EMBRYOGENESIS OF THE MAYFLY EPHEMERA-JAPONICA MCLACHLAN (INSECTA, EPHEMEROPTERA, EPHEMERIDAE), WITH SPECIAL REFERENCE TO ABDOMINAL FORMATION

Embryogenesis of the mayfly Ephemera japonica is described, with speci al reference to the abdominal morphogenesis. Cleavage is of the typica l superficial type. The germ disc is formed by the cell concentration of the embryonic area broadly defined at the posterior half of the bla stoderm. The embryo undergoes embryogenesis of the typical short germ type. Blastokinesis in which the extensive and deep invagination of th e embryo is involved is similar to those of odonatans and plecopterans . In the longest embryo stage, the abdomen is folded and is divided in to four regions (regions I-IV, from anterior to posterior). All the fi rst to eleven segments are derived from regions I and II. Regions III and IV fuse together to form the proctodaeum This manner of abdominal formation may be regarded as basic in pterygote insects, because a sim ilar manner is found in another palaeopteran group, the odonatans. The caudal filament has been interpreted as the elongation of the elevent h abdominal tergum, but it is now revealed that the caudal filament or iginates from the posterior extremity of region IV and that its origin is away from the eleventh segment, with regions III and IV or the pro ctodaeum interposing between. Thus, it is concluded that the caudal fi lament should be correlated not to the eleventh abdominal segment but to the telson. (C) 1997 Wiley-Liss, Inc.