Sensitivity to external stimuli such as chemicals, vibrations and light are crucial to animals’ survival. Light is important as organisms photosynthesise, and as animals seek such photosynthesising organisms as food, prey on other animals, seek warmth and seek shelter. Light-sensitive cells have evolved in response to selective pressures that favour their development, and we find such light sensitivity in the simplest organisms today that are arguably related to those that are very ancient.
[Image: Compound eye of a horsefly]
As we survey the range of animals of increasing complexity, we see the occurrence of more and more complex light-sensitive cells, grouped together to form what we can now call primitive eyes. Over the ages, further sophistication has occurred: packages of transparent fluid or gel can act as a focussing lens; control of the amount of light can be built into the system in the form of an iris; pigments can be used to make the cells sensitive to differing intensities of light or to different wavelengths (colours). Such a sequence of complexity is seen as we move towards the eyes of insects (that use a mosaic of images), and to vertebrates and to molluscs, like squids, that, interestingly, have evolved, in parallel, almost identical eye architectures that focus the single images with which we are familiar.
The complexity of the eye is a fascinating subject; zoologists and evolutionary biologists can discern a pattern of increasing complexity in light-sensitive organs.
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