Biologists lust for sovereign terminology: kingdom, phylum, class, order, family, genus, species. The head of a trilobite is called the “cephalon” and the tail is the “pygidium”—I believe the thinking is that “head” and “tail” aren’t precise enough. That must be why they say “operculate” rather than lidded and “operculum” rather than lid. The ironic thing is that no number of words is enough. And they must dimly realize it, because they say “superorder” and “suborder”, and so on, rather than making up a dozen new words for phylogenetic levels that KPCOFGS are not enough for. I’ve actually seen the term “superphylum”, so I think there’s an unbroken chain all the way down to subspecies. Or subsubspecies.
I rather like the sound of “optical operculum”, so no doubt the correct term is “eyelid”. I propose that we follow the principle of Orthotropous Max, and use “optical operculum” to mean lens cap. Master photographers of the future will snap their fingers, saying “Flunky! Disengage the optical operculum!”
Original version, October 1997.
Updated and added here January 2012.