Entomology definition of Thyridium -ii: small, whitish or almost transparent
spots near the anastomosis of the disc of the wings in some
Neuroptera, or in the recurrent veins in the cubital cellule in
some Hymenoptera; also the apical margin of the gastrocoeli,
often alone visible: in Trichoptera, specifically, a hyaline spot
on second fork of median vein.