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A well-developed proventriculus is present in larvae of the Polycentropodidae and Hydropsychidae. Its structure varies greatly between species but there is a common bilaterally symmetrical pattern based on a pair of posterior ventral folds. These are continuous with the sides of a ventral gap in the fold of foregut that invaginates into the midgut. Modifications of this basic structure to form either triturating or filtering devices occur, and there are others where the mechanism of food treatment is not immediately apparent and may be complex.