Ultrastructure Of The Visual Organs Of Mecoptera

The visual organs are important to the behavior of insects and the phylogenetic investigations. However, there is no research on the ultrastructure of the adult visual organs in Mecoptera hitherto. Thus, we investigated the ultrastructure of the compound eyes of five species in three families, Bittacidae, Panorpidae and Panorpodidae, using light microscopy, scanning and transmission electron microscopy. In addition, we observed the ultrastructure of ocelli in Bittacus planus and Sinopanorpa tincta using transmission electron microscopy.The compound eyes of Mecoptera sharing the following common structures, a biconvex corneal lens, a tetrapartite crystalline cone, two primary pigment cells connected with the corneal lens and eight retinular cells. While the numbers and shapes of secondary pigment cells are various. In Panorpidae there are 12 secondary pigment cells in Panorpa liui while 16 in P. sexspinosa and Sinopanorpa tincta. There are 12 secondary pigment cells in Bittacus Planus and 16 in Panorpodes kuandianensis. The secondary pigment cells of B. planus are regular in shape and connected intimately with each other, while in other four species they are narrowed in the two ends and separate from each other. Close to the rhabdom, there exists a circle of electron-lucent material. In P. sexspinosa, S. tincta, B. planus and Pd. kuandianensis, these structures are loose and called"endoplasmic cisternae", while in P. liui, they are gather together around the rhabdom and called"palisade". Except for the P. liui, there exist gaps at the corner of each two adjoining rhabdomeres. The gaps are differs in sizes and shapes in different species. They are smaller in Panorpodes than other three species.Mecoptera adults possess three ocelli and each ocellus has a corneal lens, some pigment cells at the outer rim of the lens, and retina. Beneath the corneal lens lies a layer of corneagenous cells which are arranged in a thin layer in B. planus and very thick in S. tincta. The fused rhabdom is formed from rhabdomeres of two adjacent retinular cells in S. tincta, but from rhabdomeres of four retinular cells in B. planus. Large amount of electron-lucent granules can be found in the middle zone of the retina in B. planus, while these granules are not present in S. tincta.Our research is the first attempt to investigate ultrastructure of the compound eyes and ocelli in Mecoptera hitherto. Combined with the habitat and behavior of the insects, we explained some fascinating discoveries. Furthermore, we compared the ultrastructures of the compound eyes and ocelli of different families in Mecoptera. Based on the differences and similarities, we suggest that Panorpidae and Panorpodidae are close in the phylogenetic relationships and further away from Bittacidae. In addition, our investigation provides some evidences for the primitiveness of the Panorpidae and Panorpodidae.