I was searching online for quite a while this evening, chasing a half-remembered bit of trivia, that trilobites were supposedly unique in their use of calcite for their lenses, composing the ommatidia of their compound eyes.
It must be so obvious to scientists in the field of studying insects that they never mention it in their papers…
So, what compound(s) do modern arthropods use in their compound eyes. If it isn’t calcite, what do modern ‘bugs’ use?
What I find interesting, then, is what advantage the trilobites may have gained by using a basic mineral for the lenses vs. organic chitin. Chitin must have a transparent form in order to function for the eyes in modern creatures? Hmm.
I read in one paper that trilobites may have actually formed some kind of dual-layer in their lenses to compensate for the double-refraction property of calcite.
Seems to be easier focusing with the calcite as well as durability in the scratchy sand environment.
Additionally, more derived/later species had aspherical lenses to compensate for the spherical aberration and dual layer organic→calcite lenses to allow for more of a refractive gradient to counteract the double imaging of calcite alone.
Modern arthropods probably done use it on land because of an inability to accumulate it in an efficient manner. Water arthropods like crustaceans probably done because it’s metabolically expensive to do these things when chitin alone can make for useful eyes.
Evolution is more of a “good enough” kinda situation.