CHAPTER VIII 



The Cribellate Spiders 



T 



J. HE TRUE SPIDERS THAT POSSESS 



a flat spinning organ close in front of the anterior spinnerets are 

 called "cribellate spiders." This organ, which exists in addition to 

 the usual six spinnerets, is known as the "cribellum." It is the 

 homologue of the anterior median spinnerets, and has been retained 

 as a functional spinning organ, whereas in other true spiders it is 

 represented by an inconspicuous vestige. 



The cribellum (Text Fig. 4, C) may be likened to the fused spin- 

 ning fields of two spinnerets lying nearly flat against the surface of 

 the abdomen, all but the tips of the originally paired fingers having 

 disappeared. The dual character of the organ usually is evident on 

 close examination, which shows an actual division of the field by a 

 longitudinal line or ridge, or a pinching at the point of division. The 

 spinning field itself is covered by thousands of tiny spinning open- 

 ings, which give it a sievelike appearance under magnification, and 

 from which come exceedingly fine threads of viscid silk. The ordi- 

 nary silken threads of cribellate spiders are derived from glands 

 opening on spinnerets, as in other spiders. Whenever cribellar silk 

 is combined with the regular threads, the line becomes so character- 

 istic in color and physical appearance that it is called a "hackled 

 band." 



Invariably accompanying the cribellum is an accessory comb of 

 hairs called the "calamistrum." This is a line of curved setae, dif- 

 fering somewhat in appearance in the various families, and always 

 found upon the metatarsis of the hind legs (Text Fig. 4, E). The 

 use of the cribellum and calamistrum together as a spinning and 

 carding apparatus to produce the cribellate thread is essentially the 

 same among all the spiders of the group. Let us consider the method 

 of a typical hackled band weaver of the genus Amaurobius. 



The cribellum of Amaurobius is divided longitudinally by a 



