CHAPTER II 



The Place of Spiders 

 in Nature 



RELATIONSHIP TO OTHER ARTHROPODS 



T 



J.HE VAST ASSEMBLAGE OF ANIMALS 



comprising the phylum Arthropoda includes such familiar creatures 

 as the crabs and lobsters, centipedes, millipedes, and insects, as well 

 as the spiders and their multitudinous kin. Indeed, three fourths of 

 the known animals of the world are arthropods and attest by their 

 numbers, their variety, and their occupancy of every conceivable 

 place in nature a degree of success not even closely approached by 

 any other group of animals. Present in numbers conservatively esti- 

 mated as beyond a million different species, they make up in vast 

 populations what they concede to the vertebrates in size. Most of 

 them are small, and because seven out of every ten kinds are insects, 

 the average size is perhaps as small as a quarter of an inch. Indeed, 

 it is perhaps to this small size, and to superior armament in the form 

 of a tough but light external covering, that they owe their domi- 

 nance in the world. 



The arthropods have their bodies encased in a stiffened outer 

 covering, or exoskeleton, and completely lack the type of internal 

 skeleton present in the vertebrates. The integument is made imper- 

 meable to liquids and gases and kept hard and tough by the presence 

 of amber-colored substances called sclerotin and chitin. Between 

 the body segments and the joints of the appendages the cuticle is 

 not so strongly impregnated with sclerotin and remains soft and 

 pliable, allowing movement of the legs and other articulated seg- 

 ments of the body. The problem of growth in size has been solved 

 in the arthropods by their shedding the rigid outer skeleton at 

 rather definite intervals, a process called molting. All the increase 



ii 



