INSECT CUTICLE — RICHARDS AND PIPA 25I 



various refractive indices produced no noticeable change in the ampli- 

 tude of birefringence. This was somewhat surprising since the demon- 

 stration of streaming birefringence implies that at least a sizable por- 

 tion of the birefringence is form birefringence (i.e., birefringence 

 due to the shape of the molecules). These data show that the arthro- 

 podin molecules either exhibit largely intrinsic birefringence (i.e., 

 birefringence due to the internal structure of the molecules) or, more 

 likely, that the packing of arthropodin molecules in air-dried sheets 

 and fibers is so tight that the organic immersion media could not 

 penetrate between the molecules. For reasons to be detailed later, we 

 conclude that anhydrous organic molecules of moderate size do not 

 penetrate into dry arthropodin. Similarly, saturated aqueous solu- 

 tions of potassium iodide (in which arthropodin is insoluble) have no 

 effect and hence, presumably, do not penetrate. 



2, The optical properties of cuticle. — It has been known for many 

 years that the birefringence of any cuticular structure (tendons or 

 setae are usually studied) increases on purification of the chitin in the 

 structure. The initial objective of the present study was to see whether 

 or not the birefringence of normal cuticle is a simple arithmetric sum 

 of the birefringence of its chitin and arthropodin components. 



Determination of the magnitude of birefringence shows that chitin 

 is some 6 to 7 times as strongly birefringent as arthropodin. With 

 specimens immersed in ethanol, and 9 separate determinations of the 

 magnitude of birefringence of chitin from purified leg tendons of a 

 tarantula, 7 determinations from normal leg tendons of cockroaches 

 (Blaberus), and 6 determinations from thin sheets of arthropodin, we 

 obtained the following average values : 



Chitin 0.00084 



Cuticle 0.00070 



Arthropodin 0.00013 



One may object to the above materials coming from three widely 

 different arthropods. But there is no reason to think that the extra 

 labor involved in obtaining all these from one source would change the 

 values significantly. The highly purified chitin of tarantula tendons 

 was used because it was already prepared and of tested purity. A 

 large volume of literature attests the uniformity of chitin within the 

 phylum Arthropoda (Richards, 1951 ; Rudall, 1955). Arthropodin 

 is somewhat heterogeneous even in the extract from a single species 

 as shown by Hackman's (1953) separations and by the somewhat 

 indefinite angle of isocline we found in our flow birefringence deter- 

 mination. But it is doubtful that these known differences would have 



