ANIMALS WITH JOINTED FEET-THE ARTHROPODS 



249 



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Fig. 11-49. The king crab {Limulus), or horseshoe 

 crab, is a cJistant relative of the spiders and scor- 

 pions and is not a crustacean at all. It is a 

 "living fossil" whose close relatives have all be- 

 come extinct many millions of years ago. 



Note the leaf-like flaps just back of the legs. 

 These are the book gills, so called because when 

 in use the flaps wave in the water like the pages 

 of a book. The snails {Crepidula) find it advan- 

 tageous to "hitch hike" on the crab, thus afford- 

 ing them a much more extensive feeding area 

 than they could ever attain under their own 

 power. When out of water note the track it makes 

 in the sand. While clumsy on land Limulus moves 

 effectively along the ocean floor where it shovels 

 in the mud searching for worms of various kinds 

 that make up its diet. 



more serious than a bee sting. There is one 

 species, however, which is very common 

 in the United States, particularly in Cali- 

 fornia, which can cause serious illness and 

 death in some cases. This is the black 

 widow, Lactrodectiis mactans (Fig. 11-47), 

 which is three-fourths of an inch long, and 

 glistening black. On the ventral side is a 

 bright red hour-glass shaped figure, which 

 is a positive means of identification. It lives 

 normally in piles of rocks, lumber, and more 

 recently around buildings, particularly ga- 

 rages. It has been known to cling to the 

 underside of automobiles, thus beine trans- 

 ported to all parts of the country. The bite 

 causes severe abdominal spasms and gen- 

 eral restlessness, and the mortality is about 

 5 per cent. An anti-venom has been de- 

 veloped which protects the victim from the 



more serious effects of the toxin. In spite of 

 the relatively small amount of harm done 

 by this one species of the group in our 

 country, spiders are generally set upon and 

 killed by the ordinary person. This is unfor- 

 tunate, for these friends, rather than ene- 

 mies, of man should be protected. 



The life history of the spider is rather 

 unique in some respects. The male is always 

 smaller than the female and in some cases, 

 such as the black widow, he is hardly rec- 

 ognizable because of his proportionately 

 minute size. He spins a web upon which he 

 deposits his sperms in a mass, which is then 

 picked up by his specially formed front ap- 

 pendages and carried while searching for 

 a female. Once he has found a mate, he 

 usually performs a rather weird kind of 

 dance and then deposits the sperm bundle 



