From ZooTerms (Dictionary of Invertebrate Zoology)
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tapetum (noun; Latin tapete, carpet): 1. A reflecting surface within an eye.

2. (Arthropoda: Insecta) A light reflecting surface within clear-zone eyes, formed by tracheae that run through the eye parallel with the ommatidia forming a layer around each one, and reflecting the light back into the ommatidia.