DEVELOPMENT OF ONCHOCERCA VOLVULUS (FILARIOIDEA: ONCHOCERCIDAE) IN THE WEST AFRICAN BLACK FLY SIMULIUM YAHENSE (DIPTERA: SIMULIIDAE) IN LIBERIA

Simulium yahense black flies infected with microfilaria of Onchocerca volvulus were kept in a defined insectary environment in Liberia, West Africa. A daily sample of infected flies was dissected for larvae developing in the thoracic muscles and examined for growth in stadial development. Microfilariae ingested by black flies transformed to the L1 larval stage without molting. Successive larval development included molting to the L2 stage and, finally, to the L 3 stage, which was infective in humans. The cephalic cap, consisting of a laterally located hook and central stoma, occurs in the first larval stage. The caudal appendix and the laterally located anal opening are apparent in the L 1 larva. In the L2 stage, the cephalic cap is lost and the large circular stoma becomes surrounded with elevated flaps. The caudal appendix was lost after larvae molted to the L 3 stage, and in its place, 3 terminal papillae developed. Sense organs, such as 2 opposing phasmids and 8 papillae that were arranged into 2 circles, developed in the cephalic region of the L 3 larva. The evidence of pathological consequences due to the presence of the L3 larva in the fly host are illustrated and discussed. Microscopic studies of the development and morphology of Onchocerca volvulus in Simulium damnosum s.l. black flies were first described more than 70 yr ago in Sierra Leone by Blacklock (1926a, 1926b) and later in Cameroon by Duke (1968). Scanning electron microscopy (SEM) studies, which re- veal morphologic detail characters in a vector, were reported for O. volvulus larvae by Franz and Renz (1980), who exam- ined third-stage larvae (L 3) for identification purposes, and by Franz and Schulz-Key (1981), who showed the anterior region of 3 larval stages. Gibson et al. (1976), Martinez-Palomo and Martinez-Baez (1977), and Matsuo et al. (1980) used transmission electron microscopy (TEM) to study the ultrastructure of O. volvulus microfilariae. Strote and Bonow (1993, 1995) studied the ner- vous system, sensory organs, excretory system, and genital pri- mordium of L