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The Horridge procedure (also known as leg position learning, or position learning) is an instrumental conditioning technique for invertebrates in which the movement of an appendage beyond a specified position is reinforced. This paradigm was one of the first to suggest that an invertebrate can learn. In the original version an insect was suspended above an electrified solution with a wire attached to one of its legs. When its leg (or other appendage) was lowered to a point where the wire made contact with the solution a brief shock could be administered. The result of the shock was that the animal learned to keep its leg raised. Over the years many versions of this technique have appeared. Examples include leg learning in fruitflies, cockroaches, and crabs. Appendages other than legs can be used as, for example, when the eyes of a crab are trained and the claws of a crayfish. The latter two examples are unique in that two appendages are used (both eyes or both claws). By using two appendages of the same animal each animal can serve as its own control.