- Is an insect the smallest species that we can see with the naked eye?
- Are any insects too small for the naked eye to see.
- What is the largest species the naked eye can not see.
According to this source, the human eye can see objects that are around 50-60 microns across.
The smallest adult insect is Docopomorpha echmepterygis, which is 139 microns long. This answers question #2: there is no adult insect too small for the naked eye to see.
There’s room between 50 microns and 139 microns for some other species to claim the prize for question #1. Strictly speaking, worms are not insects - and there is a nematode that measures in at 20 microns, too small to see with the naked eye. It’s hard to search Google for “adult animal larger than 50 microns but smaller than 139 microns,” but it seems very likely that such a creature exists.
#3 is likewise difficult to search the internet for, i.e. “adult animal slightly smaller than 50 microns.”
- Mites.
Mites, which are a) not insects and b) basically everywhere, including on your skin right now, range in size from 6 mm to microscopic, which means that are probably plenty of them right at the limit of what we can see with our naked eyes.
You mean we mite see them, or we mite not?
Anyway, the human ovum is about 100 microns, so we can see that. A single human cell!
But how wide is it? If it’s 139 microns long but less than 50 microns wide, it may not be visible. And judging from this photo, I think its width is less than 1/3 of its length.
Of course “can see with the naked eye” is a very poorly defined criterion. It depends on so many factors like color, lighting conditions, background, etc. Is it crawling on a leaf in the forest, or on a pure white surface in a well-lit lab? Not to mention the huge variation in human eyesight. People who are severely short-sighted (like me) can focus on something much closer, and therefore see smaller things.
Probably an amoeba that fits in the gap.
This brings up Mixdenny’s Theorem #27. If you see something moving on your skin that you can barely see, chances are good that you do not want to look at it under a microscope.
Dennis
Plankton (i.e. in the ocean) are sorted and classed by size; e.g. “picoplankton” are anything 0.2 to 2 microns, “nanoplankton” are 2 to 20 microns, and “microplankton” are 20-200 microns. There’s a table on that wiki link that list species groups each size class, so the range of species that cross the visual threshold might include diatoms, flagellates, etc. For a fish trying to eat these (I study some of this), visual acuity depends heavily on light, contrast, and shape, so I wouldn’t want to pick a number as a threshold, but it’s likely you could find a continuum of species crossing any limit in this range that you might set.
Tardigrades (water bears) range from 50-1200 microns.
I had two large chicken cells for dinner tonight, myself.
Caulerpa taxifolia is a single celled algae that looks remarkably like a plant. it has structures that resemble stems and leaves, and grows up to 12 in long, but it’s all just one cell.
I didn’t need to hear that.
Sometimes I have something stroll across my phone screen that there is no chance I would have ever seen if it wasn’t on a backlit uniform surface. They (whatever they are) are probably pretty close to the visibility limit.
…and thanks to the crashed Beresheet lander, Tardigrades have recently been spotted on the moon…
The answers to 1 and 3 will be the same: If one individual of a species is just slightly above the limiting size, then there will be some other individual of the same species that’s just slightly below the line. But there are many, many different species, of many vastly unrelated lineages, that are at that size scale.
There’s gotta be a Straight Dope rule against the flagrant use of weaponized puns outside the pit. If not there should be. 
Look, it’s not as if there aren’t billions of microscopic creatures on and in your body at any given time. So some of them are multi-cellular - what difference does it make?
The ciliate protist Stentor is one of the largest unicellular life forms, and can reach two millimeters in length. So my vote would be for a not quite fully grown Stentor.
And who said NASA was a waste of money!
Probably somebody who knows that NASA isn’t an Israeli agency?