Fig. 2

Photoreceptor arrangement in the ommatidia of Parhyale. a Transmission electron micrograph of a transverse section through a Parhyale ommatidium at the level of the rhabdom. The rhabdom, at the centre of the image, consists of the tightly packed microvilli of photoreceptor cells. Five photoreceptor cells contribute to the rhabdom, numbered clockwise R1–5 (highlighted in red). Dark granules are visible in the cytoplasm of each photoreceptor cell. The photoreceptors are surrounded by lighter stained material, which belongs to accessory cells with reflective granules. b Transmission electron micrograph of a longitudinal section through a Parhyale ommatidium. The rhabdom is visible below the crystalline cone (labelled CC) and surrounded by the cytoplasm of photoreceptor cells containing dark granules. The labelled square corresponds to an area shown at higher magnification in panel c. c Higher magnification view of the longitudinal section shown in panel B. The microvilli of neighbouring photoreceptors are arranged perpendicular to each other, and retain the same orientation throughout the length of the rhabdom. d, e Transmission electron micrographs of transverse sections at two different levels of the rhabdom: at the distal end of the rhabdom, close to the crystalline cone (d) and at a proximal level near the nuclear layer (e). The R5 photoreceptor (highlighted in red) contributes to the rhabdom at both levels. Scale bars, 2 μm