CONSTRUCTION OF AN ORBWEB. 



73 



It must be understood that I have only taken a case that may be con- 

 sidered fairly typical of the general plan of construction. It will explain 

 the ordinary principles and a common mode of proceeding ; but in point 



of fact the details continually vary, according to the local pecul- 

 v e . * n iarities of the orb site, the conditions of the wind, and I suppose 



to some extent the individuality of the spider. Some incident will 

 cause a variation; the advent of an insect, the presence of an enemy, the 

 neighborhood of another web, a falling leaf, a fright or excitement of any 

 sort may divert the spider's attention, and cause a variation in her plan. 

 In short, her " plan " is to some extent elastic, and variable by the stress of 

 accidents and circumstances; but as a general fact the exterior frame of the 

 orb will be found to be a more or less regular quadrilateral or a triangle. 



FIG. 68. Double foundation lines. 



The foundation lines most commonly observed take the form of a 



trapezoid, whose sides are directly anchored to surrounding objects. "Webs 



frequently occur, however, in which the orb with its trapezoidal 



p , frame is hung within an outer foundation of strong cords usu- 



tions. a ^7' Du t n t always, triangular in arrangement, as illustrated in 



Fig. 67. This form prevails in cases where the snare is hung 



within large open spaces, or stretched between two trees or bushes in 



woods. Some species appear to hang their snares quite habitually in this 



way, as with all the indigenous species of the genus Acrosoina whose 



industry is known to me. 



An example of this style of foundation is shown at Fig. 68, which is a 

 snare of Acrosoma rugosa. Here we have two strong cords, ab, cd, 

 united at J in a point, and joined at the base by ec, which was probably 

 the line used in securing the union of the two long cords at J. The 

 lower line, cd, is caught up by a short perpendicular line, id, and stayed 

 upon a cross line (i), which again is supported by an upper straight cord 

 attached to the leaves of the grapevine in which the orb hangs. 



