Glial cell of the retina (Edwards and Meinertzhagen, 2010; Liu et al., 2015) that produces screening pigments (Tomlinson, 2012). These pigments form layers around the retina and each ommatidium to prevent lateral transfer of light, ensuring photoreceptors of one ommatidium are activated only by light entering through one lens (Tomlinson, 2012). These cells also have metabolic interactions with photoreceptor neurons, providing alanine (Edwards and Meinertzhagen, 2010) and removing lipids (Liu et al., 2015).