Ocelli (pit-type eyes of arthropods) blur the image across the whole retina, and are consequently excellent at responding to rapid changes in light intensity across the whole visual field; this fast response is further accelerated by the large nerve bundles which rush the information to the brain. Focusing the image would also cause the suns image to be focused on a few receptors, with the possibility of damage under the intense light; shielding the receptors would block out some light and thus reduce their sensitivity. This fast response has led to suggestions that the ocelli of insects are used mainly in flight, because they can be used to detect sudden changes in which way is up (because light, especially UV light which is absorbed by vegetation, usually comes from above). Multiple lenses
Some marine organisms bear more than one lens; for instance the copepod Pontella has three. The outer has a parabolic surface, countering the effects of spherical aberration while allowing a sharp image to be formed. Another copepod, Copilia, has two lenses in each eye, arranged like those in a telescope. Such arrangements are rare and poorly understood, but represent an alternative construction. Multiple lenses are seen in some hunters such as eagles and jumping spiders, which have a refractive cornea: these have a negative lens, enlarging the observed image by up to 50% over the receptor cells, thus increasing their optical resolution.
Psorosperm (from the Greek ψωρα itch and σπερμα seed) is a former name of a number of parasitic protozoa that produce cystlike or sporelike structures in the tissue of hosts. The term is now essentially obsolete. Some that affect vertebrate hosts are now called coccidia. Others, such as the cause of pébrine in silkworms, are now recognized as microsporidians, and some are myxosporidians. The genus Psorospermium (which includes the species Psorospermium haeckeli) itself is a parasite of crayfishes, and belongs to an enigmatic group of unicellular organisms that some biologists think may be related to the common ancestors of animals and fungi.J. Müller introduced the term in German (as Psorospermien) in 1841.Psorosperm was at one point believed to be the cause of Dariers disease. "Psorospermiasis" is classified under 136.4 in ICD-9. == References ==