A simple eye (sometimes called a pigment pit) refers to a form of eye or an optical arrangement composed of a single lens and without an elaborate retina such as occurs in most vertebrates. In this sense "simple eye" is distinct from a multi-lensed "compound eye", and is not necessarily at all simple in the usual sense of the word.
The structure of an animal's eye is determined by the environment in which it lives, and the behavioural tasks it must fulfill to survive. Arthropods differ widely in the habitats in which they live, as well as their visual requirements for finding food or conspecifics, and avoiding predators. Consequently, an enormous variety of eye types are found in arthropods. They possess a wide variety of novel solutions to overcome visual problems or limitations.
Use of the term simple eye is flexible, and must be interpreted in proper context; for example, the eyes of humans and of other large animals such as most cephalopods, are camera eyes and in some usages are classed as "simple" because a single lens collects and focuses light onto the retina (analogous to film). By other criteria, the presence of a complex retina distinguishes the vertebrate camera eye from the simple stemma or ommatidium. Additionally not all ocelli (ocellus in singular) and ommatidia of invertebrates have simple photoreceptors; many, including the ommatidia of most insects and the central eyes of Solifugae have various forms of retinula, and the Salticidae and some other predatory spiders with seemingly simple eyes, emulate retinal vision in various ways. Many insects have unambiguously compound eyes consisting of multiple lenses (up to tens of thousands), but achieve an effect similar to that of a camera eye, in that each ommatidium lens focuses light onto a number of neighbouring retinulae.
Some jellyfish, sea stars, flatworms, and ribbonworms have the simplest "eyes" – pigment spot ocelli – which have randomly distributed pigment, and which have no other structure (such as a cornea, or lens).
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Explores the electromagnetic spectrum, optical sensors, human retina, color synthesis, light polarization, photon-material interaction, and X-ray interactions.
Covers the processing of visual information in the eye, the anatomy of the retina, the biochemistry of photoreceptors, and the use of implants to treat visual impairments.
Insects (from Latin insectum) are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body (head, thorax and abdomen), three pairs of jointed legs, compound eyes and one pair of antennae. Their blood is not totally contained in vessels; some circulates in an open cavity known as the haemocoel. Insects are the most diverse group of animals; they include more than a million described species and represent more than half of all known living organisms.
Arthropods (ˈɑːrθrəpɒd, (gen. ποδός)) are invertebrate animals in the phylum Arthropoda. They possess an exoskeleton with a cuticle made of chitin, often mineralised with calcium carbonate, a segmented body, and paired jointed appendages. In order to keep growing, they must go through stages of moulting, a process by which they shed their exoskeleton to reveal a new one. They are an extremely diverse group, with up to 10 million species. Haemolymph is the analogue of blood for arthropods.
Spiders (order Araneae) are air-breathing arthropods that have eight legs, chelicerae with fangs generally able to inject venom, and spinnerets that extrude silk. They are the largest order of arachnids and rank seventh in total species diversity among all orders of organisms. Spiders are found worldwide on every continent except for Antarctica, and have become established in nearly every land habitat. , 50,356 spider species in 132 families have been recorded by taxonomists.
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