The External Anatomy of Spiders 



The cribellum (cri-bel'lum). — In certain families there is, 

 in front of the spinnerets, an additional spinning organ, which on 

 account of its sieve-like appearance when slightly magnified, is 

 named the cribellum. The cribellum consists of a transverse 

 plate, which is usually divided by a delicate keel on the middle 

 line of the abdomen into two equal parts (Fig. 140 and Fig. 141). 

 On each of these areas, which are often a little convex, there are 



very many spinning tubes, the 

 number greatly exceeding the 

 number of spinning tubes borne 

 by the spinnerets. The number 

 of spinning tubes varies greatly, 

 however, in different species; 

 thus Bertkau ('82) found 500 in 

 the undivided cribellum of Dio- 



Fig. 142. THE TIP OF THE ABDOMEN ,. , , o • u u if 



of loxosceles c, coiuius hma a nd about 4800 in each half 



of the cribellum of Stegodyphus 

 lineatus, making 9600 in all. Or, to select observations based upon 

 genera represented in the United States, Bertkau found over 300 

 spinning tubes in each half of the cribellum of a Dictyna, and about 

 3600 in the undivided cribellum of an Uloborus. It is evident that 

 the threads issuing from the spinning tubes of the cribellum are ex- 

 ceedingly small. These threads are used, doubtless, in the forma- 

 tion of a part of the hackled band which is a characteristic feature 

 of the webs of spiders possessing a cribellum and a calamistrum. 



The Colulus (col'u-lus). — Immediately in front of spin- 

 nerets there is in certain spiders a slender or pointed appendage, 

 the colulus (Fig. 142, c). The function of this organ is unknown. 

 It is wanting in the tarantulas, the Drassidae, and those families 

 in which the cribellum exists. It has been suggested by Menge 

 that the colulus is the homologue of the cribellum. 



The Anus. — The posterior opening of the alimentary canal 

 is situated just behind the group of spinnerets, on the lower side 

 of the last segment of the postabdomen. 



The Stridulating Organs. — In some of the comb-footed 

 spiders, Theridiidae, the males possess a stridulating organ, con- 

 sisting of a scraper on the abdomen and a file on the thorax; 

 this organ is described in the account of that family. Stridulating 

 organs borne by the mouth-parts are described on page 2f 



136 



