They appear in chapter 8 and immediately thereafter – and in the Chapter 3 description of Bilbo’s handwriting.  The word comes from the Old English word meaning “to spin”.

• 03.041 and spidery.
• 08.041 All the time he was wondering whether there were spiders
• 08.042 and then he found spiders all right.
• 08.075 Then the great spider,
• 08.075 Then the spider jumped back,
• 08.075 The spider evidently was not used to things
• 08.075 as small spiders do to flies –
• 08.076 The spider lay dead beside him,
• 08.076 Somehow the killing of the giant spider,
• 08.080 That is why the spiders neither saw nor heard him coming.
• 08.081 that there were spiders huge
• 08.088 With that one of the fat spiders ran along a rope
• 08.089 To the fattest of these bundles the spider went –
• 08.089 and kicked the spider straight
• 08.089 and the enraged spider fell off the branch,
• 08.091 hissed the angry spider climbing back onto the branch.
• 08.092 the spider had reached Bombur,
• 08.092 The stone struck the spider plunk on the head,
• 08.093 and taking off the spider sitting
• 08.094 The idea came to him to lead the furious spiders
• 08.096 Old fat spider spinning
• 08.096 Old fat spider can’t see me!
• 08.098 no spider has ever liked being called Attercop,
• 08.098 Practically all the spiders
• 08.099 but several of the spiders had run now to different points
• 08.099 that at least was the spiders’ idea.
• 08.103 The spiders saw the sword,
• 08.104 if a spider had not luckily left a rope hanging down;
• 08.104 only to meet an old slow wicked fat-bodied spider
• 08.104 and before the spider knew what was happening it felt his sting
• 08.104 before the spiders were disgusted
• 08.108 when the spiders began to come back,
• 08.111 The spiders had caught them pretty easily the night before,
• 08.112 that some of the spiders had gathered
• 08.112 and slashed at the spiders
• 08.113 For he saw spiders swarming up all the neighbouring trees,
• 08.114 and hundreds of angry spiders were goggling at them all round
• 08.115 and again the spiders were beaten off,
• 08.115 Already the spiders were beginning
• 08.117 I shall draw the spiders off,
• 08.118 the spiders were drawing their circle ever closer.
• 08.119 That upset the spiders greatly.
• 08.119 they drove at the spiders on the left,
• 08.120 though many of the spiders were close behind.
• 08.120 and already some spiders were
• 08.121 and charged into the astonished spiders
• 08.123 The spiders swelled with rage,
• 08.123 the spiders suddenly gave it up,
• 08.124 which the spiders did not like.
• 08.130 their cries as the spiders caught them
• 08.144 The giant spiders were the only living things
• 09.001 The day after the battle with the spiders
• 09.008 to be trapped by spiders?
• 09.008 Are the spiders your tame beasts or your pets,
• 09.009 and rouse the spiders with your riot
• 09.015 from the spiders,

“spider, n.” OED Online. Oxford University Press, March 2015. Web. 11 May 2015.