Giant spiders: are they getting bigger and bigger?

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John Keats may have described fall as the “season of mists and gentle fertility,” but for many arachnophobes it is also spider season. As evening approaches, large spiders scurrying across the ground become a common sight. They’re a trusted source of the UK media’s beloved ‘fear of the wild’ stories, but are these spiders really getting fat, as some reports claim?

The large spiders that we often see in our homes are commonly referred to as “house spiders”, but scientifically they belong to two genera, Tegenaria and Eratigena. There are several different species that are broadly similar and, once mature, quite impressive. A few species can grow to a leg span of over 10cm, which is more than large enough to scare most people.

The impression that these spiders are getting bigger could have some explanations. The first is that during the summer these spiders are still growing and are not so visible in our homes. In the fall, the adult males start to move around looking for females and suddenly we see larger spiders much more frequently. Add to that the fact that a lot of people aren’t exactly spider fans, and house spiders can appear against pale rugs or white tubs, then it’s easy to see how people might think they are getting bigger.

It is also possible that people see different species of spiders. If you are used to seeing the smallest Tegenaria domestica, the biggest Eratigena atrica will come as a shock. Another possibility is that since spiders are predators, a good summer for their prey can mean that the spiders are better fed and have a better chance of growing to a larger size.

Neither of these explanations suggests that spiders get bigger. However, there is some intriguing work in Australia that lends weight to the idea that spiders could get bigger, under the right circumstances.

In the study, golden spiders that weave orbs living in and around Sydney were collected and measured. The researchers focused on mature adult females collected from a variety of sites ranging from urban parks to the bush. They measured these spiders to assess the size and condition of the body. They also dissected some of them to measure the size of the ovaries. What they found was that spiders in urban areas were significantly larger than those in less built-up areas. Not only were the bigger “city spiders”, they also had bigger ovaries, which meant they could lay more eggs.

It seems that two factors may have caused the largest urban spiders: temperature and the availability of prey. Buildings, concrete, tarmac and hard materials store heat and warm urban areas. Warmer temperatures in urban areas may have increased spider growth rates.

Urban areas may also have more prey available for spiders, or spiders may build their webs in areas that attract more prey. Street lighting is effective at attracting flying insects, and larger spiders have been associated with structures such as street lights and have been found in central areas with higher light levels.

It remains to be seen whether other spiders are similarly affected by urbanization. What is clear is that the habitats we create in our cities can have profound effects on the creatures that share our homes and gardens.

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