Novel type of sub-retinal pigment shield in the miniaturized compound eye of Trichogramma evanescens

Pigment granules, found in different cell types of the retina in insect compound eyes, fulfill important functions. They isolate the individual ommatidia from stray light, regulate the angular sensitivity, and restrict the light that reaches the photoreceptor according to ambient light intensities. Descriptions of pigment cells within the retina are included in ultrastructural eye descriptions, but knowledge of pigment cell types beneath the retina and basal matrix (BM) are relatively limited in insects. In the miniaturized parasitoid wasp Trichogramma evanescens Westwood 1833, a sub-retinal pigment shield is formed by pigment-bearing cells, which appear in two-dimensional TEM sections to form a separate population beneath the BM. By using three-dimensional reconstructions of serial-section transmission electron microscopy, it was possible to reveal that the sub-retinal pigment shield of T. evanescens is not formed by a separate cell type, but by extensions of the lateral rim pigment cells that penetrate gaps in the BM. The reconstruction is supported by evidence from a statistical analysis of pigment granule volumes of all pigment bearing cell types in the retina and rim region. The study reveals the first known case of the participation of lateral rim cells in a sub-retinal pigment shield in an insect eye. As neither pigmented extensions of secondary pigment cells, nor pigment granules in the extensions of the cone cell projections are present above the BM in T. evanescens, the sub-retinal extensions of the lateral rim cells can be seen as a functional adaptation to miniaturization in order to maintain a proximal shielding function.