The external ultrastructure of the anterior and posterior extremities of the nematodes, Strongylus asini, Strongylus vulgaris, Strongylus equinus and Strongylus edentatus, was studied with scanning electron microscopy (SEM). Fresh specimens of S. asini were collected from the caecum, ventral colon and vena portae of Equus burchelli and Equus zebra hartmannae; S. vulgaris from the caecum, colon and arteria ileocolica of E. burchelli; S. equinus from the ventral colon of E. z. hartmannae and S. edentatus from the caecum and ventral colon of both zebras, during surveys of parasites in zebras in the Etosha Game Reserve, South West Africa/Namibia, and the Kruger National Park, Republic of South Africa.
The worms were cleaned, fixed and mounted by standard methods and photographed in a JEOL JSM- 35C scanning electron microscope (SEM) operating at 12kV. The SEM showed the following differences: the tips of the external leaf-crowns varied and were fine and delicate in S. asini, coarse and broad in S. vulgaris and, in S. equinus and S.edentatus, closely adherent, separating into single elements for half their length. The excretory pores showed only slight variation, and the morphology of the copulatory bursae did not differ from those seen with light microscopy. The genital cones differed markedly: S. asini had a ventral triangular projection and laterally 2 finger-like projections: in S. vulgaris there were numerous bosses on the lateral and ventral aspects of the cone; in S. equinus 2 finger-like processes projected laterocaudally; and in S. edentatus 2 pairs of papilla-like processes projected laterally on the ventral aspects, and a pair of rounded projections and a pair of hair-like structures adorned the dorsal aspects. The most significant micrograph was the shedding of the cuticle of the 4th moult of a female S. asini.
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