The External Anatomy of Spiders 



The folium. — The dorsal aspect of the abdomen in many 

 spiders is marked by an area with scalloped margins (Fig. 128); 

 this area on account of its leaf-like outline is termed the folium 

 (fo'li-um). The folium is frequently marked with spots; and 

 its margin is often a double stripe, the inner part of which is 

 dark and the outer part light. 



The postabdomen. — At the caudal end of the abdomen there is a 

 small, conical or semicircular portion, which, when seen from above, 

 appears to consist of two or three segments; this is the greatly 

 reduced postabdomen. In the embryo, the postabdomen is much 

 more distinct, resembling in a striking degree the postabdomen 

 of scorpions (see Korschelt and Heider '99, Vol. 111., p. 50). 



The epigastrium. — On the ventral aspect of the abdomen, 

 the basal portion is usually more convex than the remainder of 

 this aspect of the abdomen and is sometimes more densely chiti- 

 nized; this area is termed the epigastrium (Fig. 129, e.). 



The epigastric furrow. — A furrow separating the epigastrium 

 from the more caudal portion of the abdomen is known as the 

 epigastric furrow (Fig. 129, ef.). In this furrow there is, on the 

 middle line of the body, the opening of the reproductive organs, 

 and at each end, the opening of a lung. 



The furrow of the posterior spiracle. — This is a transverse 

 furrow on the ventral aspect of the abdomen, a short distance 

 in front of the spinnerets, in which is situated the posterior 

 spiracle, in most of those spiders that have only three spiracles. 

 In a few cases, as in Anyphazna and Glenognatha, the third spiracle 

 is situated near the middle of the ventral aspect of the abdomen. 



The abdominal sclerites.- — Owing to the lack of segmentation 

 in this region of the body comparatively few sclerites can be recog- 

 nized on the abdomen, still in some spiders the abdomen bears 

 one or more distinct sclerites. There may be a dorsal sclerite, 

 one on the dorsal aspect of the abdomen; an epigastric sclerite, 

 one on the epigastrium; a ventral sclerite, one in the region behind 

 the epigastric furrow; and an infra-mammilliary sclerite, a semi- 

 circular one situated in front of the spinnerets, i. e., below the 

 spinnerets when the spider is hanging suspended by a thread. 



The spiracles. — The spiracles or openings of the respiratory 

 organs, are of two quite distinct kinds; one leading to lung-like 

 organs, the lung-slits, and one leading to tracheae of the ordinary 

 type, the tracheal spiracles. 



128 



