66 SPIDERS : THEIR STRUCTURE AND HABITS. 



allowing it to move inwards in a horizontal or inclined plane only. 

 AVhen not in use, the fang is folded upon a groove along the 

 inner edge of the basal joint, which is furnished sometimes with a 

 single, and sometimes with a double, row of teeth (Plate 7, 

 Fig. 8). 



Below the falces, and attached to the forepart of the breast- 

 plate, are the external organs of the mouth : these comprise a 

 pair of maxilte, or jaws, each bearing a long, five-jointed palpus, — 

 and an under and upper lip, the latter scarcely visible. The palpi 

 project from the jaws on either side of the fakes, and have each 

 five joints covered v/ith hairs and spines, and named respectively 

 the axillary joint, which is short ; the humeral, which is long ; the 

 cubital, short ; the radial, long in the female, but short in the 

 male ; and lastly the digital. In the female they strongly 

 resemble the legs, and taper towards the extremities, which are 

 armed with a single claw, toothed like a comb. In the male the 

 fifth, or digital, joint is much dilated, and has no claw, but 

 instead, a complicated set of soft membranous parts, ascertained 

 by the patient observations of Mr. Blackwall to constitute the 

 sexual organs. They are fully developed in the adult male only, 

 and afford the readiest means of distinguishing the sexes (Figs. 

 9, 10, 11). 



The legs are attached to the breast-plate, and consist each of 

 seven joints of very different lengths. The seventh joint, called 

 the tarsus, is terminated by two or more claws, usually curved and 

 toothed like a comb ; the number of these claws varying according 

 to the habits and requirements of the family. The absolute and 

 relative lengths of the legs also vary greatly, and afford useful 

 generic and specific characters. 



The abdomen is unsegmented, and is enveloped in a soft, 

 continuous skin, covered more or less densely with hairs. Its 

 upper surface, in the out-of-door species, is often variously 

 painted. At its extremity are the spinning-organs, consisting of 

 three pairs of mammulae, or spinnerets, in every British family 

 except one — that of the Ciniflo7iidcE^ — which has four pairs. They 

 are distinguished as the upper, lower, and intermediate pairs. 

 The upper pair have each two, and occasionally three, joints ; 

 the lower pair have two, and the intermediate pair but one joint. 

 Each of these spinnerets is furnished, at its extremity, or along 

 the under surface of the terminal joint, with fine moveable 

 papillae, or spinning-tubes, communicating by ducts with a series 

 of internal glands. These secrete a liquid gum, which on issuing 

 from the tubes hardens immediately by exi)osure to the air, and 

 forms numerous very delicate filaments. To produce the finest 

 possible lines, the spider employs the spinning-tubes separately ; 

 but if stouter lines are required she causes the tips of the tubes 



