652 RADIATION BIOLOGY 



summation can permit its detection at a lower threshold level than would 

 be found in the same target moving vertically in the visual field. For 

 the fly Calliphora electroretinograms bore out this relationship. It must 

 be regarded as an astigmatism of the individual ommatidium. 



The extreme of difference between omniatidia in a compound eye is 

 met in some May flies (Zimmer, 1897; Priesner, 1916; Grabenhorst, 1930), 

 in the dipterans Bibio and Blepharocera (Kellogg, 1898; Dietrich, 1907, 

 1909), and in the aleuronid bugs (Weber, 1934). In most of these the 

 phenomenon is restricted to the male, but in Blepharocera the disparity 

 in ommatidial dimensions is characteristic of part of the female popu- 

 lation. In these arthropods the upper portion of the compound eye 

 forms a sort of "turban," with elongated ommatidia having only sHght 

 angular divergence; the rest of the eye is of shorter ommatidia, diverging 

 more markedly. Kellogg observed that the females with divided eyes 

 also had piercing mouth parts and bloodsucking habits, whereas females 

 with normal eyes (all ommatidia with the same-sized facets) lacked man- 

 dibles and fed on nectar. He pointed out further that among the crusta- 

 ceans illustrated by Chun (1896) were several showing major differences 

 in ommatidial dimensions, and that all of these with "divided" eyes were 

 predators. May-fly eyes, by contrast, seem to be adaptations permitting 

 the male to be the aggressor in twilight mating dances. The male detects 

 a female against the dim light of the sky above in time to seize her and 

 carry her off. 



A very different interpretation for divided eyes was offered by Radl 

 (1901, 1902). In his view the two parts are of separate embryologieal 

 origin, and the phylogenetic dissimilarities postulated led him to draw up 

 a "duplicity theory" for the arthropod compound eye. Zavfel, studying 

 various dipterans (1907), extended the hypothesis into a "triplicity" 

 theory. But later workers have not supported either of these ideas 

 (Dietrich, 1909; Priesner, 1916; Grabenhorst, 1930). 



By sectioning arthropod heads through the compound eyes, it is possi- 

 ble to demonstrate which ommatidial axes converge toward those of the 

 opposite eye, permitting a binocular field (Demoll, 1909c). Another 

 technique, more satisfactory in some respects, is to examine the living 

 eyes for directions from which a false pupil can be seen in each. The 

 false pupil consists of a group of seven or more ommatidia facing directly 

 enough toward an observing eye or camera lens so that light entering 

 them from the direction of the observing eye or lens passes down the 

 pigment-cell sheath and is absorbed by the pigment and receptors. It is 

 evident as a dark spot that migrates in position according to the position 

 of the observer. In a cylindrical compound eye, such as that of the ghost 

 crab Ocypoda, the false pupil seen in horizontal directions is a vertical 

 line the length of the cylindrical portion of the eye. Binocular fields are 

 common among predaceous crustaceans and insects and appear to be 



