814 



CLASS V. ARACHNIDA. 



or four stigmata, or by one median stigma, the result of the 

 fusion of two primitive stigmata, just in front of the spinnerets. 

 This tracheal system is usually regarded as a development of 

 two lung-books or as having arisen independently within the 

 group (see p. 776), and not as being genetically connected with 

 the tracheae of insects. The tracheae connected with one stigma 

 do not communicate with those connected with any other stigma, 

 and they do not ramify throughout the body. Each stigma gives 

 off a bunch of comparatively short tracheae which end blindly 

 The male and female generative orifices (Fig. 526) only 

 become visible after the last moult. As a rule they are difficult 

 to see, but in the females of some families, e.g. the Dysderidae 



FIG. 534. Male and female of 

 Liryplria, during copulation 

 (after O. Hermann). 



FIG. 535. Spinning organ 

 of Arnaurobius ferox (after 

 O. Hermann). Cr Cri- 

 bellum : Spw spinning 

 mammillae. 



.and Epeiridae the orifice opens on a complicated armature, the 

 " epigyne," which is of considerable systematic importance. 

 At the posterior end of the abdomen is a cluster of 

 spinnerets (Figs. 535, 537) which may be two or four in 

 number but are more usually six or eight, two anterior, two 

 median and two posterior, and close behind the last is the 

 anal tubercle bearing the anus and terminating the abdomen. 

 The spinnerets are probably highly modified segmental appen- 

 dages. They are very mobile and have at least two joints and 

 they are pierced at their somewhat flattened end by innumerable 

 minute pores through which the viscid fluid, which quickly hardens 

 in the air into silk, exudes. This fluid is secreted by various 

 glands and the excretions of the several glands differ in com- 

 position and functions. The pores open on the ends of minute 

 projections termed fusulae. In some spiders grouped by Simon 

 into the Oibellatae, there is, besides the spinnerets and in front 

 of them, a double plate pierced by pores called the cribellum 



