Circulatory organs of abdominal appendages in primitive insects (Hexapoda: Archaeognatha, Zygentoma and Ephemeroptera)

The abdominal circulatory organs and the haemolymph supply of the terminal filament and the cerci were investigated in a total of nine species (serial semithin sections, TEM, in vivo observations). In all investigated species, the dorsal vessel features a bidirectional flow. In the 10th abdominal segment, there is an intracardiac valve preventing flow in the anterior direction. The posterior portion of the dorsal vessel differs significantly from the anterior portion in design and wall structure. Pumping actions and frequencies do not correspond with each other in the two portions. Vessels supply haemolymph to the long terminal appendages. The terminal filament vessel is connected to the dorsal vessel, the two cercal vessels originate from a transverse septum at the base of the terminal filament. All taxa show the same flow pattern through the caudal appendages. Ephemeroptera have a unique spherical body at the posterior end of the dorsal vessel functioning as a backflow valve. The results indicate that the plesiomorphic state of the circulatory organs in the abdomen of primitive insects differ distinctly from that of higher insects which serve as bases for generalized schemes common to entomological textbooks.