Presented below are some detailed photos of two screen used Flying Parasite creatures from "Operation - Annihilate!" (first airdate: Apr. 13, 1967), the final episode of the 1st season of Star Trek: The Original Series. These props are central to the storyline of the episode, in which the human inhabitants of the planet Deneva are driven to mass insanity after being attacked by these neural parasites which individually resemble enormous brain cells and are found to be all parts of a single entity with seemingly invincible resistance to heat, radiation, etc. Following the modern day images are a pair of photos of another Flying Parasite creature from the collection of Greg Jein on display at the 1992 Star Trek Exhibition at the National Air & Space Museum in Washington, D.C.; and some episode screenshots from "Operation - Annihilate!" that feature the parasites.
Many thanks to long time Star Trek fan and collector Paolo in Italy for his kind permission to display these photos.