The structure of the double-eyes of Baetis and the uniform eyes of Ecdyonurus (Ephemeroptera)

Abstract

The lateral eyes of Baetis vernus are structured similarly in both sexes. They are eucone apposition eyes with a closed rhabdom formed by eight retinula cells. The accessory pigment cells surround the ommatidia over their entire length. The morphological adaptation to different illuminations consists of a radial displacement of mitochondria and endoplasmic vacuoles. The dorsal eyes of the male Baetis, often called turban eyes, differ remarkably in structure and size from the lateral organs by a large clear zone which is only crossed by thread-like elongations of the retinula cells; the rhabdom is divided in a distal part next to the cone and a large basal part. Each of the seven retinula cells contributes microvilli to both parts of the rhabdom. The pigment cells only surround the cone and the distal part of the rhabdom, whereas the basal rhabdoms are optically isolated from each other by a ring of trachea. No morphological adaptation to different illumination is detectable in the dorsal eyes. The relation of the two different eyes of the male, being optically isolated but forming a morphologically connected complex, are discussed. With the description of the dorsal eyes of the subimago the formation of the dorsal eyes is investigated. A possible mode of function is considered. The male of Ecdyonurus venosus has larger eyes than the female, but they are structured after the same scheme. The eyes are similar to the lateral eyes of Baetis.