In the Diptera most of the positively photosensitive species occur in the Nematocera, 

 particularly in the Chironomidae, Ceratopogonidae, and Culicidae ( sens , lat .). Among 

 the B rachycer a, members of the family Pyrogotidae and the tribe Ormiini of the Tachinidae 

 are attracted to lights. These flies are parasitic on nocturnal insects and are usually 

 active at dusk or at night. Other photopositive B r a c h y c e r a occur in the Empidae, 

 Lonchopteridae, and Sphaeroceridae. 



In the Hemiptera the sternorrhynchous Homoptera are little attracted by induced 

 light, although aphids may be more strongly photopositive than has been suspected. Of 

 the auchenorrhynchous Homoptera the leafhoppers and fulgoroids, particularly the former, 

 contain many species that are positively photosensitive. Among the terrestrial Heteroptera, 

 several lygaeids, mirids, and cydnids are strongly attracted to light, as are most corixids 

 and some other aquatic species. Usually males of the terrestrial Hemiptera respond 

 more frequently than females. 



Present evidence regarding the attraction of insects to. light is unsatisfactory for a 

 critical appraisal of the phenomenon. Most published references, particularly the older 

 ones, give little information except that certain species came to light. Knowledge of other 

 species present in the area, but not attracted, is almost invariably lacking, as is in- 

 formation regarding population levels of species that show positive responses. With few 

 exceptions, the optimum conditions for attracting a given species of insect are not 

 known. 



HOW INSECTS ARE PHOTOSENSITIVE 



[SUMMARY] 

 Roy J. Barker 1 



Insect vision is basic to orientation, movement, and consequently insect environ- 

 ment. Nevertheless, entomologists have left mostly to others the problems of under- 

 standing insect vision. Basic research is needed before success can be expected even 

 from light traps. For example, intensity is usually measured in units (such as foot- 

 candles) based on "normal human observers," which have no physiological meaning when 

 applied to insects. 



General receptors such as simple dermal cells that respond to light probably occur 

 in insects. The eye spot in maggots has some elements of eyes but no iris pigment. 

 Grasshoppers have many areas of cuticle modified evidently to function as heat detectors. 



Simple eyes exhibit extensive variation in morphology. Images are in focus over a 

 wide depth of field, but such images are not necessarily used by the insect. The receptor 

 morphology of insects involves a layer of visual cells that is perpendicular to the lens 

 image, not parallel as in mammals. 



Compound eyes contain packed rods of rhobdomeres stacked radially. In Drosophila 

 each 17 by 100 micron ommatidia contains 17 rhobdomeres. Compound eyes contain from 

 1 to 28,000 ommatidia. Some iris pigment occurs even inside of the visual cells. The 

 presence of numerous mitochondria suggests high metabolism in the visual cells. 



The lens diameter as well as the angle between neighboring ommatidia seems to 

 limit the resolving power of insect eyes. The corneal lens is transparent to ultraviolet 



i Entomologist, Entomology Research Division, Agricultural Research Service, U. S. D. A. 



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