Let's make drow weirder biologically! Here's some workshopping for some headcanons, aka the only thing my veterinary technology degree and the hours watching animal documentaries is useful for these days.
Explanations of how pit organs, slit pupils, and the tapetum lucidum functions in animals and how they could be helpful for a drow. I will try to keep it concise and explain these structures accurately but keep it somewhat contained. Let me know if you have any more questions or if you have a correction to some of the information I have provided.
Pit Organs
Pit organs (also called heat pits, thermal pits, or heat sensing pit organs) in some snake species such as boas, pit vipers, and pythons are mainly comprised of a depression in the skin/scales with a membrane, the structure of which is comprised of cells that are extremely sensitive to radiant heat in the environment. It varies in its structure across species. In pit vipers, the membrane has a pocket of air behind it, leaving the membrane suspended in the pit. In boas and pythons the membrane is across the back and sides of the pit organ. They serve the same function, that being that they pick up on subtle changes in temperature around them and send that information to the brain, which overlays it with any information coming from the optic nerves as well. These two images are then layered on top of one another so both information is being interpreted at the same time.
Blood circulation is very important to the function of these organs, to cool down the receptors after the warmth is no longer there, to prevent any sort of after-image.
As for the location I would put them, a humanoid skull does have a depression in it that might fit, right above the zygomatic arch on the cheekbone. However, because the direction of the heat pit matters to where they can "see" the heat, it might be that the drow might have to turn their head to the side slightly to get an accurate reading, unless it was also angled outwards.
There's also right underneath the eyes, which would work best for the angle (as both the eyes and the heat pits would both get the same directional information, so there would be no confusion about the direction the heat source is coming from) but they would be limited by how deep they could go to the layer of flesh and muscle on the face. Take your pick, really.
A heat pit would also explain the sun sensitivity - these structures are extremely sensitive and extremely vascular, and being overlayed with their normal eye vision, all they would see would be bright whites and reds, unless they were to cover them up in the sunlight.
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Slit Pupils
Slit pupils are very good for low-light vision. When light and color enter the eye, they bend and create a blurry image. For diurnal animals with round pupils, this doesn't matter as much, because our eyes constrict the pupil down to sharpen the focus in the daytime. However, nocturnal and crepuscular animals would have their eyes wide to let in more light, and the image would be blurry and short-sighted.
Some nocturnal animals instead have different bands (multi-focal lenses) for each wavelength going from the center of the eye outwards, each tuned to a different wavelength so they can be centered roughly where they need to go. However, if the pupil is round and needs to be constricted, those other bands are no longer available for use. A slit, however, opens up those other bands for use, no matter if the eye is narrowed or dilated.
Thus, even in low-light conditions where they aren't using their heat pits for visual information (such as reading, where only using infrared will not work, or on the surface during the nighttime) they will still be able to get a full image.
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Tapetum Lucidum
A very common structure in any animal that spends its time in low-light conditions - it causes the eyeshine on dogs, cats, crocodiles, and many others, including spiders! It is an iridescent structure in the eye. though its exact location and chemical structure in the eye changes between species. It works to increase the amount of light that passes into the eye by reflecting it back. It is a retroreflector, which bounces light back exactly to its source without scattering it, which is very important as to not make the image more blurry.
Notably, while being iridescent, it also has its own color in each animal. This color may also change based on species, age of the animal (cats) and season of the year (reindeer). I headcanon that all drow have a red eyeshine, and each humanoid species with darkvision (orcs, elves, dwarves, etc.) have their own distinct color of eyeshine that you can use to tell them apart in the dark, if you know which is which, but that's a post for a different day.
All drow having a red eyeshine is one of the best and only ways to identify if you're looking at a pale elf or a Szarkai - just wait for light to shine on their eyes. The elf would have white, the szarkai, red. Unless you're Drizzt I guess, whose eyes shone purple. However, because the tapetum lucidum has a color associated with it, a mutation in the structure could cause it to reflect an odd color - such as purple - without impacting its function.
If the tapetum lucidum is particularly sensitive (cats, for example, have some of the most powerful retroreflective tapetum lucidum in the animal kingdom) it could also explain their sunlight sensitivity as any light would be reflected back tenfold.
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