There is no definitive estimate for how many individual spiders are alive in the world today. The number of individual spiders in any one area depends heavily on the terrain and the types of spiders which live in it.
One study found that up to 840 spiders lived in one square meter of English pasture. This would equal about 3.5 million spiders per acre. The same study found an average of 140 spiders per square meter across a range of habitats, which is only about 570,000 spiders per acre.
Since the Earth is usually estimated to have about 37 billion acres of land, if this average is correct, it would mean there are around 21 quadrillion spiders alive on the Earth today.
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