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.

BBC Wildlife Magazine - - Komodo Dragons - ADAM HART is Pro­fes­sor of Science Com­mu­ni­ca­tion, and ANNE GOOD­E­NOUGH is Pro­fes­sor of Ap­plied Ecol­ogy, both at the Univer­sity of Glouces­ter­shire.

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.

Why do we see more house spi­ders in our homes in the au­tumn? It’s the age-old tale – your abode is the set­ting for an eight-legged love story.

Six for­ward­fac­ing eyes and long legs make Te­ge­naria do­mes­tica an ace am­busher. Liv­ing rooms are a walk in the park for most house spi­ders.

House spi­ders’ sheet-like webs, with a fun­nel struc­ture within, should per­haps be more wel­come in our homes as they help re­duce the num­ber of un­wanted in­sects.

Bath­rooms are full of traps for the un­wary spi­der.

For most of the year you are far more likely to find house spi­ders in out­build­ings than in homes.

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