except during molts; a molting insect is nearly blind. The cone of the eye must provide a 

 clear path of a proper refractive index to conduct light from the lens to the rhobdomere. 

 The iris pigment functions to produce a super-position or an apposition image depending 

 upon the intensity of light. The identified pigments are related to pteridines and many 

 are fluorescent. Perhaps pteridines convert ultraviolet into visible light in insects. The 

 color of the reflecting pigment seems to have a secondary effect on spectral response. 



A pigment must do more than simply bleach in light to be a visual pigment. Retinene-1 

 has been isolated from honeybees. Since most insects do not require dietary vitamin A, 

 the precursors of insect retinene are unknown. Rhodopsin consists of opsin plus retinene. 

 Insect opsin is uniquely water soluble. Spectral response curves for insects indicate 

 that rhodopsin and another visual mechanism are involved. The absorption of some 

 photo-labile pteridines from Drosophila show partial correlation to the spectral response 

 of the strain tested. The complex biochemical reactions of rhodopsin initiated by light 

 occur in microseconds. In converting light to chemical energy, configurational changes 

 of opsin apparently opens a "condensor" of about 1,200 molecules in the stacked rods. 

 One quantum of light can thus release an electric impulse sufficient to depolarize a 

 sense cell with a utilization efficiency of nearly 5 percent. 



Electroretinograms of optic nerves can give a quantitative measure of responses 

 to spectra, polarization, and form. Simple and compound eyes have similar response 

 patterns. When individual ommatidia are tested, synchronous discharges show interaction 

 between the visual units. The frequency of nerve impulses depends upon intensity and is 

 modified by temperature, exposure time, adaptation, and other factors. 



Orientation mechanisms involve intensity discrimination, acuity, form perception, 

 polarization, and color distinction. Flight periods are closely related to a critical in- 

 tensity because insect vision adapts to dark by a sensitivity increase of only 100 times; 

 in man sensitivity increases 10 million times. Insects fly during periods of iris pigment 

 movement because these are the times they can see. Most insects have better vertical 

 than horizontal acuity because the ommatidial angles are smaller vertically. The angle of 

 discrimination and acuity of movement is about 1 percent of that in man. Some fast- 

 flying insects possess diphasic electroretinograms and can distinguish 250 light flickers 

 per second. Other insects have flicker vision more like man (40-50/sec.) and a higher 

 absolute sensitivity to light. This may be related to peculiarities of (insect opsin. In aform 

 perception, the most stimulating patterns match ommatidial angles l°-3°). Moving objects 

 are more attractive; more ommatidia are stimulated. Flickering light is more stimulating 

 than continuous light. (For some insects fluorescent lights are flickering.) 



The important factor in form perception is not shape, but how many ommatidia are 

 stimulated. Discrimination of polarization could result from lens reflection of peripheral 

 vision since polarized light reflects selectively. Guidance by polarized light is con- 

 ceivably guidance by the reflection pattern of the environment and needs no specialized 

 receptor. Color distinction is probably a function of differences in the activation energy 

 of opsin molecules. Pigment analyses of insects trapped at different colors could deter- 

 mine whether insects see ultraviolet because of iris pigment fluorescence or because of 

 the uniqueness of insect opsin. Acuity at some colors is affected by iris pigment color 

 in a manner analogous to wearing sunglasses. 



Loeb's theory of symmetrical stimulation guiding insects to a light source is unneces- 

 sary, because ommatidia are of unequal sensitivity. In keeping the light where it seems 

 brightest, an insect maintains a constant angle to the source. If this angle is less than 

 90°, the insect spirals toward the light, and this occurs even with only one eye function- 

 ing. 



35 



