636 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



ous hynienopteran insects. Wolsky C302) sought in 

 vain to find an analyzer function in tiie corneal lenses 

 of the coarse compound eyes of land isopod crusta- 

 ceans, von Frisch credited the actual demonstration 

 of polarization sensitivity to Autruni, although von 

 Frisch's own reports (271, 272) antedate any of 

 Autrum's published comments on the subject, von 

 Frisch has found in the honeybee's '.sky compass' a 

 basis for the ability of one bee to communicate to 

 another by dancing within the dark hi\e the direction 

 to a discovered supply of food (273). 



Since von Frisch used an octagon of Polaroid film 

 cut into svmmetrically fitted equilateral triangles, it 

 was natural that he should parallel this with the ring 

 of receptor cells in each insect ommatidium. When 

 the octagon was held toward the blue .sky, some one 

 diagonal was darkest, another lighest. The former 

 corresponded to Polaroid triangles the axes of which 

 were transverse to the plane of polarization in the 

 sky area seen through the plastic. Possibly an image 

 of comparable type was cast upon the ring of receptor 

 cells and information of this kind interpreted ijy a 

 single ommatidium. 



In the dipteran VohueUa, a ring of eight receptor 

 cells and birefringence with extinction of one ray 

 were reported (186), but the reality of the phe- 

 nomenon described is open to question. Certainly 

 each ommatidium may vary in its response to light 

 as the plane of polarization of incident radiation is 

 rotated around its optic axis, as may occur in Limulus 

 (279, 280, 282, 283) or in Drosoplula (244). But struc- 

 tural asymmetries of dioptric components in indi- 

 vidual ommatidia and the oL)liquit\ with which 

 many ommatidia in each eye meet the outer surface 

 seem more responsible for the sensitivity found to the 

 plane of polarization. The extent to which polarized 

 light is available as a cue useful in arthropod naviga- 

 tion both in air and in water has been described by 

 Waterman (280, 285). It has been shown experimen- 

 tally (12) to be significant in the free beha\ior of 

 fresh-water planktonic crustaceans. 



In the earliest comprehensive account of the arthro- 

 pod compound eye, Grenacher (81) recognized a 

 difference in the distribution of pigment cells accord- 

 ing to whether the organism was a day-active type 

 or a crepuscular and night-active organism. In most 

 of the latter, the pigment is not extended as a sheath 

 isolating each ommatidium from the next but is 

 clumped in such a way that light could pass obliquely 

 from ommatidium to ommatidium. Exner (67, 68) 

 traced the ray paths, and showed by diagrams how 

 light entering many ommatidial lenses could be re- 



fracted and fall on the receptors of a central visual 

 unit. Grenacher's terms apposition' type for the eye 

 with isolated ommatidia and 'superposition' type for 

 the eyes used in dim illumination have been retained. 



The same ommatidium may function alone by day 

 and in concert by night through migratory move- 

 ments of its pigment (145, 248). In crustaceans these 

 changes in the eye are often matched by alterations 

 in body color, the entire chromatophore system being 

 imdcr the control of hormones whose secretion is in- 

 fluenced by stimulation of the eyes by light (207). 

 The literature on this subject has become extensive 

 l)ut most of it centers on hormonal aspects. In insects 

 the corresponding shifts in ommatidial pigment may 

 be independent of hormones (44, 49). 



Normal structure of compound eyes has required 

 extensive study because of the large numJDer of varia- 

 tions within the wealth of genera in the phylum 

 Arthropoda. Where possible, many writers on the 

 subject have attempted to correlate form with func- 

 tion (30, 31, 48, 51, 61, 62, 119, 250, 268, 269). 

 Numerous crustaceans bear their eyes on movable 

 eyestalks and show compensatory movements of these 

 when the animal or its visual field is rotated. Branchio- 

 pods show all gradations between a distinct pair of 

 compound eyes and indistinguishable fusion into a 

 single mass. The fused median compound eye of 

 Daphnia consists of al)Out 20 ommatidia and is some- 

 what unusual in that it can be rotated several degrees 

 within the body through the action of a series of 

 oculomotor muscles. 



Ostracod compound eyes are commonly separate 

 if a median ocellus is present but fused if the ocellus 

 is lacking. Some lack compoimd eyes entirely. The 

 luminescent Cypndina, howe\er, has full)- developed 

 eyes. 



Copepod compound eyes range from the median 

 fused structure of Cyclops and Calamis through genera 

 in which the two are completely separate. Branchiuran 

 compound eyes must be regarded as degenerate. 

 Argulus has four eye types present in each individual. 

 Barnacles ha\e compound eyes onl\ during the meta- 

 nauplian stage (69). .'\mong chilopods only Scuttgera 

 and related genera possess compound eyes (84). Here 

 each eye consists of not more than 200 ommatidia, 

 each with two rings of receptor cells as in the thysanu- 

 ran insect Lepisina. 



Growth of compound eyes is inferred among trilo- 

 bites because of the gradual increase in number ot 

 ommatidia found to accompany increase in body 

 size within each species (226). In Limulus and other 

 xiphosurans, ijoth the number of ommatidia and the 



