The ommatidium of the pyralid moth Ephestia has ten similar retinula cells and one basal cell (with rhabdomere). A long rod of rhabdom material of high refractive index, contributed by all ten retinula cells, runs up the centre of the column of ten retinula cells and ends at the crystalline tract in the light-adapted eye. Considerable movements of cells occur on adaptation. In the dark-adapted eye the retinula cell columns stretch from the cone tip to the rhabdom layer, with no crystalline tract. In the light-adapted eye a crystalline tract, surrounded by principal pigment cells, extends from the cone. The retinula cell columns appear histologically as if they contain more protein than the surrounding accessory cells. This has an optical significance in providing a further light guide outside the rhabdom extension. A layer of bacteria-like bodies lies between the extensive tapetum of trachea and the basement membrane.