INSECT CUTICLE — RICHARDS AND PIPA 255 



when a sensitive Brace-Kohler compensator is being employed. In 

 fact, plate i, figures 1-4, were made from dry arthropodin immersed 

 in the oil-bromonaphthalene mixture of R. I. 1.555 ^o ehminate 

 refraction and hence give better photographs. 



When we found that the ampHtude of birefringence of dried arthro- 

 podin fibers was independent of the refractive index of the immersion 

 medium, the possibiHty existed that these molecules were intrinsically 

 birefringent. Alternatively they would have to be impermeable to the 

 medium since the working distinction between form and intrinsic bire- 

 fringence is the abolition of the former in media of the same refrac- 

 tive index as the substance. The birefringence of chitin is known to 

 be largely of the form variety (Richards, 1951). If the birefringence 

 of arthropodin is intrinsic, and if the arthropodin molecular chains 

 are at a right angle to those of chitin, then a reversal of sign of bire- 

 fringence should occur on successful imbibition of a normal tendon. 

 Tests showed no effect from immersion in the organic media custom- 

 arily used for these determinations even if the media were heated 

 and the specimens left immersed for several days. But these anhydrous 

 organic compounds require preceding dehydration either by drying or 

 by absolute ethanol or other agents. With dehydration the lattice ap- 

 parently tightens up so much that the media do not penetrate. Tearing 

 the tendons shows that the nonpenetration is not due to any surround- 

 ing membrane or sheath. 



We did not have any mercuric iodide available, but a saturated 

 aqueous solution of potassium iodide has a fairly high refractive index 

 (>i.5o). Immersion of fresh tendons in saturated aqueous potas- 

 sium iodide reduced the amplitude of birefringence by 60 to 70 per- 

 cent; immersion in pure glycerine (R. I. = 1.47) reduced the ampli- 

 tude about 50 percent. Drying this tendon with alcohol and passing 

 into xylol (R. I. = 1.50) returned the amplitude approximately to the 

 original value. Air-drying from xylol followed by immersion in sat. 

 aq. KI produced no noticeable change. It follows that it is only the 

 hydrated cuticle which is freely permeable. In contrast, tendons and 

 setae purified to chitin are freely permeable (Picken, 1949, and 

 others). Therefore, the impermeability of dry cuticle is due to arthro- 

 podin, and dry arthropodin is impermeable both by itself and when 

 associated with chitin in cuticle. 



Of more interest to the subject of the present paper, the large re- 

 duction in birefringence on immersion of hydrated tendons in sat. aq. 

 KI, and failure to obtain reversal of sign of the tendon, implies that 

 the birefringence of arthropodin is at least largely a form birefrin- 



