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 special reference to the abdominal morphogenesis. Cleavage is of the typical superficial type. The germ disc is formed by the cell concentration of the embryonic area broadly defined at the posterior half of the blastoderm. The embryo undergoes embryogenesis of the typical short germ type. Blastokinesis in which the extensive and deep invagination of the embryo is involved is similar to those of odonatans and plecopterans. In the longest embryo stage, the abdomen is folded and is divided into four regions (regions I-IV, from anterior to posterior). All the first 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 similar manner is found in another palaeopteran group, the odonatans. The caudal filament has been interpreted as the elongation of the eleventh abdominal tergum, but it is now revealed that the caudal filament originates from the posterior extremity of region IV and that its origin is away from the eleventh segment, with regions III and IV or the proctodaeum interposing between. Thus, it is concluded that the caudal filament should be correlated not to the eleventh abdominal segment but to the telson. J. Morphol. 234:97-107, 1997. © 1997 Wiley-Liss, Inc.