What's the connection between artillery and spiders?

Strange as it may seem, there is one and it's even written up in an official 20th Century pamphlet on artillery instruments.

Put very simply, spiders spin a tough silken thread that makes an excellent 'graticule' in an optical sight, such as those used in artillery instruments. Graticules are the fine lines in a sighting system that are used to lay an instrument accurately on a given point. It was found that the web of a common spider in the grounds of the Royal Arsenal could be used, and these were sought out so that long strands could be selected and wound on a frame. A short length would then be sandwiched between thin sheets of optical glass and inserted at the correct point in the lens system for the sight so that it would remain in focus when the sight was being aimed, whether at a close or a distant object.

I doubt whether it's a system still in use, though graticules remain an essential element of many types of sight and of military binoculars and telescopes. Spiders are probably considered old-fashioned - a bit like gravity, but that's another story!