthropodin chains similar to that dia- 

 grammed in figure 2 A. Subsequently, in a symposium paper that is 

 probably unknown to most entomologists, Rudall (1950) did point 

 out that the variability in side chain spacings of the protein chains 

 makes it conceivable that the protein might form a crossed-grid ar- 

 rangement with the chitin chains. Since we have now demonstrated 

 the existence of some kind of a crossed system it seems likely that 

 chitin and arthropodin chains in cuticle are normally each weakly 

 cross-linked by the other. In soft cuticle the protein is readily dis- 

 placed or removed. On hardening of the cuticle by sclerotization there 

 is an addition of other compounds, dehydration, desalting (Richards, 

 1956), and stabilization of the protein (Richards, 1958; Wiggles- 

 worth, 1957). We have no idea yet whether this stabilization involves 

 linkages to chitin chains as well as to the arthropodin chains, but, in 

 any case, a stabilized crossed-fiber grid will be produced. Such grids 

 break but do not tear readily (sclerotized setae break irregularly but, 

 after removal of the protein, split [Lees and Picken, 1945]). 



The details of how the crossed arthropodin and chitin chains are to 

 be fitted together remains to be determined. Clearly the chitin chains 

 extend in the b direction and the arthropodin chains in the a direction. 

 But insertion of the arthropodin chains does not produce any regular 

 displacement of chitin chains ; it makes the c reflections diffuse rather 

 than larger. Otherwise stated, the arthropodin chains must modify the 

 chitin lattice only at some points, not at all points. There results, then, 

 a mixture of chitin spacings and variously modified chitin spacings 



5 Since the above was written, Foster and Hackman (i957) have published a 

 note reporting a strong co-valent bond between chitin and protein in the crab, 

 Cancer pagurus. Interesting and important as this is, it can represent only a part 

 of the full story because their glycoprotein contained less than 5 percent protein 

 whereas insect cuticles usually contain i to 2 times as much protein as chitin 

 (Richards, 1951). 



