African spider divination is interesting to me because it uses a system of leaf-cards marked with symbolic "ideograms." The cards are usually made with tree leaves or raffia palm ribs. The ideograms are traditional symbols with specific meanings. It is interesting that the practice has changed somewhat over the years. Contemporary diviners use a reduced set of about 76 cards, but accounts from the 1960's describe sets of 200-300 cards.
These cards are used in combination with large African pit dwelling spiders. A spider hole is found and the area surrounding it is cleared of grass and debris. Generally, an old pot that has its bottom broken out is used to contain the spider.
After hearing the question, the diviner places a set of leaf-cards along with some other objects in the pot-enclosure based on the client's query. For instance, rocks might be used to represent individual people related to the question. The leaf-cards have predetermined meanings indicated by their ideograms. Other objects, usually sticks, may be added to the mix to indicate specific factors related to the question. One or two cards are placed over the spider hole that may have special pertinence to the query. The rest of the cards are stacked.
Once the cards and objects are placed in the pot-enclosure, the diviner will rub a stone around the rim of the pot and blow into it, saying "come out, come out." The question is chanted under the breath and the stone is tapped on the rim in rhythm with the chanting. Then the pot is covered and left undisturbed, sometimes overnight. Once enough time has passed for the spider to emerge and to rearrange the cards and objects by its movements, the lid is removed. The diviner interprets the answer by the pattern and arrangement of cards and objects. The ideograms on the cards have very specific meaning, but talent and experience in the diviner is required to understand the meaning of how the cards have been rearranged by the spider in relation to the other objects.
This type of divination is found primarily in lower Cameroon. Like most pre-modern traditions there is a lot of variation from place to place. For instance, near the coastal areas, land crabs are sometimes used instead of spiders. Some diviners give less weight to the meanings of the ideograms and more significance to the overall rearrangement.
The most common type of questions have to do with health or witchcraft. Often these two things go hand-in-hand as illness is often thought to be a result of witchcraft. The gradual advancement of the medical model in these areas does not necessarily negate the belief in witchcraft as people are perfectly capable of harboring dual explanations. For instance, even if I accept that malaria is caused by a parasite, I can also believe that the reason I contracted it is because my neighbor has caused a curse on me.
In Cameroon, the spider of myth has a chthonic quality. It lives in the ground and it comes out at night. It is symbolically associated with wisdom, high rank, and the dead. The spider can cross between the world of the living and the world of the dead. The association with rank might have to do with the frequent use of spider divination by the higher social ranks. Historically, the spider appears to have been a creator god in its association with weaving and has also been associated with war as well as beginnings and endings. You'll note on the Cameroon spider mask, the spider is pictured with six legs instead of eight. This is the traditional iconography and is still seen quite frequently in Cameroon.
The classic anthropological work on the subject is Paul Gebauer's Spider Divination in the Cameroons. You can still find used copies around. The link goes to Amazon.com, but there may not be a used copy posted there consistently. Gebauer goes into quite some detail about the leaf-card ideographs and their specific meanings. You'll not find this level of detail anywhere else.
A great on-line source is by another anthropologist, David Zeitlyn: Zeitlyn, D. 1993. Spiders in and out of Court or 'the long legs of the law.' Styles of spider divination in their sociological contexts. Africa 63(2), 219-240. I summarized some of his article here. He also goes into some depth on the social and political role of spider divination in Cameroon as well as the process of becoming and diviner and the details of interpretation.