From ZooTerms (Dictionary of Invertebrate Zoology)
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thyridium (noun; plural thyridia; Greek diminutive thyris, window): (Arthropoda: Insecta) 1. A small whitish spot in the wings of Neuroptera, Hymenoptera and Trichoptera. 2. In ichneumonid Hymenoptera, a scar-like area on each side of the second abdominal tergite, between the middle and base; the third tergite rarely may have tyridia.