CONSTRUCTION OF AN ORBWEB. 



73 



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

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

 the ordinary i)rinciples and a common mode of proceeding ; but in point 



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

 „ . , , 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 i)resence of an enemy, the 

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

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

 In short, her "2:»lan" is to some extent elastic, and variable by the stress of 

 accidents and circumstance^; 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. 



Fui. 68. Double foundation lines. 



Double 



Founda 



tions. 



within 



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 

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

 ally, but not always, triangular in arrangement, as illustrated in 

 Fig. 07. This form prevails in cases where the snare is hung 

 irge 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 Acrosoma 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 uj) by a sliort perpendicular line, id, and stayed 

 ui)oii a cross line (i), which again is supported by an ui>per straight cord 

 attached to the leaves of the gra|)evine in which the orl) hangs. 



