Abstract: 【Aim】 Mecoptera are unique in Holometabola in that their larvae possess a pair of compound eyes instead of stemmata, thus becoming ideal materials for uncovering the evolutionary relationship between compound eyes and stemmata in insects. This study aims to elucidate the structural features of the compound eyes of larval Mecoptera and to provide evidence for exploring the evolutionary relationship between the compound eyes of larval Mecoptera and the stemmata of other holometabolous larvae. 【Methods】 In this study, we observed the ultrastructure of the larval compound eyes of the scorpionfly Dicerapanorpa magna (Chou) using light, scanning and transmission electron microscopy and explored implications of the compound eyes of larval Mecoptera in the evolution of the stemmata of holometabolous larvae based on these structural features. 【Results】 The results showed that the compound eyes of larval D. magna are of the apposition type, each eye comprising more than 50 ommatidia. The ommatidium consists of a cornea, a crystalline cone, eight retinula cells, two primary pigment cells, and several secondary pigment cells. In the larval ommatidia, these retinula cells are arranged into four distal and four proximal cells. The rhabdomeres of the distal retinula cells extend up to surround the basal surface of the crystalline cone, so that the rhabdom assumes distally a funnel shape. 【Conclusion】 Layered retinula cells and the funnel-shaped rhabdoms are very likely the ground plan of the compound eyes in larval Mecoptera. These two characters are absent in imaginal Mecoptera, but present in many hemimetabolous insects, implying that the compound eyes of larval Mecoptera are likely homologous with those of hemimetabolous insects. We assume that the peculiar compound eyes of larval Mecoptera are the plesiomorphy of Holometabola, and the stemmata of other holometabolous larvae might be derived from compound eyes.  

Key words: Dicerapanorpa magna, larva, ommatidium, retinula cell, rhabdom, ultrastructure, evolution