For those with vivid imaginations the white markings on the black back of a noble false widow spider (Steatoda nobilis) can look like a skull and crossbones, or in other cases just a skull. Perhaps it is a warning that the spider has a nasty bite. The poison will not be fatal, however, rather on a par with being stung by a wasp and or a bee.
The species is one of six similar spider varieties found in Britain, but is not native, having arrived in 1879 in a bunch of bananas from Madeira. After more than a century of being confined to the far southwest it is now spreading northwards as the climate warms. Noble false widows tend to live in houses, round the back of washing machines being a favourite, presumably because of the warmth.
Source: Guardian Environment