PHOTOSENSITIVITY IN INVERTEBRATES 



631 



pycnogonids — all arising from a dorsal ectodermal 

 mass in the embryo; 6) the lateral ("secondary") 

 ocelli of modern arachnoids and all eyes of diplopods 

 and chilopods, arising through degeneration from 

 the ommatidia of compound eyes produced by the 

 lateral ectodermal mass of the embryo; and f) the 

 ventral ocelli of trilobites and xiphosurans and the 

 median ("primary") eyes of eurypterids and arachnids, 

 arising from a ventral ectodermal mass in the embryo. 

 No clear correlation can be noted between these 

 categories and the detailed anatomical features of 

 the ocelli in postembryonic life — features described 

 and illustrated with great care by Grenacher (81). 



In many planktonic crustaceans the median ocellus 

 is the only eye present. Fundamentally it appears to 

 be a double structure, but fusion may be remarkably 

 complete. Many crustaceans which metamorphose 

 lose their ocelli as they grow. An extreme example is 

 found among barnacles (6g): newly hatched nauplii 

 have a bilobed median ocellus; a pair of compound 

 eyes appears at the metanauplian stage, only to be 

 extruded or to degenerate at metamorphosis; until 

 this time the median ocellus remains unchanged, but 

 then it separates into two, each half migrating into a 

 lateral position and continuing as the sole photo- 

 sensory specialization of the adult. 



The ventral position of the median ocellus in 

 Branchipus, Artemia and other branchiopods, many 

 copepods, some trilobites and larval xiphosurans sug- 

 gests that inverted swimming may be an ancestral 

 habit. Inverted swimming is characteristic of Limulus, 

 Branchipus and Artemia, and probably was also of 

 trilobites. A median ocellus must be of help while 

 dorsal compound eyes are directed toward the bot- 

 tom rather than the sky. Persistent nauplian ocelli 

 are known in some decapod malacostracans. In 

 Artemia the ocelli can serve alone in mediating essen- 

 tially all normal adult responses to light stimulation 

 (171); exceptions, which depend upon function of 

 the compound eyes, are the visual following of females 

 by males and a convulsive reflex when a dark-adapted 

 animal is suddenly illuminated. 



Waterman (281) has provided a convenient table 

 showing the groups of arthropods in which a median 

 ocellus is known. At the same time he presented 

 evidence from electrical recordings indicating that 

 messages pass along the optic nerve fibers from 

 Limulus ocelli comparable to those from the com- 

 pound eyes. Their use by the animars central nervous 

 system remains a mystery. 



Eight ocelli or less are characteristic of spiders 

 (208); the arrangement and actual number varies 



from one genus to another. One pair, the 'primary 

 eyes," are simpler in having a direct retina and no 

 tapetum, although the entire retina may be moved 

 within the body through the contraction of paired 

 muscles — perhaps in following the progress of prey 

 or potential mate. The 'secondary eyes" usually have 

 an inverted retina and often a tapetum; no move- 

 ments of the retina are possible. Nervous connections 

 to the two types of ocelli are consistent with this 

 difference in structure and with Hanstr6m"s generali- 

 zations (82). The role of vision is difhcult to demon- 

 strate (128-130, 217, 294) in spiders, except in jump- 

 ing spiders (127). These have been recommended as 

 ideal laboratory material because they seem so un- 

 aware of confinement. 



Among centipedes and millipedes which have 

 ocelli, no responses to light ha\e been described 

 which could not be accounted for adequately on the 

 basis of a general photosensitivity in the body surface. 



Two, or at most three, ocelli are present in many 

 insects (136, 137), but their role in normal living 

 habits has been a puzzle (23, 52, 53, 108, 118, 126, 

 224). When only the ocelli are exposed, insects usu- 

 ally behave as though completely blinded. Some show 

 responses which cannot be accounted lor on the basis 

 of general photosensitivity (25). Demoll & Scheuring 

 (52) found considerable correspondence between the 

 visual fields of the compound eyes and of the ocelli. 

 This discovery, together with the observation that 

 many insects with their ocelli covered respond more 

 slowly to events in the visual field of their compound 

 eyes, led to the notion that ocelli serve to measure 

 general intensity of illumination and to control the 

 level of tonic contraction in locomotor muscles. 



Variation in proportion of parts and arrangement 

 of retinal cells seems to have little effect in determin- 

 ing the role of insect ocelli (167-170, 303, 304). 

 Some ocelli show a strong retinal astigmatism (fig. 5, 

 lower right'), those in some dragonflies (order Odonata) 

 being particularly pronounced (252). In the orthop- 

 teran Acridium the ocelli are dimorphic in that those 

 of the female alone show a double curvature on the 

 proximal surface of the corneal lens — like a bifocal 

 spectacle lens — producing two images at different 

 distances (253). No explanation is available. 



Ocelli in which the components of three-part 

 lenses lie side by side, like the top of a clover-leaf 

 roll, are found in the larvae of many urochordates. 

 Mast (181) reported photosensory responses of this 

 type of larva until the time of metamorphosis when 

 the ocelli degenerate. Whether the remarkable lenses 

 indicate fusion from an originally triplicate ocellar 



