646 RADIATION BIOLOGY 



Demoll and Scheuring (1912) analyzed the visual fields of ocelli and 

 compared these with the fields of the corresponding compound eyes. 

 Since there is so much correspondence, they concluded that the ocelh 

 must furnish supplemental information. Negative response to light 

 appeared to be mediated through the ocelli of May flies (Alverdes, 1923, 

 1924, 1927). Dragonflies soar and land with reasonable precision when 

 their compound eyes are blackened but their ocelli are left intact (Tirala, 

 1923). The well-developed ocelh of the cockroach Periplaneta seem 

 somewhat more reliable than dermal photosensitivity in allowing estab- 

 hshment of conditioned responses to light stimuh (Brecher, 1929). In 

 Drosophila the photokinetic effect of light stimulation appears stronger 

 when the ocelli are blackened, but no orientation takes place if the com- 

 pound eyes are covered and only the ocelli are exposed (Bozler, 1925). 

 Evidence of ocellar function in ants seems conflicting: Forel (1904) 

 believed the ocelli to be valuable for near vision and in dark cavities; 

 Caesar (1913) considered them useful only in respect to far objects while 

 in flight; Mueller (1931) found that orientation to light vanished if only 

 the ocelli were exposed, but the number of ocelli exposed (if any one was 

 uncovered) made no appreciable difference in the reactions mediated 

 through the compound eyes; blinding all ocelli caused changes in response 

 similar to those reported by Bozler (1925) for Drosophila. 



Wolsky (1930, 1931a,b) tried to correlate the dioptric system of insect 

 ocelli with the activities of the respective insects. In the honeybee the 

 lenses operated at f/2.3, f/2.6, and f/3 in the queen, drone, and worker, 

 respectively. But since the receptors are not at the focal point for dis- 

 tant vision, Wolsky concluded that no image could be interpreted through 

 the ocelU. This corresponds to the calculations of Tuempel (1912) and 

 the resolution estimate of Hess (1920c). Among Hymenoptera, Goetze 

 (1927) suggested that the primitive number of ocelh was three, that they 

 were usually larger in males than in females, and that degeneration was 

 frequent among nonflying forms. In these, however, whenever com- 

 pound eyes had degenerated to rudiments, ocelli often remained. Other- 

 wise in flightless forms well-developed ocelli and well-developed com- 

 pound eyes commonly went together. 



Aggregated ocelli take the place of compound eyes in the males of 

 Xenos (Strepsiptera), according to Strohm (1910) and Roesch (1913). 



Compound Eyes. According to Hanstroem (1926), arthropod com- 

 pound eyes all arise from a lateral ectodermal mass in the embryo. They 

 are found in crustaceans, trilobites, xiphosurans, eurypterids, many fossil 

 chilopods and diplopods, the "house" centipede Scutigera and a few 

 related modern genera, and most insects (including the degenerate 

 "aggregated ocelli" of Strepsiptera). More degenerate still are the lat- 

 eral eyes of certain crustaceans, certain trilobites, the remaining modern 

 arachnoids, and most chilopods and all diplopods. The steps whereby 



