A SPIDER 19 



those of the male, and the two sexes may be distinguished in this 

 way. In the female the pedipalp looks exactly like a small leg ; 

 in the male the terminal portion is expanded and very complex 

 in structure, being used by the animal in the act of pairing. 



The third, fourth, fifth, and sixth pairs of appendages are the 

 legs ; they are used by the spider for a variety of purposes be- 

 sides walking. They are important as tactile organs, their great 

 length increasing their usefulness in this respect, and they un- 

 doubtedly compensate the animal in a certain degree for the lack 

 of antennas. The legs are also of use in spinning and manipulat- 

 ing the web, the complex structure of the claws being associated 

 with this function. 



The median plate between the maxillae on the ventral side of 

 the body is the labium ; the one between the bases of the legs is 

 the sternum. 



The Abdomen. The dorsal surface is usually marked by several 

 pairs of depressions, which mark the points of attachment of mus- 

 cles. At the hinder end, on the ventral surface, are three pairs of 

 spinnerets. Study these carefully with the aid of a hand lens. At 

 the end of each spinneret are numerous microscopic holes, from 

 which is exuded the semifluid silk. This is made up of many soft 

 strands, which harden as they unite to form the thread. 



A study of the embryology of the spider shows that the spin- 

 nerets are homologous to abdominal legs. 



Note the spiracles, the external openings of the respiratory 

 organs, the tracheae and the lungs. A short distance in front of 

 the spinnerets in the ventral surface of the abdomen is the single, 

 median, minute tracheal spiracle, which it is often difflcult to see. 

 The lung spiracles are a pair of large slits near the anterior end of 

 the abdomen, each one at the lateral end of a transverse fold of 

 the integument. Between them in the median line is the genital 

 pore. In the female spider it is covered by a large and complex 

 plate called the epigynum. 



Exercise 1. Cut off the legs on the right side of the body and draw 

 an outline of a side view of the spider on a scale of from 5 to 10, 

 putting in only the basal portion of the legs but all the pedipalps 

 and the mandibles. Carefully label all the parts observed. 



