Why are insects getting bigger?

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http://homepage.mac.com/sonicboy/photos-au/DSC03145.jpg
http://files.myopera.com/Ravo/blog/giantspider.jpg
http://www.diacritic.org/blog/archives/images/spider_hand.jpg
http://media.urbandictionary.com/image/page/clockspider-1746.jpg
http://media.urbandictionary.com/image/page/clockspider-1747.jpg