The Internal Anatomy of Spiders 



ducing organs have arisen independently in different races of 

 animals. 



The silk organs of spiders are the most complicated silk 

 organs known. This condition might be expected from the fact 

 that a single species of spider spins several distinct kinds of silk; 

 thus, for example, an orb-weaving spider spins five different 

 kinds of silk. 



A detailed account of the different kinds of silk spun by 

 spiders is given in the following chapter; and in the preceding 

 chapter the external spinning organs are described. In this we 

 have to consider the internal silk organs, the glands that produce 

 the silk. 



As spiders spin several kinds of silk it is obvious that there 

 are several kinds of silk glands differing in function. Seven 

 different kinds of silk glands have 

 been recognized. These differ in 

 form, in number, in colour, in the 

 structure of their ducts, and in 

 the nature of their products. 



No spider has been found to 

 possess all of the seven kinds of 

 silk glands; but three of the kinds 

 have been found in all species 

 studied from this point of view; 

 and a fourth is wanting in only 

 two families. The three other kinds are each characteristic of 

 a particular group of spiders, and no two of them are found to- 

 gether. Each of the three groups of spiders that possesses a 

 characteristic kind possesses also the first four kinds. Hence 

 the presence of five kinds in a single spider is common. 



Much has been written on this subject; but the paper that 

 has been of the most service in the preparation of this account is 

 one by Apstein ('89). I have followed Apstein quite closely 

 in my account of the structure of the glands; but in several cases 

 my own observations do not confirm his conclusions regarding 

 their function. 



The different kinds of silk are spun from different types of 

 spinning tubes (Fig. 172), which are described on page 135. 



The aciniform or berry-shaped glands. — There are four clusters 

 of these glands, one for each of the hind and middle spinnerets 



SPINNING TUBES 



171 



