Scanning Electron Microscopy of Third- and Fourth-Stage Larvae and Adults of Brugia pahangi (Nematoda: Filarioidea)

Third- and fourth-stage larvae and adults of the filarial worm Brugia pahangi were examined by scanning electron microscopy. All stages studied have closely spaced, cuticular annulations over the entire surface, except at the extremities. Cuticular annulations of recently molted worms are narrower than annulations seen late in the preceding stage, but widen as the worm grows. The head bears eight cephalic papillae in two circles of four papillae each. There are two subdorsal and two subventral papillae in each circle. Third-stage larvae bear a central, nipplelike protuberance atop each of the inner, cephalic papillae. Three prominent, terminal, caudal papillae are lost at the third molt. Molting third-stage larvae show irregularities in the cuticular surface and desquamation of the old cuticle. The fourth-stage larva shows a globular head region, numerous tubercles in the anal region, and a pair of terminal, caudal papillae. No sexual dimorphism is seen in either third- or fourth-stage larvae. The vulva appears after the final molt, and there are major changes in the male at this time; tubercles shift to the first coil of the tail about 1 mm anterior to the anus, there are 10 to 12 nipple-bearing, perianal papillae, and the cloaca of the adult appears. The terminal, caudal papillae of adult worms are difficult to visualize and count. The orifice of the mouth is slitlike in the infective larva, but becomes triradiate after the final molt. The external orifice of amphids and phasmids could not be visualized.