Incy wincy spider
As the nights draw in, leggy arachnids invade our homes. Adam Hart and Anne Goodenough suppressed the desire to run and gave them a close look to find out why.
This time of year, when summer draws to an end and autumn begins to take hold, is wonderful for exploring nature. There are amazing colours in our parks and gardens, a bounty of hedgerow berries, spectacular starling murmurations and fabulous fungi. However, for a substantial number of people – including many wildlife enthusiasts – this is also the start of an eight-legged nightmare in our homes, because September is spider season!
The first sign that the season has arrived might be a big, hairy arachnid in your bath in the morning, or one scuttling quickly across your living room carpet as you watch TV in the evening. From that first sighting many wary householders – even those who are not arachnophobic – start to approach darkened rooms with trepidation. Spider sightings continue, seemingly relentlessly, into midautumn. But then, as abruptly as it started, spider season comes to an end. So what’s going on? It was an irresistible challenge to a pair of scientists such as ourselves.
Many species of spider have been identified in British homes, but what we tend to call ‘house spiders’ are the brown, long-legged species of the genera Tegenaria and Eratigena. These are among the largest spiders in the UK. Indeed, Tegenaria parietina with its 14cm leg-span (also called the cardinal spider because, so the story goes, it terrified Cardinal Wolsley at Hampton Court), is the country’s largest. The well-named T. gigantea approaches a similar size, while Eratigena
atrica is only slightly smaller. In common with other ‘house spiders’, all three of these species have long hairy legs, large low-slung bodies, and move very rapidly.
Probably the most abundant of the house spiders in the UK (and indeed throughout Europe and North America) is T. domestica, characterised by sooty markings on its abdomen, which can look like chevrons. As its species name domestica suggests, it has become more or less reliant on our homes, garages and outbuildings, though it can also be found living in tree hollows, rock clefts and cave entrances.
“Spiders are rank opportunists,” says Peter Smithers, a spider expert at Plymouth University, and fellow of the Royal Entomological Society. “They moved in with us as soon as we began to build houses. Those early homes offered ideal supports for their webs or retreats, and the other invertebrates that entered houses to take advantage of the food we stored there provided easy prey.”
One of the key features of many spiders is
their intricate webs. Indeed, it is thought that the word ‘spider’ comes from the old English term ‘spithra’, which means spinner, and reflects this part of spider biology. All of the house spiders make a two-part web to catch their prey. The first part is a flat, sheet-like web, made from multiple silk strands. Within this, usually towards the edge, the spider creates a funnel-shaped structure.
Such webs are often seen on neglected garage windowsills. Once the web is built, its owner waits in the funnel until an unwary invertebrate stumbles onto the silk sheet. The spider then rapidly moves out of the funnel, ambushes the prey, drags it back into the funnel and consumes it.
The ambushing tactic is greatly helped by house spiders having six of their eight eyes facing forwards in the classic predator arrangement to give excellent depth of field, and by having long legs that allow for rapid movement. The ‘sit and wait’ approach is low-energy and effective, and the favoured strategy for a great many spider species. Unlike orb-weaving spiders, such as the garden orb spider, Araneus diadematus (which is also very obvious in early autumn), house spiders rarely store prey in their webs.
HOME INVASION
Though we often refer to Tegenaria and Eratigena as house spiders, for most of the year nowadays we are actually far more likely to come across these species in our gardens, garages and outbuildings. These are areas where we tend to be far less scrupulous in our cleaning and tidying – and where there is therefore much more potential prey. If spiders are left undisturbed, females can live for several years and may stay in the same web complex throughout this time, repairing the sheet and enlarging the funnel as necessary.
So, if house spiders are more commonly seen in locations other than our houses for most of the year, why do we see so many of them in our bedrooms, bathrooms and living rooms in autumn? The reason, as with so many activities involving animals, is sex.
During spider season, male spiders are on the lookout for females and it is their romantic wanderings that bring them into our homes. In common with many different animals, it is male spiders that are the dispersive sex, tending to have shorter life spans; by contrast, females are more sedentary and live longer. Because egg production is related to body size, female spiders are larger than males. Body size is
THE AUTUMN VISIT OF SPIDERS TO OUR HOMES MIGHT BE CREEPY FOR US, BUT IT’S BLIND DATE TIME FOR THE ARACHNIDS.
one way to tell the sexes apart, but going on this alone can be difficult, since you could be dealing with an unusually small female, an atypically large male or even a juvenile.
TAKING A CLOSER LOOK
A far more reliable method to distinguish sex in spiders is to look at their front end. Although they have eight legs, they also sport a pair of appendages in front called pedipalps, which can sometimes be long enough to look like an extra pair of legs. The male uses his pedipalps to transfer sperm into the female and, in some species, the ends are clubbed or bloated so they resemble tiny boxing gloves. The presence of these enlarged pedipalps is a sure sign that you’re looking at a male.
The annual appearance of spiders in our homes might be creepy, or even terrifying, for some, but for biologists interested in the timing and ecology of this sort of mass event it provokes many questions. When you start to ask some of these questions you quickly realise that, while a great deal is known about house spider biology, detailed information about their sudden mass appearances in homes during spider season is lacking.
What was needed was a UK-wide census of house spiders throughout the autumn to document when and where spiders were appearing. With such a dataset you could answer some basic questions, such as when does spider season start and end, what times of the day are spiders particularly active, and where in the house are you most likely to encounter one?
The problem was that two biologists, such as ourselves, simply couldn’t be everywhere at once. Luckily, with the rise of mobile technology we could call on one of the best resources available to the modern scientist: the public. Together with the Royal Society of Biology – which, in collaboration with the University of Gloucestershire, had already organised a hugely popular citizenscience survey into the timing of flying ant emergences each summer – we set up a new survey into house spiders.
SPIDER SIGHTINGS
We developed an app for mobiles and tablets called ‘Spider in da house’. Following its launch in August 2013, thousands of people downloaded it and logged their sightings. Records could also be submitted online via a website, and we built up a detailed picture of house spiders in the UK.
Overall, we received almost 10,000 records of spider sightings up to the end of January 2014. It was immediately apparent that spider season is not just a piece of media hysteria. There is indeed a very pronounced increase in sightings from the end of August, and they reach a peak in mid-September and then tail off relatively rapidly towards the
end of October. Indeed, in mid-September we were getting around 1,800 sightings per week, whereas by the end of October this was down to around 350. So if you are seeing a lot of spiders in your home in September, this is only temporary.
WEB OF INFORMATION
We were keen to establish whether the classic place to find house spiders – the bath – was actually where the majority of the individuals were seen. In fact, most of the 10,000 house spiders reported were found in living rooms, with bathrooms the second most likely location. Since we spend a lot of time in our living rooms, you might have expected a bias in observations towards them (they accounted for 27 per cent of records). But we generally spend far less time in our bathrooms, so despite the ‘spider-in-the-bath’ scenario, it was still a surprise to see them produce 21 per cent of records.
Our data suggest that most observations occur during the evening, which makes sense as we tend to be at home more, but likely also reflect spider biology, with males tending to wander more just after dusk. Given this, and the fact that we spend a fair amount of time in bedrooms, it was another surprise to find that these areas produced just 17 per cent of records.
We wondered whether the uncluttered nature of most bathrooms compared to the wealth of places in typical bedrooms where a spider can hide from sight might be important in explaining our results. Bathrooms also have some well-designed spider traps – smooth-sided baths and basins – that make it difficult for the lanky intruders to escape but easy for us to see them.
Spider season isn’t everyone’s favourite time of year, yet we should remember that arachnids are essential to healthy, balanced ecosystems. As predators, they perform a valuable pest control role. Arachnologist Peter Smithers points out: “In our cleaner modern houses the abundance and diversity of prey is lower than in the past, but spiders still play an important role in keeping down the numbers of invertebrates that stray into our homes.”
Spiders are not just predators, of course: as prey they help to sustain a host of other animals. Spiders are also endlessly fascinating in their own right, and if you take the time to really look at them, incredibly beautiful, too. When you next encounter one of these creatures in your home, just remember that it’s harmless and most likely a lonely male looking for love.