1912] Evolution of the Webs of Spiders 
CO 
A much more elaborate provision for capturing prey made 
by a burrowing spider by nee the lining of its tube is 
the purse-web of Atypus. 
Another remarkable device to aid in the capture of prey is 
made by one of the Dysderide, the species known as Ariadna 
bicolor. ‘This spider lives in a silken tube built in the hole that 
serves as a retreat for the spider. This tube is suspended from 
a frame-work of threads, built at the entrance of the retreat, in 
such a way that any disturbance of the exposed parts of the 
nest is communicated to the occupant of the tube. From the 
frame-work at the entrance of the retreat there extends a 
series of radiating lines each of which passes over two or more 
piers which keep it suspended a short distance from the face of 
the supporting object; so that any insect walking on this object 
is sure to disturb one of these lines. The touching of one of the 
trap lines by an insect results like the touching of the spring of 
a “ Jack-in-the-box.’”’ The spider comes forth with amazing 
swiftness, seizes the unlucky insect, and retreats with it in- 
stantly to its lair. 
Let us pass now from this glance at unusual devices for the 
capture of prey to a study of the more common forms of spider 
webs. 
How did the web-making habit arise? What were the steps 
by which the gap between the use of silk for the protection of 
eggs to its use in the construction of an elaborate web for the 
trapping of prey was bridged? With our present knowledge 
our answer can be only an hypothesis. 
The most important step I believe to have been the acquir- 
ing of the habit of spinning a drag-line, the thread which most 
spiders spin wherever they go. The first drag-line may have 
been a thread which a spider was using in the construction of 
an egg-sac and by which the spider found it could drop from an 
elevated position to a lower one; from this all of the well-known 
uses of the drag-line could be evolved. 
The complete history of the development of this activity is 
not so simple, however, as this statement might indicate. For 
there has been evolved a special kind of silk for use as a drag- 
line which differs in structure from that used in making egg-sacs 
and which is secreted by a distinct set of glands. But this is 
only one of several differentiations that have arisen; for now at 
