SPIDERS— WEB-BUILDING* 
209 
gous structures should never have been developed in the 
case of any other animal having predaceous habits — 
especially, perhaps, in that of the imago form of preda- 
ceous insects. It is easy to see how, if there were any 
original tendency to secrete a viscid substance in the 
neighbourhood of the anus, this might be utilised in de- 
scending from low elevations (as certain kinds of slugs use 
their viscid slime as threads whereby to let themselves 
down from low branches tc the ground); and so we can 
understand how natural selection might thus have the 
material supplied out of which to develop such highly 
specialised organs as the spinnerets of a spider. But if 
we are inclined to wonder why this should not have 
happened among other animals, we must remember that 
any expectation that it should rests on negative grounds ; 
we have no reason to suppose that in any other case the 
initial tendency to secrete a viscid substance was present. 
One inference, however, ip the case of spiders seems per- 
fectly valid. As this comparatively rare faculty of web- 
spinning occurs so generally throughout the class, it must 
have had its earliest origin very far back in the history of 
that class, though probably not so far back as to include 
the common progenitors of the spiders and the scorpions, 
seeing that the latter do not spin webs. 
I shall now give a few details on the manner in which 
spiders’ webs are made. Without going into the ana- 
tomy of the subject further than to observe that a 
spider’s c thread ’ is a composite structure made up of a 
number of finer threads, which leave their respective 
spinneret-lioles in an almost fluid condition, and immedi- 
ately harden by exposure to the air, I shall begin at once 
to describe the method of construction. 
The so-called 6 geometric spider ’ constructs her web 
by first laying down the radiating and unadhesive rays, 
and then, beginning from the centre, spins a spiral line of 
unadhesive web, like that of the rays which it intersects. 
This line, in being woven through the radii in a spiral 
from centre to circumference, serves as a scaffolding for the 
spider to walk over, and also keeps the rays properly 
stretched. She next spins another spiral line, but this 
