How many spiders are in my house? Pest control companies will often give a figure of 40 spiders in an average house, but it's hard to find anything in the published literature that directly backs this up.
A 2016 study of 50 houses in North Carolina, US, found cobweb spiders in 65 per cent of all rooms sampled, but this was looking at biodiversity, and didn’t count the number of individual spiders.
If a typical house has 5 or 6 rooms, plus hallways, the average figure would be around 10 spiders per room, which seems like a lot.
To add some research of my own, I’ve just hoovered my entire house (a rare event) and only found 26 spiders. But that’s just the parts of the house that we can easily scrutinise.
If you consider under the floorboards, behind the fridge, the loft, the garage and so on, there could easily be that many more still hiding.
If we consider the average density of spiders outdoors, a widely cited figure from Nyffeler and Birkhoffer’s 2017 paper is 131 spiders per square metre (11 square feet) of land. That’s of the order of 10 quadrillion spiders worldwide, or about a million spiders for every human.
A typical house actually has relatively little spider food available. But it also has almost none of a spider’s main predators, such as centipedes and birds, so each spider may live longer indoors than out.
Spiders also lay anything from 10–250 eggs at a time, and house spiders may lay as many as five clutches per year.
Less than one per cent will reach adulthood, but even so, if we count baby spiders in our household population census, the figure may be as many as a couple of thousand spiders at any one time.
This article is an answer to the question (asked by Marie Chelmsford, via email), 'How many spiders are in my house?'
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