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I hate spiders but I know they are here to keep fly numbers down so I don't kill them if I can help it...
No tigerlily11, They are not here to keep fly numbers down. Spiders exist because they have been successful in perpetuating their genes. They have found a niche where they survive by consuming flies. If there was to be a boom in fly numbers, this would be no harm at all to spiders - in fact it might lead to an increase in spider numbers. There has therefore been no evolutionary pressure to evolve spiders that wipe out (or seriously reduce) fly populations.
In short, spiders are NOT there to keep down fly numbers. Simply, by being there, they do result in a reduction in the number of flies, but this is a result, not a purpose.
If you were to apply your logic to other species then you would be saying that malaria exists to keep down the number of humans.