272 



COLLEGE ZOOLOGY 



Jumping spicier 

 (Sahicus) 



House spider 



(Theridion) 

 Figure 164. Some common spiders. 



Crab spider 

 (Misumena) 



which is attached to the web or to a plant, or 

 carried about by the female. The young leave 

 the cocoon after hatching, as soon as they 



Figure 165. The web of an orb-weaving spider. 

 (Photo by Spencer. Courtesy of Nature Magazine.) 



can run about. Several molts occur before 

 maturity. 



The spinning organs of spiders are three 

 pairs of appendages called spinnerets (Figs. 

 162 and 163); they are pierced by hundreds 

 of microscopic tubes through which a fluid, 

 secreted by a number of abdominal silk 

 glands, passes to the outside and hardens in 

 the air, forming a thread. These threads are 

 used to build snares, spin webs, and to 

 form cocoons. Some spiders, while hunting, 

 play out a dragline. Spiderlings disperse 

 themselves by spinning a long thread on 

 which they are carried away by the wind. 



Many spiders possess an accessory spin- 

 ning organ, the cribellum (Fig. 163) in ad- 

 dition to the spinnerets. A special kind of 

 silk is emitted from this organ and is combed 

 out by a row of bristles on the fourth 

 metatarsus. The silk threads of spiders are 

 stronger than steel threads of the same size. 

 They are no longer used as cross hairs in 

 the eye pieces of optical instruments; mod- 

 ern technology has produced better mater- 

 ials for the purpose. 



An orb web, such as is shown in Fig. 165, 

 is spun in the following manner. A thread 

 is stretched across the space selected for the 



