I'd never heard of divination by spider being an actual practise until Jevne suggested that I look at some of the African divinatory techniques which are known to use spiders in their work. I researched it a bit and it's fascinating stuff.
The native Mambila people (living in the area of Nigeria and Cameroon) don't use pet or tame spiders - what they do is look to find a hole in which a spider is actually resident. They clear the area around the hole and then place a pot with the bottom cut off over the spider's lair. Inside the pot they place a stick and a stone at opposite sides of the hole. From a pack of divination cards, two are selected and placed over the hole with the remaining cards in the pack set beside the hole.
After a period of waiting in which the spider (referred to as the witch as the people of Mambila
believe only witches can be involved in divination) the spider is inspected. The findings can be read in several different ways. Sometimes the spider has made a “choice” and moved one of the cards covering the hole onto the stick or stone (both of which have been given specific meanings in relation to the working being undertaken) and sometimes the spider has disturbed the cards so they are “pointing” at the relevant item. The shapes that the cards make are also considered to be relevant. Sometimes the spider will disturb the pack of cards near the hole, in which case the pattern and spread of the disrupted cards is also seen as significant and interpreted accordingly.The procedure is known as Nggum.
African divinatory cards are often replaced with light shavings of tree bark, with the pattern the spider makes in disrupting the stacked pile yielding the answer to the question being asked.
Apparantly divination by spider is even common enough to have its own name – Arachnomancy. The ancient Incas used to place a large spider in a cup and then withdraw it before undertaking any form of divination. The spider's legs were taken as an indication as to whether the circumstances for undertaking divination were favourable. If the spider's legs were bent, it was an indication not to proceed, if the spider's legs were straight, the Incas would proceed to the next stage which was placing the spider on a dish of leaves, letting it move them around and then reading the pattern of the leaves the spider left behind to foretell the future.
Spider webs in China were also used to predict omens. A spider was captured and held captive overnight. If it span a web the future looked good, if it didn't, the spider was considered indolent and there was a suggestion that good times would be slow to arrive.
Ancient Greeks used to observe the behaviour of spiders to determine the future. There is a particular method (I've seen several claims this comes from Ancient Greece but the link is hard to establish conclusively) where the spider is placed underneath a glass or jar and the glass is then broken, frightening the spider which runs madly at objects and cards placed nearby. The objects it chooses and the order it chooses them in form the basis of the divination.
The shed skins of spiders have also been used to predict the future. The time a spider chooses to shed, the position in which it does so and the shed it leaves behind can all be linked to divinatory purposes. Generally a full complete shed would be taken as a positive omen.
And other witches are still using prophetic spiders today in modern times. I didn't know this but apparantly it is not uncommon to use a pet spider in conjunction with a ouija board (guess what I'm doing tonight...). The idea is that the spirit finds it easier to work through a living creature than it does an inanimate object.
Really interesting to see how important this creature has been to witches and magical practitioners throughout history and into modern times.