2gg Confparative Animal Physiology 



ula cells. In many nocturnal insects (e.g., Noctuid moths), tracheae are 

 grouped around the retinula cells and especially at the basal end of the 

 rhabdom. These tracheae reflect light distally through the ommatidium and 

 serve the same function as the tapetum lucidum of vertebrates (page 398). 

 In higher Crustacea the retinula cells contain guanin crystals which act as 

 reflectino surfaces just as they do in the tapetum lucidum of vertebrates. 



In inspects the whole ommatidium is surrounded by a sheath of pigmented 

 cells made up of two sections: the "primary iris cells" around the crystalline 

 cone, and the "secondary iris cells," which may cover both the primary iris 

 cells and the retinula cells. The proximal ends of the ommatidia rest on a 

 fenestrated membrane through which the axons and tracheae pass. In prim- 

 itive Crustacea (e.g., Gavimarus^, the retinula cells contain all of the iris 

 pigment and extend distally to surround the crystalline cone. In higher 

 Crustacea (e.g., shrimp, crayfish), the basal iris pigment is in the retinula 

 cells, but the distal pigment is in separate cells. 



Diurnal Movements of Eye Pigments. In all compound eyes there is a 

 movement of pigment with light and dark adaptation.^^- The arrange- 

 ment is often such that the eye functions as an appositional eye"*" when light 

 adapted and as a superpositional eye when dark adapted (see page 390). In 

 addition, there are rhythmic diurnal pigment migrations which are independ- 

 ent of light intensity; these persist independendy of direct changes in the 

 environmental illumination. 



Persistent diurnal rhythm of pigment migration occurs in the eye of Palae- 

 monetes, a shrimp.^"^ On the left in Figure 289 (Ch. 22) is shown the posi- 

 tion of the pigment in animals kept in constant light, and, on the right, that 

 in animals kept in constant darkness. The upper figures show the position 

 of pigment during the day, and the lower ones during the night. In animals 

 kept in constant darkness the distal pigment moves outward at night and 

 inward during the day; the basal pigment in the retinula cells moves out- 

 ward during the dav and inward, largelv below the basal membrane, at 

 night. Light adaptation brings about the diff'erences apparent upon compar- 

 ing the right and left diagrams in the figure. In dark-adapted animals it is 

 evident that the type of image formed is appositional during the day and 

 superpositional during the night. In light-adapted animals the image is cer- 

 tainly appositional during the day and probably also at night. The reflecting 

 pigment (R) is unprotected from light during the night but is covered dur- 

 ing the dav. This permits the eye to glow at night but not during the day. 

 The effect is readilv observed in cravfish, which, if kept in constant dark- 

 ness, will exhibit an orange eyeshinc at night but not during the day. This 

 diurnal rhythm of eyeshine may persist for months under conditions of con- 

 stant darkness and temperature.'^- ^'^'^ 



The persistent diurnal rhythm of pigment migration was discovered in 

 the noctuid moth, Phtsifl,^^^ aud later was seen in numerous crustaceans: 

 Macrohrachium,^*'*'- ^""^ CaviharnsJ two macrurans and tour brachyurans,"^ 

 and Anchistioides, Camhanis, and Hounnis.^*"''* Such a rhythm may be estab- 

 lished by diurnal environmental changes. These changes must determine 



* In an appositional eye the retinula cells of an ommatidium are stimulated only by 

 light entering; through that ommatidium, whereas in a superpositional eye the retinula 

 cells are stimulated by light coming through neighboring ommatidia as well. 



