what kind of spider has stripes on its legs and a silver back?

Answers:
That would be Argiope florida, the Silver Backed Argiope spider.

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Spider identification can be really tricky. There's a website called spiderbiz that may help you.

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Try this link it may help you identify your spider

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Many varieties of spiders are known to have this striping pattern on their legs. Several spiders come to mind that have a silver back. I have attached a page here with many different spiders on it. I think spider you are talking about is actually (Argiope trifasciata), they are a very common "garden spider" that are completely harmless to humans. They build beautifult webs which are surprisingly efficient at snaring mosquitoes, and other biting flies.
I hope this helps!

Main page (click on the images to enlarge and to get the name):
http://bugguide.net/index.php?q=search%26amp;k.

Argiope trifasciata:
http://bugguide.net/node/view/33307.