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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY 



cluster is not clear from embryological studies (78). 

 A comparable suggestion of multiple origin for the 

 one to three ocelli in pelagic salps has received no 

 support. 



COMPOUND OCELLI OR AGGREGATE EYES. The grouping 



of separate ocelli, each with its own retina and pig- 

 ment cup, into a pattern with roughly radial diver- 

 gence of the optic axes seems to have arisen inde- 

 pendently in many phyla through convergent 

 evolution. In coelenterates a number of scyphozoans 

 (cubomedusae) exhibit this arrangement (132). It is 

 characteristic of chaetognaths in which three ocelli 

 are clustered in each of two groups (120, 124). 

 Among arthropods, compound ocelli resemble a 

 compound eye in many millipedes and in males of the 

 insect order Strepsiptera (227, 246). The millipede 

 Narceus is hatched with only a single ocellus but adds 

 others in a triangular area on each side of the head 

 until a total of between 40 and 50 are present a few 

 instars before sexual maturity (32). 



Many starfish (asteroid echinoderms) bear a cluster 

 of ocelli at the tip of their arms. Muscular movements 

 of the arm tissues can alter somewhat the relative 

 orientation of the separate ocelli (296), Ijut the form 

 of each photosensory unit appears fixed. A large 

 central dioptric body must serve to concentrate radia- 

 tions upon the receptor cells (242, 258), and photo- 

 sensory functions seems indicated by the slow action 

 potentials which develop upon illumination of the 

 ocellar area (90). Some movements related to the 

 direction of lateral illumination and of shadows have 

 been described, but at least some of these may be 

 due to a general photosensitivity in the dermis. The 

 presence of carotenoid pigments in the compound 

 ocellar tissues (192) could relate to a photosensitive 

 substance. Alternatively these pigments may serve as 

 filters which affect the spectral sensitivity of the 

 organism. 



STEMMATA. In the larvae of many members of the 

 holometabolous insect orders, Neuroptera (j. lat^, 

 Coleoptera, Lepidoptera, Trichoptera, Diptera and 

 Hymenoptera, photosensory structures resembling 

 isolated ommatidia are present. They disappear at 

 metamorphosis and have no relationship to the 

 compound eyes of the adult stage. Their distinctness 

 from an ontogenetic standpoint led Landois (160) to 

 consider them as an independent type of eye; he 

 called them 'composite eyes,' but the term stemmata 

 has been approved more widely. 



Anatomical details have been described for those 



of a larval water beetle Acilius (236), a lepidopteran 

 caterpillar Isia (54, 55), a mosquito wriggler Cukx 

 (35) and several sawfly larvae (Hymenoptera) in a 

 comparative study by Cornell (36). For the cater- 

 pillar, Dethier considered the diopteric system and 

 found that a stemma with a one-part lens had an 

 effective aperture between f 0.5 and f/i.o, whereas 

 those with a three-part lens were slightly less spec- 

 tacular collectors of li»ht with effective apertures be- 

 tween i.o and 1.5. In all instances the caterpillar 

 stemma had seven receptor cells arranged at two 

 levels, a distal clump of three and a proximal group 

 of four. No matter whether the corneal lens was 

 simple or tripartite, only a single crystalline body was 

 below it, close to the distal group of receptor cells. 

 The stemmata were fi.xed in the firm head capsule 

 at such angles to each other that their fields did not 

 overlap. Dethier concluded that a coarse type of 

 mosaic vision was possible. 



Many caterpillars show clear responses to distant 

 trees. Those of the nun moth Lymantria under ex- 

 perimental conditions will react to and approach 

 pillars and vertical stripes of paint (131), whereas 

 horizontal patterns seem to be ignored. Hundert- 

 mark, who explored this problem thoroughly, con- 

 cluded that dark vertical patterns stimulated the 

 larvae while their heads were being swung from side 

 to side — a characteristic gesture of these caterpillars. 

 Stimulation would then correspond to patterns cross- 

 ing the visual field of stemma after stemma, and the 

 astigmatism noted would have a basis in behavior 

 rather than in structure. 



COMPOUND EYES. True compound eyes are restricted 

 to arthropods (fig. 6, lower right) and are represented 

 among crustaceans, trilobites, xiphosurans, euryp- 

 terids, inany fossil chilopods and diplopods, the 

 centipede Scutigera and close allies, and most insects. 

 Holometabolous insects possess them only as adults. 

 In all situations they present a much more eflfective 

 organization than compoimd eyespots or compound 

 ocelli Ijut show the same quasi-radial dixeraience of 

 visual units. 



According to Hanstrom (82), all true compound 

 eyes arise from a lateral ectodermal mass in the 

 embryo. In following these embryonic steps toward 

 the final battery of ommatidia, Watase (278) recog- 

 nized no major variants in development. One rather 

 fundamental difference has been overlooked in these 

 and subsequent studies: in Limulus (and presumably 

 all xiphosurans, perhaps eurypterids as well), the 

 entire dioptric mechanism is molted. Other arthro- 



