PHOTOSENSITIVITY IN INVERTEBRATES 



641 



violet, are more sensitive to this part of the solar 

 spectrum than to the region visible to man (17, 18, 

 1 74, 1 75). In consequence it becomes important for 

 man to learn more of what reflects ultraviolet, and 

 hence may be visible to insects though not to him 

 (26, 38, 175, 176). 



Amebas travel as rapidly in the presence of radia- 

 tions of long wavelengths (red) as in darkness but are 

 increasingly sensitive as the wavelength of a stimulus 

 is shortened (86). Paramecium tends to swim upwards in 

 darkness, downward in light, and the direction is 

 altered most effectively by shorter wavelengths (73). 



The platyhelminth Planaria has been studied ex- 

 tensively in responses to spectral distribution in light 

 stimuli. Erhardt (64) was able to account for earlier 

 claims (19, 115) that Planaria had color vision upon 

 intensity discrimination. Werner (292) concluded 

 that much of the flatworm's response to ultraviolet 

 arose through general photosensitivity and not the 

 eyes; but Merker & Gilbert (187) found only non- 

 directional kinetic responses when the eyes were 

 removed, compared to a definite orientation with a 

 single ocellus intact. They were able to plot the visual 

 fields of Planaria toward ultnn iolet and believed that 

 responses were to the wavelengths used (366 to 313 

 m/j) rather than any secondary fluorescence. 



Two separate receptor systems were described for 

 the earthworm (255). One, mediating the shadow 

 reaction, was most sensitive in the yellow portion of 

 the spectrum and depended upon receptors distrib- 

 uted uniformly in the skin. The other, a more general 

 photosensitivity related to rate of locomotion and the 

 like, showed greatest sensitivity in the blue and was 

 most developed toward the two ends of the body. In 

 the leech Pisricola, pigment migration in surface chro- 

 matophores is an effector demonstration for which a 

 spectral action curve can be drawn (140). 



Using the threshold for retraction of the siphon as 

 a kinetic cue to photosensitivity in the pelecypod Mya, 

 Hecht (95) obtained a spectral action curve with 

 limits somewhat short of those for the human eye. 

 Its maximum fell at 500 inn, suggesting that the 

 neuronal photoreceptors in the mantle tissue of the 

 clam have a photosensitive pigment similar to tho.se 

 extracted from organized eyes. 



The fresh-water planktonic crustacean Daphnia ap- 

 pears to have at least three photosensory systems, one 

 with greatest sensitivity in the ultraviolet (257), one 

 in the yellow and the third in the blue. Only the 

 latter two can have much importance under natural 

 conditions (240). The response with maximum sensi- 

 tivity to yellow is a positive horizontal swimming 



toward the radiant source. The response to blue is 

 negative. Baylor and Smith at the University of 

 Michigan have used the yellow and blue responses in 

 an underwater trap which catches a wide variety of 

 plankton organisms, crustaceans, acarid arachnids and 

 insect larvae. Possibly photosensory mechanisms of 

 this kind are involved in the \ertical migrations made 

 daily by many types of plankton, down during day- 

 light, up at night. 



Although the arthropod cuticle transmits freely a 

 wide range of radiations from infrared to ultraviolet, 

 only certain fireflies (nocturnal Coleoptera) have been 

 found to respond to infrared stimuli (28). By painting 

 the eyes of various butterflies with a clear red lacquer, 

 Eltringham (63) was able to show that some kinds 

 flew about naturally — able to see in red light — 

 whereas others behaved as though blinded. 



Sensitivity to ultraviolet is pronounced in most 

 insects, and shown by many lar\ae as well (139, 184). 

 Bertholf (17, 18) found a bimodal curve represented 

 the spectral sensitivity of the honeybee. The peak in 

 the ultraviolet was far higher than that in the spectrum 

 visible to man and explained why these insects re- 

 spond more to cues in ultraviolet components of sun- 

 light than to reflectances visible to man. Lutz (174- 

 176) examined the ultraviolet world of the insect and 

 was aljle to produce conditioned responses in tropical 

 hymcnopterans (175) to patterns in white paints when 

 one white reflected ultraviolet and another did not. 



Conditioned responses in honeybees demonstrate 

 that these insects do have color vision (270, 273). They 

 can be trained to come for food to line spectra regard- 

 less of relative intensity (154-158). But a majority of 

 insects, particularly the night-active ones, probably 

 show no color vision, merely intensity discrimination 

 based on a simple spectral-.sensiti\ity curve (288). 

 This may be modified from one genetic strain to 

 another according to the eye pigments present and 

 changes in the eye structure itself (71). 



A neural basis for color vision has been described 

 in insects (222, 230, 231); but whether even day- 

 active species, operating in good light, make use of 

 cues reaching them from differential mechanisms in 

 the ommatidia is a point which inust be established 

 separately for each kind. 



Form Perceplinn and Pattern Recognition 



If both the photosensory mechanism and the nerv- 

 ous system are sutticiently well organized and co- 

 ordinated, the animal can give evidence of an aware- 

 ness of surrounding events that is close to, if not 



