Morphology of the compound eyes of two ancestral phyllopods, Triops cancriformis and Lepidurus apus (Notostraca : Triopsidae)

The kidney-shaped compound eyes of adult tadpole shrimps of the genera Trio ps and Lepidurus are composed of about 310 and 170 ommatidia, respectively. Each ommatidium has a flattened cuticle on top, and is made of 4 cone cell s that form a crystalline cone of the eucone type, 2 corneagenous cells and 8 retinula cells which form a fused rhabdom. Among the retinula cells, 6 a re arranged in 3 mirror symmetrical pairs (referred to as R1/R6, R2/R5, R3/ R4) on either side of the axis of the ommatidial cross section, while 2 unp aired cells (referred to as R7 and R8) are positioned one behind the other along the axis. The rhabdomere of R7 sits on top of the rhabdom, and is rep laced by that of R8 more proximally. In its distal region, the rhabdom is f ormed like a pi in Triops and is of a triangular shape in Lepidurus. The st ructure of the dioptric apparatus, the retinula and cone-cell patterns, the orientation of the rhabdomeric microvilli, the array of the ommatidia, and the arrangement of the ommatidial main axes are discussed with regard to t he compound eyes of other arthropods.