86 



ZOOLOGY 



commonly known as trap-door spiders. The lid of the 

 nest is covered with hardened dirt, and when closed, looks 



exactly like the ground around 



FG it. Some of these spiders gain 



a great size and capture birds 

 (Fig. 82). 



2. Orb-weavers (Orbite- 

 lariee 1 ). These, which are 

 typically represented by 

 Argiope, are well known ; yet 

 few persons have studied the 

 almost mathematical precision 

 with which the webs are built. 

 Foundation lines (Fig. 83) 

 of unusual strength are first 

 laid down, to form the periph- 

 ery or frame of the web. Then 

 radii are spun from a central 

 little ball of floss to the frame. The radii are, often at 

 least, laid down alternately on opposite sides of the 

 centre. The number of radii formed by a species of 

 spider is not perfectly constant, but varies within limits. 

 It would be an interesting occupation to sketch a number 

 of webs of Argiope showing the variations in the number 

 of radii and the other details of form. After the radii are 

 placed, the spiral lines are laid down. In the completed 

 web four regions are distinguishable, as follows, passing 

 from the centre outward : (1) the notched zone, consist- 

 ing of four to eight turns of a spiral at the centre ; (2) 

 the free zone in which no spiral is laid down ; (3) the 

 spiral space, the main part of the spiral framework ; (4) the 



1 orbis, circle ; tela, web. 



rs 



FIG. 83. Diagram on nomencla- 

 ture of parts of an orb-web. FS, 

 foundation space ; SS, spiral 

 space; CS, central space ; FZ, 

 free zone ; NZ, notched zone ; H, 

 centre. From McCook. 



