Spider webs are one of the most fascinating examples of animal architecture. The most beautiful and structurally ordered are the round webs. The main function of the web is to stop and hold flying insects,long enough for the spider to catch them. In order to do so,the threads of the web have to stand the forces from large and heavy insects as well as environmental forces from wind and rain for at least a day in most cases. The round web is found to have two main characteristics. The first is its geometry,which consists of an outer frame and a central part from which threads radiate outward. Enclosed in the frame are capture spirals winding round and round from the web center out to the frame. The whole web is in tension and held in place by anchor threads,which connect the frame to the surrounding plants or objects. The second and perhaps most important characteristic is the material with which it is built. Spider silk is a kind of natural material that gives this lightweight fiber a strength comparable to that of steel, while at the same time making it very elastic . Two types of silk threads are used in the web. One is highly elastic and can stretch to almost twice its original length before breaking and, for most types of spiders, is covered in glue. This type is used in the capture spiral for catching and holding insects. The other is stiffer and stronger,and is used for the radius,frames and anchor threads,which allows the web to stand forces and to keep its structural strength through a wide range of environmental conditions. Which of the following is used for catching and holding insects?
The center of the web. The capture spirals. The anchor threads. The radius.