256 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I37 



geiice. This is as it should be to obtain a measurable positive birefrin- 

 gence of flow. 



One seemingly anomalous result we are not prepared to explain is 

 that air-drying from water produces a decrease in amplitude of bire- 

 fringence of about 50 percent. The original value is regained on 

 rewetting with distilled water. 



DISCUSSION 



If an optical unit had been found in normal cuticle that was not 

 explainable as simply an arithmetic sum of the chitin and arthropodin 

 values, this would have been strong evidence for cuticle being a good 

 glycoprotein. Since to a first approximation the magnitude of cuticle 

 birefringence does equal the value of chitin minus the value of arthro- 

 podin, the optical data tell nothing about any possible glycoprotein 

 linkage. 



Nonetheless some form of chitin-protein linkage seems inescapable, 

 but it must be a very weak linkage. Fraenkel and Rudall (1947) ob- 

 tained good chitin crystallites after steam treatments, repeated wetting 

 and drying, or grinding cuticle to a powder in water. Various authors 

 have found microfibers by electron microscopy (pi. 2, figs. 3, 4) only 

 after treatments with acid, alkali, digestive enzymes, oxidizing agents, 

 or hot water, all of which disrupt or remove the arthropodin (see 

 Richards, 1958). Recently, Sanborn and Young (in press) stained 

 protein fibrils in cuticle sections but they, too, had to use a pretreat- 

 ment with butyl alcohol which is thought to disrupt glycoprotein bonds. 

 Using a different type of approach to the problem, Hackman (1955a) 

 showed that acetylglucosamine can be made to react with arthropodin, 

 and that some protein can be adsorbed from solution by purified 

 chitin. The ease with which the protein could be eluted from chitin 

 agreed with other data in implying a very weak bonding — less strong 

 than a hydrogen bond and hence likely some type of induced dipole 

 attraction. 



We cannot imagine the chitin and protein both being oriented in a 

 definite relation to one another unless some bonding is involved. We 

 have demonstrated such orientation by these birefringence studies. 

 Also, oriented proteins have already been reported in cuticles from 

 X-ray diffraction data but without demonstration of a constant rela- 

 tionship between the orientation of chitin and protein (Picken and 

 Lotmar, 1950). Although no one has yet positively identified a glyco- 

 protein bond in cuticle, we consider that documentation of a regular 



