The business end
This is where the magic happens - the spinnerets of the lace web spider, Amaurobius similis.
I had not intended this shot at all, as it does seem a fairly rude place to stick a camera, but in her attempts to hide, this spider hid itself under a leaf leaving this part of its anatomy exposed. As it started to twitch and wriggle I realised that she was producing silk and making the beginnigs of a web. I noticed the purpose of the pedicel, the very thin 'waist' that attaches the spider's head to its abdomen - it is the pedicel which gives the abdomen flexibility to move from side to side in the production of silk while keeping the head steady. This is particularly noticeable in orb weavers like the garden spider Araneus diadematus, whose wiggling while it makes a web resembles an elaborate dance.
Spiders are not the only invertebrates to produce silk, but they are the only creatures to use silk throughout their entire adult lives (as other silk producers tend to use them for coccoons). All spiders produce several types of silk for different purposes, and the number and appearance of the spinnerets also varies greatly. It is thought that the more highly evolved spiders can produce more different types of silk, and certainly the uses that silk is put to is what has characterised the evolution of spiders. The earliest spiders are likely to have built a burrow, and used the silk to form triplines to alert them to passing insects. Sticky silk used to construct webs came later, although some spiders have now evolved past web-building, so the most evolved spiders such as jumping spiders don't build a web at all.
Amaurobius similis is a cribellate spider. The cribellum is an organ which produces very fine strands of silk which, when combed out by the specially adapted hairs on the hind legs, form a kind of wool. Cribellate silk does not contain the adhesive glue like some orb weavers, but its strands are so fine that it can entangle insects, a bit like velcro.
I think the cribellum must be that triangular feature just below the four spinnerets.