Ultrastructural Comparison Of The Compound Eyes Between Larvae And Adults Of Panorpidae(Mecoptera)

Compound eye is the typical visual organ of Arthropoda, and is frequently used as amodel to understand the evolution of complex traits. In Insecta, the majority of adults andhemimetabolous nymphs have a pair of compound eyes. By contrast, almost all theholometabolous larvae possess only one to seven stemmata.The Mecoptera are unique in the Holometabola in that the larvae possess a pair ofcompound eyes instead of stemmata. The knowledge of the fine structure of the compoundeyes of larval Mecoptera, however, is still considerably fragmentary hitherto. The Panorpidaeare the largest family in Mecoptera. We investigated the ultrastructure of larval compoundeyes of the Panorpidae using scanning and transmission electron microscopy and comparedthe compound eyes of the larvae and adults of the Panorpidae.The compound eyes of larval panropids consist of approximately30ommatidia. Eachommatidium is oval in shape, and contains a corneal lens, a eucone tetrapartite crystallinecone, a pair of primary pigment cells and an undetermined number of secondary pigmentcells. Eight retinula cells are arranged in two tiers of four proximal and four distal retinulacells and contribute microvilli to a centrally fused, tiered rhabdom. Remarkably, the fourdistal retinula cells extend distally to accommodate the base of the cone so that the distal partof rhabdom assumes a funnel shape.Compared with the larval compound eyes, the compound eyes of adult panorpidscontain thousands of ommatidia. The eight retinula cells of each ommatidium are not in thesame level. Two retinula cells are respectively situated in the distal level and proximal levelso that the ommatidial rhabdom is invariably composed of the rhabdomeres of seven retinulacells. In addition, the rhabdom appears rod-shaped and extends up to insert into the base ofthe crystalline cone.Based on the present study, the compound eyes of larval Mecoptera are almost identicalto those of hemimetabolous insects in structural aspect. Therefore, the two similar compoundeyes are more likely to be homologous structure, and the evolution of the larval stemmata ofholometabolous insects has the compound eye as its starting point.