Branch Arthropoda 73 



pairs. Directly below the eyes is the pair of broad, 

 thick mandibles, or jaws. Find a sharp-pointed claw 

 on the tip of e$ich mandible. It is within these claws 

 that the poison glands are situated. Directly below the 

 mandibles is another pair of jaws, or maxillae. The 

 maxillae themselves are short and thick, but each pos- 

 sesses a long jointed appendage, or palpus, which looks 

 very much like a foot. See the mouth between the 

 pair of mandibles and the pair of maxillae. 



III. — Appendages of the Thorax. The thorax bears four 

 pairs of legs, a number characteristic of spiders. The 

 maxillary palpi just mentioned, though smaller than the 

 legs, are sometimes taken for a fifth pair of legs. See 

 that this point is clearly understood. Determine the 

 number of segments in the legs. Beginning with the 

 one next the body, they are coxa, trochanter, femur, 

 patella, tibia, metatarsus, and tarsus. The trochanter 

 and patella are generally very small. 



IV. — Appendages of the Abdomen. Near the posterior end 

 of the abdomen, notice a blunt bunch of finger-like pro- 

 jections, the spinnerets, usually six in number. To see 

 them well it will be necessary to use a low magnifying 

 power. The spinnerets are the organs that form the 

 spider's silken thread. In the end of each spinneret 

 there are a great number of small tubes. A pecuhar 

 fluid is forced out through these tubes which hardens as 

 soon as it strikes the air, thus forming the silk thread. 



Try to find a hard rough structure surrounding a 

 pore or spiracle just in front of the spinnerets. This 

 opening leads to a breathing apparatus. Two small 

 openings to another breathing system may be found on 

 the ventral side of the abdomen, near its union with 

 the thorax. 



