Ultrastructure of the dorsal ocellus of Bittacus planus larvae (Mecoptera: Bittacidae) with evolutionary significance

The Bittacidae are unique in holometabolous insects in that their larvae bear a dorsal ocellus on the frons. The fine structure of the dorsal ocellus, however, has not been investigated to date. Here, the ultra-structure of the larval dorsal ocellus was studied in the hangingfly Bittacus planus Cheng, 1949 using light, scanning, and transmission electron microscopy. The dorsal ocellus of the larvae comprises a cornea, corneagenous cells, and retinula cells. The cornea is a laminated structure. A layer of corneag-enous cells is located below the cornea. Numerous retinula cells are arranged tightly beneath the cor-neagenous cells. The retinula cells modify their adjacent membranes into numerous linear microvilli, which form an analogue of the rhabdom among adjacent retinula cells. The results show that the dorsal ocellus of larval Bittacidae is a highly vestigial organ and appears to be degenerating during the post-embryonic development. The presence of the vestigial dorsal ocellus is likely to represent an ancestral plesiomorphy of holometabolous insects, providing new evidence for exploring the evolutionary origin of holometabolous larvae.(c) 2023 Elsevier Ltd. All rights reserved.