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EVOI.UnON OF ANIMALS 



Part V 



etatarsus 



k lungs 



rrow 



/ [ly JC tracheal spiracle 

 r- ^^spinnerets 



Fig. 30.30. External anatomy of a spider. (Courtesy, Gertsch: American Spiders. 

 New York, D. Van Nostrand Co., 1949.) 



constructing webs. Spines and other finer projections, many of them sensory, 

 project from the surfaces of the body. Spiders usually have eight simple eyes, 

 in some species fewer. The majority of spiders have poor eyesight, at its best 

 in the runners and jumpers. Smell and taste are also weak. They know their 

 environment through their extraordinary sensitiveness to touch and vibration. 

 Near the posterior end of the abdomen are two or three pairs of spinnerets 

 from the tips of which the silk glands open. Spinnerets are flexible fingers that 

 a spider continually extends, withdraws and manipulates as the slender streams 

 of silk pour from their tips. 



The respiratory system also opens on the ventral side of the abdomen in 

 front of the spinnerets. The openings of the two leaflike book lungs are located 

 one on either side of the opening of the reproductive organs. 



The short esophagus leads to the sucking stomach operated by powerful 

 muscles that attach it to the skeleton of the cephalothorax (Fig. 30.31 ). These 

 contract and enlarge the stomach thus creating the suction. It usually takes a 

 spider about an hour to suck in the juice of a fly. Digested food is absorbed 

 from a series of blind pouches extending from the stomach and from numerous 

 glandular extensions of the intestine that branch and rebranch through the 

 abdominal cavity. Waste substances accumulate in a pocket (stercoral) that 

 opens from the hindgut and are afterward discharged from the anus. 



The ovaries and the silk glands make great demands for food, the ovaries 

 to build up a store of yolk in the eggs, and the silk glands to provide the sub- 

 stance, mainly protein, in the constantly expended silk. Wherever a spider goes 

 it plays out a silken thread, the dragline. As a house spider drops from ceiling 

 to floor, it descends gently on a dragline making it longer and longer as it 

 drops. Before a spider jumps, it fastens a dragline down to some object and 



