56 AMERICAN SPIDERS 



quartz fibers and far greater than steel, goes hand in hand with great 

 elasticity. The threads will stretch one fifth their length before they 

 break. 



The strength of the threads is to some extent dependent on the 

 manner in which the spider draws them out, greater speed increas- 

 ing it. When they are drawn speedily, the fibroin chains attain a 

 maximum orientation, which contributes greater strength to the 

 lines. The cocoon silk of the silkworm is essentially equal in strength 

 to that of the orb-weaving spider. However, spiders produce sev- 

 eral varieties of silk, and some differences are found among them 

 in strength and elasticity. The viscid line of the orb-weaver snare 

 is not very strong but extremely elastic; whereas the foundation 

 lines of these webs are of great strength, exceeding even that of 

 the cocoon silk. 



Most spider threads are not single fibers, although they may 

 appear so to the naked eye. The dragline thread readily separates 

 into two rods of equal thickness, but often elements from other 

 glands lie parallel to these elementary strands and mar the uniform- 

 ity. Under ordinary magnification, single fibers are rather uniform 

 rods, but when photographed by the electron microscope at 35,000 

 diameters even the finest threads are not completely uniform, and 

 show tiny enlargements and irregularities. Not much detail of the 

 internal structure of the silk can be seen even at this great magnifi- 

 cation. The finest single fibers attain a thinness of 0.03 micron, or 

 about one millionth of an inch, and are invisible to the naked eye. 

 Much thicker threads are relatively large, being o.i micron, or one 

 quarter-millionth of an inch in thickness. Many molecules are larger 

 than the width of these spider threads. It is possible that the spider 

 can draw out its filaments to a degree equal to the thickness of its 

 protein molecule, and that the finest threads represent a single chain 

 of molecules. 



THE SILK GLANDS 



The silk glands of spiders are secreting organs located within 

 the abdomen. Differing in size, form, and location, these organs are 

 classified largely on the basis of their physical characters. Thus, 

 the pyriform glands are pear-shaped, the aciniform are berry-shaped, 

 and the other kinds are similarly identified by their contour. At 

 least seven distinct kinds of glands are known to occur in the whole 

 group of spiders, but not all of them are found in any single family. 



