Morphology And Ultrastructure Of The Larval Stemmata Of Arge Pagana(Hymenoptera:Argidae)

Stemmata are peculiar visual organs of most larvae in holometabolous insects.A stemma consists of corneal lens,retinula cells,pigment cells,corneagenous cells,crystalline cone,and so on.However,the number and fine structure of a stemma are different among holometabolous larvae,and even the structure is different among different stemmata of a larva.Stemmata of the larvae of Lepidoptera,Coleoptera,and Megaloptera have been exclusively studied.However,stemmata of Hymenoptera are only present at the larval stage of Symphyta,whose structure is only investigated in the larvae of Pergidae and Tenthredinidae,but not in the larvae of Argidae hitherto.In this study,we used light microscopy,scanning electron microscopy and transmission electron microscopy to investigate the morphology and fine structure of larval stemmata of Arge pagana(Panzer,1978)(Hymenoptera: Argidae).Based on fine structure of these stemmata,we tried to explore the possible functional significance of Arge pagana stemmata,and combined with early studies on stemmata of Holometabolan to discuss the origin and evolution of stemmata.Stemmata of the larvae of Arge pagana are composed of cornea lens,retinula cells,pigment cells,corneagenous cells,without crystalline cone.The corneal lens is transparent and strongly biconvex,and its surface has shallow depressions;the corneal lens is surrounded by a black circle,which has an incision at its posterior area.Corneagenous cells situate under the corneal lens,their distal parts contain numerous vesicles forming a relative electron-lucent area,and their proximal parts extend to the basement membrane.Retinula cells situate beneath the electron-lucent area.Each stemma contains approximately 1000 retinula cells;eight or nine neighboring retinula cells get together forming nearly a hundred rhabdoms.Rhabdoms that formed by eight or nine retinula cells locate at two different regions,Region I and Region II.Region I situates at the centre and occupies the large area of the stemma with rhabdoms formed by eight retinula cells.Region II situates the posterior of the stemma alongside the margin like a stripe with rhabdoms formed by nine retinula cells.Clusters of pigment cells situate at the posteriormost part of the stemma or the peripheral of Region II.These pigment cells extend from the distal part of corneagenous cells to the proximal part of retinula cells.Because at the posterior of the stemma,there are a few rhabdoms,each of which is formed by nine retinula cells with roughly perpendicular arranged rhabdomeres,we speculate that the larval stemmata of Arge pagana might be able to detect polarization light.The larval stemmata of Arge pagana are similar with the larval stemmata of Pergidae,and these stemmata are composed of corneal lens,corneagenous cells and retinula cells,without crystalline cone.The stemmatal model of Pergidae is considered as developed from the fusion of corneae of many ommatidia with the reduction of crystalline cone,but their rhabdoms that each formed by eight retinula cells remain traditional situation.In present study,there are rhabdoms formed by eight or nine retinula cells.Rhabdoms that formed by eight retinula cells occupy the central and large area of the stemma,and rhabdoms that formed by nine retinula cells occupy the posterior and small area of the stemma.Thus,we propose the larval stemmata of Arge pagana are probably evolved from the fusion of two types of ommatidia.