In studying the various kinds of orb-webs one should pay particular attention to the center of the web; for this part differs greatly in the webs of the different species. There is usually a hub composed entirely of dry and inelastic silk woven in an irregular manner; outside of this there are several turns of a spiral thread which is also dry; this constitutes the notched zone, a name suggested by the fact that the spiral line is attached for a short space to each radius it crosses, thus giving the line a notched course. In many cases it is here, on the hub and the notched zone, that the spider waits for its prey; and it is obvious that sticky silk in this place would be objectionable. Between the notched zone and the spiral zone, the part furnished with the sticky spiral thread, there is a clear space, the free zone, crossed only by the radii. This gives the spider an opportunity to pass from one side of the web to the other without going around the entire web.
Fig. 92. Lace-like hub of an orb-web.
Some orb weavers do not wait upon the hub but have a retreat near one edge of the net, in which they hang back downwards. While resting in these retreats they keep hold of some of the lines leading from the net, so that they can instantly detect any jar caused by an entrapped insect.
When an insect in its flight touches one of the turns of the sticky line the line adheres to it, but it stretches so as to allow the insect to become entangled in other turns of the line. If it were not for this elasticity of the sticky line, most insects could readily tear themselves away before the spider had time to reach them.
In running over its net the spider steps upon the radii, carefully avoiding the sticky line; otherwise it would destroy its own net. The rapidity with which a spider can cross its net without touching the sticky line is remarkable.
In making its web an orb weaver first spins a number of lines extending irregularly in various directions about the place where its orb is to be; this is the outer supporting framework. Often the first line spun is a bridge between two quite distant points, as the branches of two separate bushes. How did the spider cross the gulf? It has no wings.
The bridge building can be easily seen on a warm summer evening, the time at which the spiders are most active repairing their old nets and building new ones. The spider lifts the hind end of its body and spins forth a thread; this is carried off by the wind, until, finally striking some object, it becomes fast to it. The spider then pulls in the slack line, like a sailor, and when the line is taut fastens it to the object on which it is standing, and the bridge is formed.
Fig. 93. Nearly completed orb-web.