Ultrastructural 3d Reconstruction Of The Smallest Known Insect Photoreceptors: The Stemmata Of A First Instar Larva Of Strepsiptera (Hexapoda)

Stemmata of strepsipteran insects represent the smallest arthropod eyes known, having photoreceptors which form fused rhabdoms measuring an average size of 1.69 x 1.21 x 1.04 mu m and each occupying a volume of only 0.97-1.16 mu m(3). The morphology of the stemmata of the extremely miniaturized first instar larva of Stylops ovinae (Strepsiptera, Stylopidae) was investigated using serial-sectioning trans-mission electron microscopy (ssTEM). Our 3D reconstruction revealed that, despite different proportions, all three stemmata maintain the same organization: a biconvex corneal lens, four corneagenous cells and five photoreceptor (retinula) cells. No pigment-containing cell-types were found to adjoin the cor-neagenous cells. Whereas the retinula cells are adapted to the limited space by having laterally bulged median regions, containing mitochondria and the smallest nuclei yet reported for arthropods (1.37 mu m(3)), special adaptations are found in the corneagenous cells which have cell volumes down to 1 mu m(3). The corneagenous cells lack nuclei and pigment granules and bear only a few mitochondria (up to three) or none at all. Morphological adaptations due to miniaturization are discussed in the context of photore-ceptor function and the visual needs of the larva. (C) 2021 Elsevier Ltd. All rights reserved.