Six strands of barely visible thread radiate out from a hole in the oolite limestone wall. After nightfall each thread is touched by a long black leg, detecting even the slightest movement. Deep within the hole, just visible, is a tiny hint of metallic green. Once you've noticed one, look around a little and you'll see the same sight in almost every small hole in almost any given wall in this city. And yes, these creatures are aliens. An unsuspecting moth, attracted by the nearby streetlight, comes to rest on the wall, unknowingly tripping a thread as it does so. Lightning fast, the legs fully emerge from the hole, bringing with them a large dark body with green fangs heading towards the moth. The moth puts up a feeble struggle, then settles down and is dragged back into the lair of the beast, just seconds after having landed. A group of friends pass on their way to the pub, none of which even notice the legs or the threads.
Far from being the plot of a low budget science fiction movie, the events described above are very real and happen on a nightly basis on nearly every street in Bath. The alien in question is not from outer space, but from the continent, thought to have been brought over on ships in the 19th Century. It's called Segestria florentina and it's one of Britain's largest spiders. At first they became established around ports, such as Bristol and Exeter. In 1958 the arachnologist W. S. Bristowe commented on their absence from Bath. But they are absent no longer. It's been some years since I found my first one, in the wall of my house, right by the door. It had probably been there for several years. I didn't tell the other residents of my house for a few years, and when I did they were surprised. As was the great Bristowe himself, who recounted that he walked unawares past a wall full of them on a daily basis, possibly for years. As menacing as they sound, they are of little threat to humans. On rare occasions they have been known to bite, but only when threatened, and even rarer than this do they break the skin. Having lived among them for years I can vouch that they are not interested in humans in the slightest. The one in my wall doesn't even flinch when I close the door, and I have even had one sit on my knee - it didn't even know I was there. Like most creatures, if treated with respect they won't bother you. Any it's very unlikely that their presence will do much harm to the wildlife. The same may not be true for the next species.
They landed in 2004, in just a few years they colonised the UK, devouring whatever they could, including their nearest rivals, and invaded ordinary homes. If asked the question "would a giant spider or a ladybird be a bigger threat to Britain's wildlife?", I'm sure the spider would be the popular choice. But while the spider utilises a largely unexploited niche in the urban environment and feeds only on insects that come close enough to it’s web, the harlequin ladybird is as hungry for aphids as our native species, if not more so. They have also been known to devour other ladybirds. As well as this, they breed very quickly (each female can lay over 1000 eggs), are active later into the year than native ladybirds and have greater powers of dispersal. In the winter they congregate in buildings, often in large numbers, and have been known to bite humans in their search for food. Their large larvae and pupae cases are obvious everywhere, particularly on sycamore and lime trees in the parks and gardens of Bath. The old pupal cases turn white and look like small pieces of cotton wool, lasting for some time after the adult has emerged.
They originate from Asia and have been used as a form of natural pest control in the United States, where they are now themselves considered a pest species. From their arrival in London in 2004, they have now spread out over most of the UK. I recall finding one in Keynsham during a Phase 1 Habitat survey for Ecosulis in December 2006, one of the first records for the Bath area. By the next summer I saw them as frequently as the seven-spot ladybird, and these days I see them more frequently than all of the other ladybird species in the Bath area put together. Last year, sat by the Avon Gorge in Bristol, I saw clouds of them swarming around walls and large crevices on the cliffs. The spread of the harlequin ladybird is being monitored by the Harlequin Ladybird Survey, and research is being undertaken into methods of control. But being so firmly established, they may not be easy to get rid of.