378 APPLIED BIOLOGY 



taceans and insects. When the animal is alive the " tail " 

 is carried over the back. The mouth is very small and the 

 scorpion must suck blood from its prey. All the body back 

 of the walking legs belongs to the ab- 

 domen. They do not spin webs, as 

 do spiders. 



Mites and ticks have the same ap- 

 pendages as have spiders. No seg- 

 ments can be seen. Many of them 

 are parasites on skin of domesticated 

 animals and sometimes on men. 



The daddy-long-legs, or harvestmen, 

 FIG. 129. Scorpion. Pair resemble long-legged spiders. They 

 ""S- " P^ectly harmless, and suck the 



has five segments in tail blood from Small insects. 



and a poison-sting at end The Ung-crob or horse-shoe crab 



Openings of four pairs of \ \ * j i xi. A AI A - 



lung-sacs shown on ante- (Limulus), found along the Atlantic 

 rior part of the abdomen. CO ast, is the last of an ancient race of 



(From Krngsley.) , A ,, ,. ., 



animals. Another species lives on the 



southern coast of Asia. The fossil Trilobites were related to 

 the king-crabs which lived in the past ages. Many points 

 in their structure show that the king-crabs are related to 

 scorpions. The spine of the king-crab has no sting. 



320. Economic Relations of the Arachnids. The spiders, 

 scorpions, and daddy-long-legs prey on insects, many of which 

 are injurious to useful plants or in other ways harmful 

 from the human point of view. The king-crabs are not 

 known to have any important harmful relations. Their 

 flesh is not inedible, as is commonly assumed because they are 

 related to spiders and scorpions; and in fact is said to be 

 used to some extent in making a " chowder " in some fishing 

 villages. 



Most important of the arachnids in their direct economical 

 relations are the mites and ticks which, as parasites on the 

 skin, irritate and suck blood and thus interfere with the health 



