In the uterus of the adult female, embryos develop into first-stage larva, which remain in an eggshell (sheath). The female deposits these sheathed larva in the tears of the mammal or bird definitive host, & the larva are ingested by tear-feeding flies.
In the fly, the larva hatch, penetrate the gut wall, & migrate to the body, testes, or egg follicles.
They develop into third-stage larva & migrate to the head of the fly. They wiggle out of the straw-like feeding apparatus of the fly when it feeds on the tears of another mammal or bird host. They develop into adults in the eye or surrounding tissues of the host, where they may live for over one year.
Thelazia are found in various tissues of the orbit (socket) of the host's eye, within the eyelids, in the tear glands, tear ducts, so-called "third eyelid," or within the eyeball.