The chelicerae of spiders are their two-component mouth parts consisting of a base segment that articulates with the cephalothorax and a fang portion that articulates with the base segment. Think of a two-segment finger, the outer segment of which consists of a hollow fang. Except for two very small families of spiders, they all have venom which gets injected into their prey through openings in the tips of their fangs. The glands that produce this poison are located in the two segments of the chelicerae, and, in most spiders, extend beyond the chelicerae and into the cephalothorax. Chelicerae are also found in related creatures such as the scorpions.

This drawing shows the chelicerae in black, the surface of the cephalothorax in brown, the spiders legs in reddish brown, and the poison glands and surrounding muscle tissue (used to squeeze out the venom) in green.The fang portion of the right chelicerae can be seen projecting into the space between the two chelicerae.

The second drawing shows the cephalothorax of the spider in brown (with the legs removed) and the two portions of the chelicera.