258 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 1 37 



without the appearance of a sharply defined new spacing (except the 

 larger 33 A spacing treated below ) . 



But insertion of the arthropodin chains parallel to the (001) plane 

 (= parallel to the a axis) does not automatically account for diffuse- 

 ness of the a axis itself. Somehow spacings must be distorted in this 

 direction also. One of the striking features of arthropodin is its high 

 percentage of large side chains (phenyl, carboxyl, and heterocyclic 

 groups). Whereas silk fibroin is composed of about 90 percent simple 

 amino acids, arthropodin has only about 50 percent simple residues 

 (Duchateau and Florkin, 1954; Johnson et al., 1952). With so many 

 large side groups the arthropodin molecules must be very lumpy chains. 

 This composition is presumably important to cuticle development but 

 it would be expected to produce considerable irregularities in molec- 

 ular lattice spacings. If we assume that these side groups (like the 

 side groups of the chitin chains) project in the c direction, then dis- 

 tortion giving diffuseness along the a axis is understandable. 



Such orientation with resulting distortion in both a and c axes is 

 diagrammed in figure 2, C and D. For those unaccustomed to 

 plane diagrams a three-dimensional construction of the chain arrange- 

 ments and postulated distortions is presented in figure 3. 



The above is not the only way diffuseness could be produced in both 

 the a and c directions, but it seems the simplest postulate and adequate 

 for existing data.^ 



The preceding discussion omits consideration of the 33 A spacing 

 perpendicular to the cuticle surface. This is the spacing that increases 

 greatly when the cuticle swells on being rewetted with water. Al- 

 lowance will have to be made for variable spaces for water molecules 

 largely limited to this axis (the c axis). But even without allowing 

 for swelling by water it is not at all obvious what or how many units 

 need to be added to produce a 33 A spacing which includes the 

 19.25 A of the c axis of the chitin unit cell. 



Another enigma that likewise must await further study is what hap- 

 pens in cuticles where the chitin content is low or undetectable 

 (absent ?). 



Finally, demonstration that arthropodin chains extend perpendicu- 

 lar to chitin chains makes it seem less surprising that most workers 



^ For instance, one could postulate more complicated situations such as some 

 or even all of the arthropodin chains extending obliquely through the chitin 

 unit cell (e.g., in the (lii) plane), or one could think in terms of helical coils. 

 Another possibility suggested by the recent paper of Parker and Rudall (1957) 

 is that the arthropodin particles really represent folded protein chains. 



