The Mopane worm is a large caterpillar. It can be up to 15 cm (six inches) long. It lives mainly on the leaves of the Mopane tree, Colophospermum mopane. After pupating, the worm becomes called the "Anomalous Emperor moth". Technically it is Imbrasia belina, in the Order Lepidoptera, family Saturnidae. The moth then lays its eggs on the leaves and stem of the Mopani tree. It is found in the north of South Africa, in the Limpopo and Mpumalanga provinces, and in Botswana and Zimbabwe.
The mopane worm is edible, like the Australian witchety grub. It is not just a traditional food, but is now the basis of a multi-million Rand industry, and so is a source of income as well as food. Like the silkworm, the silk is used for garments. As an economically important animal, it is thus also studied.
Too prepare the worm for eating, first squeeze out the worm's gut (i.e. disembowel it). They can be boiled in salty water, fried; or sun-dried or smoked for later. The dried worms are eaten as a snack, or rehydrated to be cooked. They are often served with pap en sous (maize meal porridge and tomato chilli sauce). Prime season to harvest the fully-grown worms is November to January, midsummer.
The worm is claimed to be "tasty and nutritious", and is apparently full of protein (almost 60%). I wouldn't know about the taste, I've never eaten them. It is reportedly mild and unobjectionable, or even "flavourless", and crunchy when dried. However the whole idea of eating caterpillars does upset some people.
Over-harvesting is a danger to the caterpillars, and the population has collapsed in some areas. This can be avoided by the traditional practice of not harvesting the worms that are too high to reach. The region is prone to droughts, so the supply is not steady. "Farming" the caterpillars is being studied, and is practiced to some extent.
The worm is also sometimes called a "Mopani worm" in English, a Masonja in North Sotho, mashonzha in Tshivenda and phane in Setswana.
Further reading: Google for "Mopane worm" will give you good information.