In all 17 species, the commonest kind of light receptors were "rod" cells, which are highly sensitive to light and allow night vision but cannot tell colors apart, they found.
Retinas use two main types of light-sensitive cells to allow animals to see: Rod cells help measure brightness, while various types of cone cells help distinguish colors.
It is shown that the resonant frequency of the composite system is sensitive to the alteration of the geometrical dimensions of tool rod with the wears in processing.