Optical Scanning in the Lateral Eyes of the Copepod Copilia

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Abstract
The lateral eyes of the female Copilia are exceptionally large for a creature of its size but have very few receptors. These receptors appear to scan the image plane of the anterior lenses of the eye, but until now there has been no behavioural evidence linking their movement positively with the functioning of the eyes, though there is some (disputed) evidence that it is merely a side-effect of peristalsis. On channel-capacity grounds we might not have expected optical scanning to occur in biological systems, so any positive evidence for scanning, in the engineering sense, is particularly interesting. Optical measurements of the position of the image plane, in live intact female Copilia quadrata, generally corroborate Exner's (1891) finding that an image is in focus at the plane of the distal ends of the receptors, though the position of best focus seems slightly in front of a second lens, behind which the receptor structure lies. An alternative optical schema of Wolken and Florida (1969) is disproved. The position of focus satisfies minimal optical conditions for scanning. Movements of a coloured stripe pattern, used as a test object for the optical measurements, elicited the putatively scanning movements of the receptor structure, suggesting strongly that these really are an intrinsic part of normal visual functioning, not an epiphenomenon. It was also discovered that intermittent swimming movements of the limbs tend to be preceded by these photoreceptor movements. Both findings support the hypothesis that Copilia's eye is an optical scanning system.