PHOTORECEPTION 207 



the receptors can still separate separate stimuli. Motion is perceived 

 as a vector without spatial connotations when two stimulated omma- 

 tidia are separated by an unstimulated one (Hassenstein, 1951). 



OCELLI 



Many insects possess dorsal ocelli in addition to compound eyes. The 

 distribution of ocelli within the Class is erratic and is of no assistance 

 in the problem of interpreting the role that these organs play. 



The structure is basically similar in all ocelli. It consists of a trans- 

 parent cornea which may be perfectly flat on both surfaces, biconvex, 

 or plano-convex. Underlying the cornea there is frequently a trans- 

 parent layer of corneagen cells and beneath this a layer of retinal cells 

 very similar in structure to those of the compound eyes. In the more 

 complex ocelli the corneagen cells may be thickened to form a aux- 

 iliary refractory organ. The retinal cells are grouped together in units 

 of two, three, or four, and each cell of a unit contributes to a central 

 rhabdom. In the lateral ocellus of Sympetrum there are about 675 

 photoreceptor cells. Beneath them lies a reflecting layer, the tapetum. 

 As the sensory axons proceed proximally they soon form synaptic 

 connexions with the intra-ocellar terminals of the ocellar nerve fibres 

 (Cajal, 1918; Ruck, 1957). The ocellar nerve of the cockroach consists 

 of about twenty-five fibres, of which four are very large - 6-10 \l. 

 In the dragonfly {Sympetrum) there is one giant fibre (25-38 [x) and 

 three or four ranging in diameter from 4-13 [x. The remainder are 

 1 [I or less. The neurocytes lie in the brain (Cajal, 1918 ; Satija, 1958 a, 

 1958 b). 



Although the corneal lens is capable of forming fairly sharp images, 

 everyone who has studied the optics of the system agrees that the image 

 has little significance, since it falls a considerable distance behind the 

 retina (Homann, 1924; Parry, 1947; Cornwall, 1955). 



Electrical events have been studied in the ocelli of Locusta migra- 

 toria migratoroides (Parry, 1947; Hoyle, 1955; Burtt and Catton, 

 1958), Periplaneta americana (Ruck, 1957, 1958 a; Goldsmith and 

 Ruck, 1958), Blaberus craniifer (Ruck, 1957, 1961a), Melanoplus 

 bivittatus (Ruck, 1957), Apis mellifera (Goldsmith and Ruck, 1958; 

 Ruck, 1958 b). Pachydiplax longipennis and Phormia regina (Ruck, 

 1958 b), Libellula luctuosa (Ruck, 1961 a), Libellula vibrans (Ruck, 

 1961 b), Anax Junius, Aeschna sp., and Sympetrum ribicundulum 

 (Ruck, 1961 c). Because different workers employed different record- 

 ing situations it is difficult to compare their results and to reconcile 



