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Fun fact about eye evolution, the dark spot you see on a dragonfly’s eye is not a pupil following you, or even a change in the eye itself. You’re actually just

Fun fact about eye evolution, the dark spot you see on a dragonfly’s eye is not a pupil following you, or even a change in the eye itself. You’re actually just looking directly down into the columns of the dragonfly’s eye. They capture light so efficiently that they appear to be black. They’re close to the absolute physical limit for efficiency.

tonetegeatinst 2y ago
So dragonfly's hit a scaling law?
- kadushka 2y ago
  Not a scaling law - a law of physics: how efficiently light can be captured by any optical system. Dragonflies capture ~99.7% of incoming light.
  - zelphirkalt 2y ago
    They capture so strongly, not even light can escape, from behind the Schwarzfly radius.
- bglazer 2y ago
  As another commenter mentioned, this is more like a physical limit for efficiency of light capture. However they also follow a scaling law of angular resolution versus diffraction. Smaller lenses means more blur from diffraction but smaller lenses also allow better angular resolution. Many insects with compound eyes follow this scaling law, having ther same (provably optimal) ratio of eye radius to lens size, despite having very different sizes of eyes. William Bialek is a biophysicist who uses this frequently as an example.
  - bozhark 2y ago
    but only dragonfly's also possess the optimum flight capabilities
hammock 2y ago
Not sure I understand. Aren’t human pupils black also because light goes in and doesn’t come back out?
- hmottestad 2y ago
  Would be interesting to know if dragonflies get red eyes when you take a picture of them with flash. Human eyes reflect some of the red light from the white flash, making it look like the pupil is red instead of black.
  - seszett 2y ago
    I guess you're being downvoted because people think this is obvious, but dragonflies won't get red eyes because they don't have red blood to reflect the light. Red eyes are also caused by a particularity of vertebrate eyes, which have their blood supply in front of the retina. This is also the reason why you can sometimes see moving dots when looking at a bright, blue-coloured thing like the sky: your eye sees the whole capillaries filled with red blood cells, but the brain processes them out because they're always there. The large white blood cells then appear as "less red" dots in the processed-out streams of red, and the brain interprets them as bright dots. Insects have none of this, they don't have blood or blood vessels but a transparent haemolymph that doesn't really circulate through a complete circulatory system like us. So there really is nothing in front of the insects' light sensors to reflect light.
  - wingspar 2y ago
    Went camping recently, and used one of those head mounted flashlights. Walking around there were glowing spots in the grasses. Thought at first it was some sort of firefly. Upon inspection, it was the light reflecting off the eyes of spiders. Dozens and dozens of spiders everywhere in the grasses. Never could get a picture. So light does reflect from compound eyes, but in ‘silver’.
- deleted 2y ago
  [deleted]
- daemonologist 2y ago

tonetegeatinst 2y ago

So dragonfly's hit a scaling law?

kadushka 2y ago
Not a scaling law - a law of physics: how efficiently light can be captured by any optical system. Dragonflies capture ~99.7% of incoming light.
- zelphirkalt 2y ago
  They capture so strongly, not even light can escape, from behind the Schwarzfly radius.
bglazer 2y ago
As another commenter mentioned, this is more like a physical limit for efficiency of light capture. However they also follow a scaling law of angular resolution versus diffraction. Smaller lenses means more blur from diffraction but smaller lenses also allow better angular resolution. Many insects with compound eyes follow this scaling law, having ther same (provably optimal) ratio of eye radius to lens size, despite having very different sizes of eyes. William Bialek is a biophysicist who uses this frequently as an example.
- bozhark 2y ago
  but only dragonfly's also possess the optimum flight capabilities

hammock 2y ago

Not sure I understand. Aren’t human pupils black also because light goes in and doesn’t come back out?

hmottestad 2y ago
Would be interesting to know if dragonflies get red eyes when you take a picture of them with flash. Human eyes reflect some of the red light from the white flash, making it look like the pupil is red instead of black.
- seszett 2y ago
  I guess you're being downvoted because people think this is obvious, but dragonflies won't get red eyes because they don't have red blood to reflect the light. Red eyes are also caused by a particularity of vertebrate eyes, which have their blood supply in front of the retina. This is also the reason why you can sometimes see moving dots when looking at a bright, blue-coloured thing like the sky: your eye sees the whole capillaries filled with red blood cells, but the brain processes them out because they're always there. The large white blood cells then appear as "less red" dots in the processed-out streams of red, and the brain interprets them as bright dots. Insects have none of this, they don't have blood or blood vessels but a transparent haemolymph that doesn't really circulate through a complete circulatory system like us. So there really is nothing in front of the insects' light sensors to reflect light.
- wingspar 2y ago
  Went camping recently, and used one of those head mounted flashlights. Walking around there were glowing spots in the grasses. Thought at first it was some sort of firefly. Upon inspection, it was the light reflecting off the eyes of spiders. Dozens and dozens of spiders everywhere in the grasses. Never could get a picture. So light does reflect from compound eyes, but in ‘silver’.
deleted 2y ago
[deleted]
daemonologist 2y ago