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?
This is an entire category of proteins known as Crystallins. Crystallins of one kind or another seem to be used when pretty much any living species needs to grow a lens. They aren’t exclusive to lenses, either; many crystallins are found elsewhere in an organism’s metabolic pathways, such as the nervous system.
I found this paper from 1996 titled “Lens Crystallins of Invertebrates” which I’d say is exactly what you’re looking for. There wasn’t much for arthropods, but it mentions Drosocrystallin for the Drosophila fruit fly’s corneal lens, and antigen 3G6 as “present in the ommatidial crystallin cone and central nervous system of numerous arthropods”.
Chitin.
Ah! So the same as the rest of their hard parts, I suppose. I suspected as much, but couldn’t seem to find any paper that explicitly stated this.
Thank you!
In regards to use of calcite vs. chitin: doing a quick search: https://www.ualberta.ca/en/earth-sciences/facilities/collections-and-museums/treasury-of-trilobites/index.html
Were trilobites also unique in using calcite in addition to chitin in their exoskeleton? Do any extant arthropods use calcite in any significant way?
From my searching, it would seem that trilobites used calcite to reinforce their hard parts as well. Crustaceans have amorphous calcium carbonate reinforced exoskeletons through mineralization, but no calcite land arthropods do not seem to have this mineralization tho.
I’m no expert. Just like using Google Scholar to answer questions I find interesting.
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.
