SILK SPINNING AND HANDIWORK 59 



THE DRAGLINE 



No better illustration of the dependence of spiders on silk is 

 afFored than the habit of laying down a dragline or securing thread. 

 Wherever the spider goes, it always plays out behind from its spin- 

 nerets a silken line, which is anchored at intervals (by means of the 

 attachment disks) to the substratum, as the climber lets out a rope 

 when he enters the recesses of a deep cave or moves down the slope 

 of a precipitous mountain. The dragline is a constant companion 

 of spiders of all ages and all kinds, excepting a small group of 

 primitive forms of the family Liphistiidae. It is the fundamental 

 thread of most spinning. 



The sedentary orb weaver, committed largely to an aerial life 

 in the confines of its web, outlines the zones of its snare with this 

 thread. Long strands floated in the air form bridge lines from tree 

 to tree or across streams. On draglines, the spider balloons for 

 long distances. Great sheets and flakes of gossamer are mostly the 

 discarded draglines from many spiders. The orb weaver again, hid- 

 den in its leafy retreat, holds a trap line and uses it to detect the 

 presence of an insect in the web. 



The dragline is the lifeline of the spider. It is an aid in prevent- 

 ing falls from precipitous surfaces, and may also serve as a means 

 of escaping enemies. Web spiders often drop from their webs on 

 these lines and hide in the vegetation. Or they drop down and hang 

 suspended in midair until the danger is past, whereupon they climb 

 up hand over hand to their original position. The hunting spiders 

 jump headlong over cliffs or leap from the sides of buildings to 

 escape capture, and float down gently on their silken ropes. Most 

 of the spinning in our houses is dragline silk, which the house spiders 

 lay down in great profusion and which soon is transformed into 

 the familiar cobweb, heavy with air debris. Even the framework of 

 the retreats is put up with dragline silk, and on this base other types 

 of silk laid. 



Not a single filament, as the name implies, the dragline in its 

 simplest form is composed of two relatively large threads that ad- 

 here so closely together that only one line is apparent. On occa- 

 sion, the dragline may be made of four strands, or even of a great 

 many threads drawn from several spinnerets. 



