582 



THE EYE IN EVOLUTION 



Caterpillar 



Sarcophagn 



ear consists of a pit filled with air or fluid across the opening of which is stretched 

 a drum-like membrane. In some Orthoptera the ears are on the shanks of the 

 front pair of legs or on the sides of the abdomen above the base of the third legs ; 

 in others on the first segment of the body ; in blow-flies under the bases of the 

 wings ; in gnats on the bases of the antennae ; and so on. In all the sense organs 

 there is a considerable variation between species, while there may well be one 

 or more types of sense organs with which we are not familiar that have no 

 counterpart in the vertebrate sensorium. 



The visual function of the larv^ of insects is relatively crude, 

 a necessary corollary of the simplicity of the structure of the stemmata. 

 In the more simple forms a crude sensitivity to light is the only possible 

 response, but in the more elaborate forms, particularly when the eyes 

 occur in groups, a coarse mosaic imagery with some degree of form 

 vision is possible.^ It may well be that the pendular movements of the 

 anterior part of the body exhibited by so many caterpillars are an 

 expedient to mediate form vision by scanning movements with the 

 simple apparatus available, the visual impressions being perhaps 

 coordinated with proprioceptive stimuli derived from the motion. The 

 entire group of stemmata functions as a unit and if all are covered 

 except one, form perception is lost and only phototactic responses 

 remain (Friederichs, 1931 ; Dethier, 1942-43). The fact that the 

 caterpillars of butterflies {Va7iessa) are attracted by green leaves or 

 paper of the same colour suggests the possibility of a crude colour 

 sense (Gotz, 1936). Finally, the stemmata of some species are capable 

 of utilizing the pattern of polarization of light as a means of orientation. ^ 



The function of the dorsal ocelli of adults is more proble- 

 matical ; since their principal focus does not coincide with the retinal 

 plane, they are ill-designed for image-formation although well adapted 

 to admit hght (Homann, 1924 ; Wolsky, 1930-31 ; Cornwell, 1955). 

 Any capacity for the perception of form is therefore probably negligible. 

 In view of the facts that some insects with only their ocelli uncovered 

 behave as if blind and that the reflex responses of the compound eyes 

 to light are less rapid when the ocelli are covered, it has been suggested 

 that the ocelli are stimulatory organs which accentuate, although they 

 do not initiate, phototactic responses. ^ In other species, however, 

 they have been shown to participate fully in the activities of the 

 animal,* while they are the only effective organs in those species in 

 which compound eyes are lacking.^ Moreover, it was shown by Welling- 

 ton (1953) that the ocelli of the flesh-fly, Sarcophaga, are sensitive to 



1 Larvse of the tussock-moth, Lymantria — de Lepiney (1928) ; of the beetle, 

 Cicindela — Friederichs (1931). 



2 Saw-fly, butterfly— Wellington et al. (1951), Wellington (1953) (p. 66). 



3 In ants — Homann (1924) ; bees — Mliller (1931) ; the fly, Drosophila — Bozler 

 (1925), Parry (1947), Cornwell (1955). 



^ In the bug, Cryptoti/mpana — Chen and Young (1943) ; the flesh-fly, Sarcophaga 

 --Wellington (1953). 

 5 p. 221. 



