640 



HANDBOOK OF PHVSIOLOGV 



NEUROPHYSIOLOGY I 



pillar (235); in the ommatidia of crustaceans and 

 insects (15, 46, 248, 254, 289, 309); and in the retina 

 of cephalopod camera-style eyes (103, 106, 223). The 

 redistribution serves to reduce the proportion of in- 

 tense light reaching the receptor cells and to increase 

 the proportion of dim light passing to retinal level. 



In most insects that are active by day the pigment 

 lies between the receptor cells when light intensity is 

 high, and migrates below the basement membrane 

 when the intensity is reduced. In mantid orthopterans 

 and sphingid lepidopterans the mechanism is more 

 like that in decapod crustaceans. During daylight the 

 pigment is spread parallel to the crystalline cones and 

 maintains isolation of one ommatidium from the ne.xt 

 in typical apposition-eye organization; at night the 

 pigment becomes concentrated distally, giving the eye 

 a far darker appearance and permitting it to function 

 on the superposition principle. 



Either type of pigment movement may expose a 

 reflecting layer in the eye. This may be either a 

 'basement tapetum' which serves to increase sensi- 

 tivity and contrast at low light intensities by reflecting 

 nonabsorbed incident light back through the receptor 

 cells, or an 'iris tapetum' which reflects energy out of 

 the eye again before it has reached the receptor-cell 

 level. The latter is more developed among crustaceans 

 (291), although found in some insects as well. If a 

 basement tapetum is hidden by pigment movement 

 at higher intensities of light, it is an 'occlusible tape- 

 turn' analogous to that found in some fish. No regu- 

 larity is noticeable in either the chemical nature of 

 the reflecting pigment or its systematic position. In 

 Limulus the iris tapetum contains only guanine (146); 

 the closely-related .xiphosuran Tachypleus lacks a tape- 

 tum of any kind (284}; the crayfish Astacus has an iris 

 tapetum of uric acid (147); and the lobster Homarus 

 one in which uric acid is supplemented by at least 

 three additional substances, none of which is guanine 

 (146). 



So far, tapeta have been recognized either from eye 

 histology or 'eyeshine' in only two phyla. Among 

 moUusks it is present in the pelecypods Pecten (233) 

 and Cardium (216). Among arthropods it is widespread 

 in crustacean and insect ommatidia, in the ocelli of 

 certain insects (216) and in the secondary ocelli of 

 many spiders. 



Spectral Sfnsitivity and Color I 'isiun 



Paralleling the spectral absorption characteristics 

 of the photosensitive pigment in a receptor system is 

 a spectral sensitivity shown through ner\e impulses or 



responses in effector systems. With care an action 

 spectrum can be plotted showing the energy required 

 in a light stimulus at each wavelength in a series of 

 tests to find the threshold of response. This graph is a 

 spectral sensitivity curve; it regularly shows one or 

 more maxima. The only exception reported, Hydra'f. 

 response to light (91), appears to be a uniform reac- 

 tion at all wavelengths. 



Even where two receptor systems are present in the 

 same eye, there is no a priori rea.son to expect them 

 to have different photosensitive pigments and hence 

 a single action curve. In many vertebrate eyes the rod 

 mechanism and the cone mechanism are known to 

 have different spectral sensitivities, evident as a 

 'Purkinje shift' in the wavelength of maximum sensi- 

 tivity and in the limits of the effective spectrum as the 

 intensity is altered — reduced until the cones are in- 

 active or raised until they dominate. A Purkinje 

 shift has been detected in only one invertebrate so far 

 (72), the fruit fly Drosopliita. 



A dual mechanism in the eye and a Purkinje shift 

 does not indicate color vision; the dog has a Purkinje 

 shift yet is color blind. Color \ision depends upon dif- 

 ferential mechanisms in the brain to which nerve 

 impulses go separately from two or more unlike series 

 of receptors active in the same intensity range. Color 

 \ision enables an organism to distinguish between 

 radiant stimuli on the basis of inequalities of energy 

 content at diiTerent wavelengths rather than upon 

 intensity alone. A color-blind organism may distin- 

 guish between a series of grays but will confuse any 

 color with some one shade of gray since only intensity 

 discrimination is possible. The xiphosuran Limulus has 

 been shown to have the peripheral basis for color 

 vision (76) in that some ommatidia have greater 

 sensiti\ity toward longer wavelengths, .some toward 

 shorter \va\elengths; apparently this differential sen- 

 sitivity at the ommatidial le\el is not used by the 

 central nervous system since no discrimination be- 

 tween a spectral hue and a neutral .source seems pos- 

 sible except on an intensity basis. 



Scarcely any two individuals, let alone any two 

 species, show the same range of spectral response. The 

 human eye is regarded as sensitive to wavelengths 

 from the extreme violet sensation at 400 m/i to the 

 extreme red at 700 niju. Many invertebrates are .sensi- 

 tive to wavelengths designated as ultraviolet (shorter 

 than 400 m/u), even when these are not a normal part 

 of their environment (as among aquatic organisms 

 which are protected from radiation of this type by the 

 spectral absorption characteristics of water). Many 

 insects, which are active in sunlight containing; ultra- 



