60 AMERICAN SPIDERS 



SPIDER THREAD IN OPTICAL INSTRUMENTS 



The use of spider silk for reticules in various optical instruments 

 is a direct consequence of the fineness of the fibers and of their 

 great strength and ability to withstand extremes of weather. Prior 

 to World War I, spider silk was very extensively used for cross 

 hairs and sighting marks in a great variety of engineering, labora- 

 tory, and fire-control instruments. For transits, levels, theodolites, 

 astronomical telescopes, and many other optical devices there is 

 nothing much superior to spider silk. Most people who use such 

 instruments are familiar with the fibers, and often replace them in 

 the field, using old spider silk or drawing a supply from living 

 spiders. 



Since World War I there has been a slackening in the use of 

 this material. The finest threads are useless for cross hairs because 

 of their fragility and the difficulty of installation. Because dragline 

 silk is most often used, the joined fibers must first be separated 

 so that the primary line will be a single uniform thread. This can 

 be easily done, since the two or four threads are discrete, and the 

 resultant single strand, averaging 1/20,000 of an inch in diameter, 

 is usable. Even finer fibers can sometimes be used. But the lines 

 spun by spiderlings and small spiders, as well as the finer fibers of 

 larger ones, are usually quite useless. 



The cocoon silk of the large Argiopes can often be employed 

 for telescopes. The floss beneath the tough outer covering is pulled 

 out easily, and single strands of considerable length procured. This 

 cocoon silk is spun from different glands and is not quite as strong 

 as the dragline silk, which is the most commonly used fiber. The 

 silks of many spiders are suitable for reticules. In Europe the 

 favorite species are large orb weavers such as Aranea diadema and 

 Zilla atrica. Many other spiders provide suitable silk, even those 

 belonging to quite different families. In the United States most silk 

 comes from the common house orb weavers, Aranea foliata and 

 dumetorum, from the numerous humped araneas, from the argio- 

 pids, particularly Argiope aurantia, and many others. The silk of 

 the black widow has also been used extensively. 



Silk is usually reeled from the spinnerets of living spiders and 

 placed upon suitable frames for storage. It is easy to secure and 

 retains its properties for many years. During World War II there 

 was an increased demand for spider fiber for laboratory and sur- 



