200 ARACHNIDS, CENTIPEDES, AND MILLIPEDES— ARTHROPODS 



ing the web to the ceiling and spinning web out as it comes down. It 

 can also wind this web in and go back up. It can move from one tree 

 top to another by spinning a little tuft of sticky web and throwing it 

 out in the breeze while spinning a single strand attached to it like a 

 boy flying a kite. When the tuft hits a solid object and sticks, the spider 

 has a cable on which to cross. This accounts for the many single 

 strands of web which a person contacts, usually across the face, on a 

 walk through the woods. 



Courtesy General Biological Supply House 



Fig. 14.1. Trade-mark of the black widow. The red or orange mark in the shape of 



an hourglass on the ventral surface of the abdomen of the female black widow is a 



reliable means of identification of this, our most dangerous spider. 



Although a housewife that has to clean cobwebs from the ceilings 

 may not think so, spider web is of value to man as well as to the spider. 

 It is an ideal type of thread to place in optical instruments to form what 

 are called "cross hairs." These enable a person using a gun sight or 

 similar instrument to center the target. Two strands of web are placed 

 in the instrument so that they cross at right angles in the exact center. 

 Spider web is ideal because it is very slender, yet possesses strength 

 greater than steel ; a steel thread the same diameter as a strand of spider 



