Scaling up compound eyes, is there a size limit?

Question

I'm designing an alien creature that looks insectoid and had a question about compound eyes. The creature is about human in size, maybe a little larger, but I'm wondering if there is a size limit on compound eyes?

I know with vertebrate eyes (camera-type eyes?) there is a limit as to how large the eye can get before getting bigger won't add any benefits and therefore they don't go beyond a certain size.

In my case, the alien has a mantis-looking head with large mantis-looking eyes and is roughly the size of a man. Would this scale up fine and be functional or would my mantis-man have weirdly small eyes for its proportions in order for its eyes to still work?

Also, my insectoid alien does not have an exoskeleton, not sure if that is an automatic deal breaker for compound eyes or not.

Answer 1

Some types of flies, such as dragon flies have compound eyes that are very large in proportion to its head.

Bathynomus giganteus are extant bottom dwelling marine creatures that have compound eyes that measure 18 mm.

The largest compound eye, so far discovered, was 4 cm in diameter. This was for an ancient marine creature called a radiodonts. They lived during the Cambrian Period, 541 million to 485 million years ago. They resembled "frankenprawns" and were up to 50 cm in length. Depending on the species, each eye contained more than 24,000 lenses. If they were scaled up to be human sized (200 cm), their eyes might have been 16 cm in diameter. That would be huge!

There also the issue of how much would a large compound eye weight and would a creature be able to move its head with very eyes. I'm tending to think that large compound eyes might not be achievable.

Answer 2

Large compound eyes are completely possible. The main limiting factor of vertebrate eyes is they are hollow which makes them weaker the bigger they are. compound eyes however do not have this problem they are very structurally sound. the only reason we don't see large compound eyes is we don't have any large animals with compound eyes. You will have to stop and think about how a organism without an exoskeleton supports a large compound eye.

More likely it will have several separate compound eyes like spiders with minimal overlapping field of view from each "eye". The main benefit of a very large compound eye is a greater field of vision, but with a big head you end up with a lot of wasted "eye" since the angle between facets will be very small. Also having your whole head covered in eye is pretty venerable. So for a big animal smaller individual compound eyes are better.

there are several kinds of compound eyes as well, in trilobites for instance the individual facets are separate and not connected to each other. this can give you a bigger eye without having a ridiculous number of facets.

You can also have spider style compound eyes which works a bit more like vertebrate eye.

Answer 3

Well, last question first - not having an exoskeleton is no problem, since some annelid worms (Sabellidae) have compound eyes -- which apparently are a case of convergent evolution, not directly related to the arthropod ones.

"They display a surprising diversity of eyes of varying levels of sophistication, ranging from scattered single ocelli to compound eyes with up to hundreds of facets." https://pubmed.ncbi.nlm.nih.gov/27453305/

On the main question - I think it is not so much 'compound eyes only work when small'. It is more 'compound eyes are not terribly good in the first place, and for something human-sized the disadvantage becomes ridiculous'.

"To give a compound eye the same (about 1 arc-minute) resolution as our eyes would require millions of lenses each as large as a human lens." Visual Acuity in Insects, Land, 1997

IE - the eye would have to be larger than an entire human body to have equal resolving power.

This doesn't mean that small is better, though - I actually found a reference to a minimum size for the more efficient 'superposition' type of compound eye: https://www.sciencedirect.com/science/article/pii/S0042698904001944?via%3Dihub

So if you want compound eyes for a human-sized creature, it might be best to make it something evolutionarily-recent and still pretty crude (like the annelid ones) for a creature whose primary sense is something else (e.g. echolocation).