This study asks whether photomechanical movements in the retinal cells of the lateral eye of the American horseshoe crab, Limulus polyphemus, are controlled locally within each ommatidium, or whether they are a retinal array property involving lateral communication between ommatidia. Three experiments were performed. A small spot, a vertical slit down the center, or the anterior third of an otherwise masked eye was illuminated. The contralateral eye was fully illuminated in each experiment and served as a light-adapted control. Morphometric analyses of aperture length and rhabdom dimensions were made from serial 1-microm plastic sections. The results suggest there is a different spatial threshold for photomechanical movement for aperture lengthening than for rhabdom lengthening. When only a few ommatidia are illuminated, the aperture does not change. When about 10% are illuminated, they lengthen, but the masked ommatidia do not. When about a third are illuminated, all the ommatidia in the eye lengthen together, including the two thirds that were masked. When either only a few ommatidia or about 10% of the ommatidia are illuminated, rhabdom shape is unchanged. When a third of the eye is illuminated, the illuminated rhabdoms lengthen, but the masked rhabdoms do not.