John Keats might have described autumn as the “Season of mists and mellow fruitfulness” but for many arachnophobes it’s also the season of spiders. As evenings draw in, large spiders scuttling across the floor become a common sight. They are a reliable source of the ‘nature scare’ stories so beloved of the UK media, but are these spiders really getting bigger, as some reports claim?
The big spiders we often see in our homes are commonly called ‘house spiders’ but scientifically they belong to two genera, Tegenaria and Eratigena. There are several different species that are broadly similar and, when fully grown, pretty impressive. A couple of species can reach a leg span in excess of 10cm, which is more than big enough to give most people a scare.
The impression that these spiders are getting bigger could have a few explanations. The first is that, during the summer, these spiders are still growing and are not so conspicuous in our homes. By autumn, adult males start moving around looking for females and so we suddenly see larger spiders much more frequently. Couple this with the fact that many people are not exactly spider fans, and that house spiders may appear against pale carpets or white bathtubs then it is easy to see how people could think they are getting bigger.
It’s also possible that people may be seeing different species of spider. If you are used to seeing the smaller Tegenaria domestica, the larger Eratigena atrica is going to come as something of a shock. Another possibility is that, since spiders are predators, a good summer for their prey species may mean that spiders are better fed and have more chance of reaching a larger size.
None of these explanations suggest that spiders are getting bigger. However, there is an intriguing piece of work from Australia that lends some weight to the idea that spiders could get larger, given the right circumstances.
In the study, golden orb-weaving spiders living in and around Sydney were collected and measured. The researchers focused on mature adult females collected from a variety of sites ranging from city parks to bushland. They measured these spiders to assess body size and condition. They also dissected some of them to measure ovary size. What they found was that spiders in urban areas were significantly larger than those from less built-up areas. Not only were ‘city spiders’ bigger, they also had larger ovaries meaning they could lay more eggs.
It seems that two factors may have resulted in the larger urban spiders: temperature and prey availability. Buildings, concrete, tarmac and hard materials store up heat and make urban areas warmer. The warmer temperatures of urban areas could have increased spider growth rates.
Urban regions may also have more prey available for spiders, or it may be that spiders are building their webs in areas that happen to attract more prey. Street lighting is effective at attracting flying insects, and larger spiders were associated with structures like lampposts and were found in central areas with higher levels of lighting.
Whether other spiders are similarly affected by urbanisation remains to be seen. 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.
- This article first appeared inissue 371ofBBC Science Focus Magazine–find out how to subscribe here
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