Morphological changes in the ommatidia of an ant in the day and night states

Morphological changes in dioptric structures, in the position of screening pigments and in the microvillar arrangement of the rhabdom were observed in Atta sexdens ant ommatidia exposed to different light/dark schedules. During the day there was a funneling of the crystalline cone and the pigments were close to the rhabdom. At night the crystalline cone became thicker and shorter than in the day state and the pigments moved away from the rhabdom. Endogenous control for these changes was demonstrated in ants kept in continuous darkness. A small but significant contribution of extraocular brain structures to the modulation of pigment position was also demonstrated.