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Spiders fascinate me, but what intrigues me the most is just how big some spiders can get, which leads me to my question. What biological constraints (if any) would limit how large a spider could grow, given ample food and no predators?

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up vote 8 down vote accepted

The standard view on this is that the size of terrestrial arthropods like insects and spiders is limited by the atmospheric concentration of oxygen. This is because they rely upon diffusuion of oxygen into the 'blood' or haemolymph via a system of tubes called trachea that open on to the body surface at the spiracles. As the body grows larger the proportion that has to be taken up by the trachea gets larger. Consistent with this is the presence of fossil giant dragonflies, Meganeura dating from the Carboniferous era when oxygen levels were at an all time high. However you will find that if you go to the Wikipedia page for Meganeura that this idea has been disputed, so the answer may be that no-one really knows.

In relation to alternative explanations; a recent paper by Clapham & Karr (2012) compares fossil insect wing lengths and oxygen concentrations to study the historical drivers of evolution of body size. They conclude that oxygen seems to be the main driver until the end of the Jurassic, but after that biotic interactions (such as bird predation) could be more important.

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That is one big dragonfly! Beautiful looking creature. I wonder if there is any evidence of giant spiders during the Carboniferous. – user3795 Jul 1 '13 at 13:10
@Alanboyd We recently had a seminar on a relevant paper, which I added to your answer. Hope you are ok with this. – fileunderwater Jul 1 '13 at 14:06
@fileunderwater. Great! Thanks. – Alan Boyd Jul 1 '13 at 14:27
+1 and accept, thank you for the answer Alan Boyd and for the excellent addition, fileunderwater. I will ready that paper and post any questions that may arise from it. – user3795 Jul 2 '13 at 0:35

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