154 



Typically the 



infrequently it is horny, and occasionally it is segmented. The 

 generative organs and anterior pair of breathing-organs open 

 ventrally along a curved groove, the genital fold, which marks off 

 the fore part of the abdomen as the epigastric area. These 

 breathing-organs or pulmonary sacs are covered by plates or 

 opercula, and the integument in front of the genital aperture 

 is in many females horny and variously modified to form an organ, 

 the vulva or epiyyne. The breathing-organs of the posterior pair 

 are either operculate pulmonary sacs or tracheal tubes with simple- 

 apertures, which rarely open separately behind the genital fold, 

 but more commonly unite to form a single aperture situated just 

 in front of the spinning-mamillse. There are usually three 

 pairs of spinning-mamillcK attached to the hinder end of the 

 lower surface of the abdomen an anterior, a posterior, and an 

 intermediate pair. Each of the latter consists of a single segment, 

 the others of two or three segments. In one instance (Liphistius) 

 there is an additional intermediate pair. In other cases the rnamilla3 

 may be reduced to four or two pairs. In some species there is 

 a spinning-plate, the cribellum, situated in front of the anterior 

 mamillae. This organ occurs only in those spiders which possess 

 the calamisti-um (p. 153). In other species there is a mamilliform 

 process, the colulus, lodged between the anterior mamilla3, and 

 behind the posterior mamillae there is a small prominence, the 

 anal papilla. 



Sexual characters, etc. Males are almost invariably smaller, 

 sometimes very much smaller, than females, and may be always 

 recognized by the modification of the tarsus of the palp to act as 

 an intromittent organ. This organ appears only at maturity. 

 Similarly in the female the genital aperture is opened and the 

 vulva fully formed only at the last moult, when maturity is 

 reached. 



All Spiders are oviparous. The eggs are enveloped in a silky 

 cocoon, and are disposed of in various ways by the mother. 



Distribution. Spiders are cosmopolitan, occurring in all oceanic 

 islands, and as far north as Greenland and Spitsbergen. 



Measurements, etc. In the following descriptions the total length 

 is taken from the anterior end of the carapace to the posterior end 

 of the abdomen ; that of the legs from the base of the femur to 

 the tip of the claws. Normally the anterior two pairs of legs 

 project forwards, and the posterior two pairs backwards. Hence 

 the surface nearest the body is regarded as inner, and the opposite 

 surface as outer. Sometimes, however, the legs are considered as 

 extending at right angles to the long axis of the body. In this 

 case they have an anterior and a posterior side. In many species 

 the legs are normally depressed backwards in such a way that the 

 posterior surface rests upon the ground. These spiders are capable 

 of moving sideways, and are hence termed " laterigrade." 



