FIRST LESSONS IN ZOOLOGY 



the lines leading from the net, so that they can instantly 

 detect an)' jar caused b\' an entrapped insect." 



Find one of these orb-webs in good condition, i.e., not 

 torn and ragged, but new and complete. Examine it, 

 and note the regularity of its construction (fig. 162). Trace 

 the sta\'-lines to their attachments; note the shape of the 

 outer pol)'gon ; note the " spiral zone," i.e., that part of 

 the snare filled with lines laid down in apparently con- 



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Fig. 163. — Spider putting in foundation lines for an orb-web; the spider 

 shown at different positions in the work. The first (uppermost) line is 

 carried across by an air-current. (After McCook.) 



centric circles ; note that these are not separate circles, 

 but are spiral, and that the line composing it is contin- 

 uous; between the outer polygon and the spiral zone 

 there is a region crossed by the radii, but without other 

 lines, the "outer free zone"; betA\'een the spiral zone 

 and the center of the snare there is another zone free 

 from spiral or circular lines, or with these lines very far 

 apart, the " inner free zone " ; the central part or central 

 zone of the snare has a close spiral in it, a-nd here the 



