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Diffraction depends on the size, and limits the performance of an eye. As creatures become smaller, visual mechanisms become more challenging if sight is to be one of the primary sensory modalities, especially if the intended prey is yet smaller. Hence, a small creature risks sensory disability simply by being small, particularly if it is a predator. Resourceful solutions, however, have been found and perfected.
Mysid shrimp are a family of small decapods, each species generally being no longer than 5–15 mm. There cannot be much of an eye in such a creature. All shrimp have compound eyes, and the larger, long-bodied decapods have reflecting superposition eyes (Br J Opthalmol, November 2006). However, Dioptromysis paucispinosa, one of the smallest of the mysid shrimp at 5 mm in length, has refracting superposition eyes similar to those of moths (Br J Opthalmol, September 2003). This particular species of mysid shrimp has another eye within the primary eye, as its generic name would suggest, providing two different ocular forms in the same eye. Moreover, this second eye is …