Missing sectors

Missing-sector Orbweb Spider (Zygiella x-notata)

While rummaging around the part of the garden near the house recently, looking for likely suspects to photograph, I came across this spider strolling unsteadily along the windowsill. It was a rather chilly, windy morning, so she might have been a bit cold. For whatever reason, she was in the open rather than hiding away, so I was able to get her picture. This is a Missing-sector Orbweb Spider, named for its habit of leaving an empty section in its otherwise normal orb web. (Think Charlotte's web with the radial lines missing in one of the upper sections.) A single thread crosses this empty section from the center of the web to its edge, where the spider sits in a retreat it has built there. The thread acts as a signal line, with its movement alerting the spider, which is holding the thread between its front legs, to the presence of any prey caught by the web. This line allows the spider to zip quickly to the middle of the web without having to negotiate sticky threads. Interestingly, if the plane of the web varies from the angle of the signal line by more than 40 degrees, the spider doesn't leave a sector missing. Presumably, that's because it can parachute in on the line without having to worry about getting caught in its own web.

In most examples of this species, the pattern on the abdomen is silvery-grey with darker edges; in fact the spider's other common name is Silver-sided Sector Spider. (This particular individual appears to lack that colouring.) The silver comes from very thin crystals of guanine beneath the spider's skin — the same substance as that found in fish scales. While the silver of a fish's scales help to camouflage it from below, scientists aren't sure how such colouring helps spiders.

As with most of the spiders we've been able to identify so far, this one is very common and widespread across Britain. It's found across pretty much all of Europe and the Middle East, as well as along both coasts of North America, throughout the Japanese islands and in other scattered locations in Asia and South America. This species is nearly always found in association with us humans, and we have undoubtedly carried it around the world with us. It's particularly common in the upper corners of window frames and doorways, occasionally venturing as far as roof eaves and fences, or even into garden bushes. It's also regular around boats and docks. Away from humans, it's sometimes found near cave entrances. As the British Arachnological Society posits, perhaps it moved from caves to houses with us!