Store Is Closed
Size of Organism and the Eye, Scaling
Jul 18 2011 5:00:00 pm EST
Many biological systems vary in proportion to the size of the animal. For example, as an animal increases in size, the skeletal system must increase in proportion to handle the added weight. However, the eye has ranges and limitations of magnitude of its own, resulting in less straightforward scaling.
From the simple eyespots of a unicellular organism, to the diverse range of simple and compound eyes of multicellular organisms, the eye has evolved to detect a narrow band of electromagnetic radiation: visible light. In the case of the unicellular organism, it might only be able to detect the presence of light and perhaps its direction. Whereas vertebrates have evolved to differentiate wavelengths of light as color, and determine shape and movement. In all cases, the shape and size of the eye is determined more by what the animal needs to see and less by the animal’s size. As D’Arcy Wentworth Thompson points out, “A big dog’s eye is hardly bigger than a little dog’s; a squirrel’s eye is much larger, proportionately, than an elephant’s; and a robin’s is but little less than a pigeon’s or a crow’s.”
According to a 2004 study in the journal, Vision Research, Cornell biologists found a relationship between animal’s body weight and eye size for all vertebrates, in general. However, when vertebrates were broken into smaller groups, such as birds, fishes, reptiles, and mammals, or further into smaller groups, a general model was found to be less reliable. In smaller groups, vision is tailored for different purposes and results in deviations from other groups. For example, birds, and especially owls have relatively large eyes in order to gather more light, as they rely more heavily upon visual input. In a similar vein, nocturnal animals require larger eyes to gather dim light. In these instances, size is influenced more by the properties of light, such as the interference patterns of waves and refraction.
In insects, a human type eye would be too small render a clear image. Instead, the insect integrates thousands of simple eyes into a compound eye. Compared to a simple eye, a compound eye possesses a much larger viewing angle and is capable of detecting fast movement. However, due to the small size of the individual lenses, diffraction imposes a limit on the possible resolution that can be obtained. Thus, scaling of the compound eye reaches practical limits on the high end; in order for a human to maintain a comparable resolution to simple eyes, a compound eye would reach the size of his or her head. Evolution has thus maintained a creative balance between the optical qualities of light and the size and requirements of the organism.