112 



AMERICAN SPIDERS AND THEIR SPINNINGWORK. 



Poison 

 Gland. 



The poison glands after the first moult are situated with Attus in the 



mandibles, and do not pass beyond those limits. Further on, as with 

 the Lycosid (Trochosa), the inferior end of the gland is placed 

 rather below the median line of the mandible. (Fig. 71, gl.v.) 

 Afterwards the superior parts of the gland issue beyond the 



mandible for a third of their length (Fig. 72), and their inferior extremi- 



ties do not reach the median line of the mandible. Finally, after a series 



of moults, the gland is no longer found within the mandible, but extends 



wholly beyond it (Fig. 73), and is withdrawn farther and farther with 



every moult. At the same time the dimensions of the gland increase as 



well as the number of muscular fibres ; a fact which gives us a measure 



of the phylogenetic development of any spider thus tested. 



With the genus Epeira the poison glands are relatively large. They are 



placed in the anterior part of the cephalothorax at some distance from the 



mandibles, with which they are joined 



by quite a long conduit. With the Clu- 



bionidae these glands are of a much 



smaller size than with Epeira, and only 



a part thereof is found within the ceph- 



alothorax, the remainder of the gland 



being situated within the basal joint of 



the mandible. With the Mygalida? the 



whole gland is located with- 



in the basal joint. It will 



thus be seen that the modi- 



fications in the poison gland 



accomplished by successive 



moults in advancing age 



are, first, the increase in 

 iyp-mrl f>r>Anrl 



size, ana, st ona, 



Of position 



Fin. 73. 



MOULTING OF POISON GLAND IN TROCHOSA. 

 rVmr>o-f> Flu - 71 - Venom gland, gl.v, at an early stage, inside the mandible ; 



cnange CTJ the duct FK . 72 G iand at iurther stage. FIG. 73. aiand 



after final moult, withdrawn beyond the mandible. 





A study of the rejected moult of a spider shows well the thickened 

 points of insertion of the muscles of the abdomen, which in that organ 

 are, characteristically, immediately upon the cuticle. Upon the 

 inferior or ventral part of the abdominal moult, according to 

 Wagner, there are two median rows of thickenings, consisting of 

 sixteen pairs, a little removed from one another (Fig. 74, Nos. 

 1-16) ; then two rows of lateral thickenings. (Fig. 74, Nos. 17-39.) These 

 rows consist of twenty-three pairs of thickenings disposed first (near to the 

 lungs) irregularly, and then in lines almost straight and parallel to the 

 two medians. In all, there are on the inferior face of the abdomen thirty- 

 nine pairs of thickenings, representing the points of insertion. Besides 



