112 



AMERICAN SPIDERS AND THEIR SPINNINGWORK. 



X. 



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

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

 . the Lj'cosid (Ti»chosa), the inferior end of the gland is placed 



Gland rii'ther 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 witliin 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 wholly beyond it (Fig. 73), and is withdrawn farther and farther with 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 phj'logenetic development of any spider thus tested. 



With the genus Epeira the j^oison 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- 

 bionidse 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 Mygalidse 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 

 size; and, second, the change 

 of position. 



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, characteristicallj^, 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 



Fl 



Fin. 72. 



Fir.. 73. 



Moulting of Poison Gland in Trochosa. 

 71. Venom gland, gl.v, at an earl.v stage, inside the mandible ; 

 cr, the duct. Fiii. 72. Gland at further stage. Fir,. 73. Gland 

 after final moult, withdrawn beyond the mandible. 



Abdomi 

 nal Mus 

 cles. 



