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- 
The Plan .__.,. : : if - a 
Variable, it ities 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. 
Fic. 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 
lia is } ithin an outer foundation of strong cords usu- 
Founda. ime is hung wi an outer founda of strong cords vu 
qigne: ally, but not 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 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 up by a short perpendicular line, id, and stayed 
upon a cross line (i), which again is supported by an upper straight cord 
attachetl to the leaves of the grapevine in which the orb hangs. 
