ARACHNIDA 129 



It is remarkable with what tenacity different groups of 

 animals cling to the " styles" laid down for them by nature. 

 There is no particular reason why an arthropod should 

 have three or four pairs of legs yet, there is no adult 

 insect which does not have three, and no arachnid without 

 four. Spiders all have spinning glands on the abdomen; 

 insects possess such organs only during larval stages. 

 Many insects have wings; no arachnids bear such append- 

 ages. The exact ancestry of spiders is somewhat un- 

 certain. The earliest spider-like animals known lived in the 

 shallow waters along the seashore. The fact that some 

 arachnids (ticks, mites) have larvae with three pairs of 

 legs, however, has led some men to believe that spiders 

 descended from insect stock. 



In making adjustments to their surroundings, spiders 

 display great acuteness in the development of their sense 

 organs. They can perceive objects at a distance and jump 

 at them with great speed and accuracy. They have many 

 structural adaptations which enable them to vary their 

 activities with changing conditions of life. But their 

 mental powers are not highly developed. Their usual 

 activities are simple movements or series of movements, 

 which are repeated over and over if a certain set of condi- 

 tions is repeated. Adjustments involving mental processes 

 are simple. Peckham tells of a spider in his garden which 

 dropped off its web when a tuning fork was sounded. It 

 fell to the ground five or six times each day and then re- 

 sponded no more to the sound. It kept this up for fifteen 

 days and then ceased to respond at all. Such behavior 

 shows some ability to learn and remember. The processes 

 involved are very simple, however, and do not allow us to 

 assume that the mental powers of the spider rank very 

 high among those of animals in general. 



There are a number of different kinds of spiders and 

 many differ greatly from the orb-weavers in their habits. 

 The following are among the most important families: 

 Thomisidce, crab-spiders, spin no webs, but lurk in flowers 



