1912] - Evolution of the Webs of Spiders 
Or 
viscid silk differs greatly in the different families producing it; 
and several types of it will be discussed before I close. 
Among the spiders that spin a simple web are the members 
of the Theridiide of which the common domestic spider, 
Theridion tepidariorum, is a familiar example. The webs of 
this spider abound in the corners of neglected rooms, and are 
nearly as simple as those of Pholcus. They probably represent 
a slight elaboration of the primitive type. 
In the family Theridiide is found the simplest form of viscid 
silk, this is merely a viscid liquid which is flung over the entan- 
gled prey. But although the silk itself is simple, correlated 
with its production there has been developed a specialized 
organ fitted for flinging the silk. This consists of a comb on 
the tarsus of the fourth legs. 
The presence of this comb is a distinctive family character- 
istic. And the silk is produced by a set of glands, the lobed 
silk-glands, which have been found only in this family. The 
Theridiidz, therefore, judged by their biological features and 
the correlated anatomical structures represent the tip of a 
distinct line of specialization. 
In the other families of spiders that make use of viscid silk, 
there is produced a thread or a band that is viscid, and which 
forms a part of the web. 
The webs in the construction of which two kinds of silk are 
used vary greatly in complexity of structure; and in different 
families the direction of specialization has been very different. 
Two quite different lines of specialization can be recognized. 
In one group of families the foundation of the web, the part 
made of dry silk, is comparatively generalized in structure, 
while there has been evolved a highly specialized band for 
supporting the viscid silk. In another group of families the 
foundation of the web is very regular in form, that is to say, 
highly specialized, while the structure of the viscid silk has 
remained comparatively simple. 
The first of these two lines of specialization, that in which 
the foundation of the web remains simple while. the viscid silk 
becomes complex is found in a group of families known as the 
Cribellatz. These are so called because they possess in addi- 
tion to the silk glands possessed by other spiders a large number 
of small silk glands which open through a sieve-like plate, the 
cribellum, situated in front of the spinnerets. These glands 
