254 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 1 37 



One further fact can be determined from these cross sections. These 

 cockroach tendons are really greatly elongated apodemes, that is, hol- 

 low invaginations of the body wall. As such they have a distinct 

 lumen of elongated elliptical shape in cross section. Hence there is a 

 recognizable "surface." Rotation of the stage of the microscope re- 

 sults in brightening and darkening the cross sections, the greatest 

 brightness occurring when the long axis of the elliptical lumen is at a 

 45° angle to the Nicols (pi. i, fig. 7). It follows that, as one would 

 expect, the long axes of the arthropodin particles are probably parallel 

 to the cuticle surface. Therefore we can say that the arthropodin 

 particles most likely extend between chitin chains along the a axis of 

 the unit cell. Presumably herein lies the explanation of the cuticle 

 micelle having axis dimensions 6>a>c (Fraenkel and Rudall, 1940). 

 The great length of the h axis of the micelles is related to the length 

 of chitin chains, the length of the a axis to the length of arthropodin 

 chains, and the shortness of the c axis to the average number of aligned 

 chains. 



3. The imbibition of cuticle, arthropodin, and chitin. — In the course 

 of studies of chitin birefringence various authors have commented on 

 the desirability or even, in some cases, the necessity for purifying the 

 chitin, i.e., removing the other components, before success can be had 

 in obtaining an imbibition curve where amplitude of birefringence 

 is plotted against refractive index of the immersion medium (e.g.. 

 Castle, 1936; Lees and Picken, 1945; Picken and Lotmar, 1950). 

 We find this to be a property associated with the protein arthropodin. 



It is simple to measure the refractive index of air-dried fibers or 

 films of arthropodin by the Becke line technique, but we had no success 

 in attempting to determine the R. I. from the minimum of an imbibi- 

 tion curve. With the Becke line technique and a series of mineral oil 

 -bromonaphthalene mixtures, the R. I. was found to lie between 1.552 

 and 1.555 but to be definitely closer to the latter. This value was 

 not noticeably altered by oven-drying at 105° C. Immersion of dry 

 fibers in a series of C.P. fluids of known R. I. gave similar results. 

 Accordingly we can say that the R. 1. of dry arthropodin is close to 

 1.554. This value is close to the R. I. of both chitin and silk (Richards, 

 1951) but is considerably higher than the values recorded for waxes 

 of insect cuticles, for collagen, and for insect connective tissues such 

 as the basement membrane and neural lamella. 



While the fibers and films can be made virtually invisible (except 

 for their light absorption — -they are a light amber color) in media of 

 R, I. 1.55 to 1.56, the birefringence is not measurably altered even 



