A Preliminary Research On Adaptive Characters And Molting Processes Of Mayfly Subimagos

The mayflies(Insecta:Ephemeroptera)are characterized by their subimagos in the life history.This means they have two flying instars in their imaginal stages.This unique prometamorphosis development pattern is only preserved in the Ephemeroptera.However,historically only very limited researchs focused on the morphological difference between subimago and imago,the molting place,time,longevity,funcation and evolution of subimagos.No specific or systematic research was conducted.In order to know more about adaptation and evolution of mayfly subimago and development,we investigate and observe them by video recording and rearing.Totally 65 short emergence videos of nymphs and 16 molting movie clips of subimagos,which are from 8 families 18 genera 20 species,are filmed.Through analyzing these videos and related informations in the previous literatures,we propose that the evolution of mayfly emergence place may be from outside of water(the nymphs climb out of the water to molt)to water surface then to underwater.This idea supports the hypothesis that the mayflies are secondary aquatic.Morphologically,we illustrate and compare the wing size,length of marginal setae between subimagos and imagos of 11 families 28 genera 52 mayfly species.The data show the subimagos who have shorter body and hindwings usually have longer marginal setae on forewings.Considering the fossil mayflies usually have larger body and hindwings,we suggest that the wings of mayfly subimago trend to become shorter but the marginal setae of forwings trend to be longer.We also measure the ecdysone level of young nymph,mature nymph,subimago and imago of mayfly species Cloeon viridulum(Baetidae)and Parafronurus youi(Heptageniidae).Data shows the ecdysone changing pattern in mayfly life history has its own characteristic.The general fluctuating model is similar to that of holometabolan insects.