Cysticercoids of three species of Tatria (Cyclophyllidea: Amabiliidae) including T. octacantha sp.nov. from the haemocoele of the damsel-fly nymphs Pyrrhosoma nymphula, Sulz and Enallagma cyathigerum, Charp.

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Abstract
Cysticercoids of three species of Tatria have been recovered from the haemocoele of two species of damsel-fly nymphs from a lake in Mid-Wales. This is the first record of Tatria in Britain. The cysticercoids are enclosed each in a transparent capsule of host origin permanently attached to the midgut of the host. The cysticercoids of the three species are indistinguishable superficially. Specific diagnosis depending on the armature of the scolex. Tatria octacantha sp.nov. differs from the other eight species by the presence of eight apical rostellar hooks and 48 small spines on the rostellar surface. One mature adult was found in a little grebe Podiceps ruficollis ruficollis (Pallas) from the same lake and is described briefly. The cysticercoids are described in general terms and the scolex of T. octacantha in detail including the mode of operation of the rostellum and the ultrastructure of the surface layers. A layer of fibres occurs below the superficial longitudinal muscles of the rostellum providing additional support for this mobile organ. A revised key to the nine species of Tatria is included.