Ultrastructure Of The Larval Compound Eyes Of Bittacus Planus And Terrobittacus Implicatus(Mecoptera:Bittacidae)

Compound eyes are the visual organs of most adults and hemimetabola nymphs.It plays significant role in insect foraging,courtship,navigation,enemies avoiding,and direction determining.The larvae of Bittacidae,a cosmopolitan family in Mecoptera,have a pair of true compound eyes.However,the ultrastructure of compound eyes of Bittacus leavipes Navás is only briefly reported.In this paper,the external morphology of the larval compound eyes of Bittacus planus Cheng,1949 and Terrobittacus implicatus(Huang & Hua,2006)and internal ultrastructure of larval compound eyes of T.implicatus were investigated using light microscopy,scanning and transmission electron microscope.Based on the fine structure of compound eye,the possible function of the compound eyes is inferred,and the origin and evolution of the compound eyes in the visual organs is discussed.The larvae of B.planus and T.implicatus all have a pair of semi-ellipsoidal compound eyes with a smooth surface.The compound eyes of T.implicatus consist of seven separate ommatidia.Each ommatidium is composed of a cornea lens,a eucone crystalline cone,eight retinula cells,two primary pigment cells,an indeterminate number of secondary pigment cells and basement membrane.The outer surface of the cornea is convex while the inner surface is relatively flat.The crystalline cone represents structure that are formed axially by four contributing cone cells.The nuclei of the cone cells were clearly visible at the distal end of the cone cells without any organelles inside.The cornea and crystalline cone are separated by the inner surface of the distal end of the primary pigment cells.According to the location of retinula cells,they can be divided into four distal retinula cells and four proximal retinula cells.The rhabdomere formed by the specialized microvilli of the distal retinula cells extend upward to wrap around the proximal end of the crystalline cone,while the rhabdomere of the retinula cells at the lower end form a fused and unbranched rhabdom that extend down to the top of the basement membrane.The rhabdom presents a funnel-shaped layered structure.The retinula cells contain a large number of electronic transparent vesicles of various shape,which are gathered around the rhabdom.Two primary pigment cells surround the crystalline cone,and the secondary pigment cells are distributed in the space between adjacent ommatidia.The results show that the microvilli of the same retinula cell in the ommatidium of the larval compound eyes of T.implicatus are in the same direction,and the microvilli of different retinula cells are perpendicular to each other,and the pigment granules in the retinula and pigment cells are lacking.Therefore,it is speculated that the compound eyes may not form a clear image and are sensitive to polarized light.There are two main characteristics in the larval ommatidia of T.implicatus and Panorpidae,the layered retinula cells and funnel-shaped rhabdom.The four distal retinula cells of the larval compound eye of Panorpidae larvae are only involved in the formation of the distal rhabdom,which is considered to be homologous to the hemimetabola compound eye.This study also found that the larval compound eyes of T.implicatus had a precursor to disintegration,and the four distal retinula cells of the ommatidia were also involved in the formation of the proximal rhabdom.Based on these observations,we speculate that the larval compound eyes of T.implicatus may be lie in the transitional stage during the evolution from the ommatidia to the stemmata.