Recent studies on the biology and ecology of adult blackflies in relation to the transmission of human onchocerciasis in Guatemala are reviewed. First, earlier studies on the transmission of the disease since its discovery by Dr. R. Robles in 1915 are outlined. Second, eleven blackfly species caught on humans are evaluated for vector status on the basis of their natural and experimental infections with third-stage larvae of Onchocerca volvulus, and Simulium ochraceum,* S. metallicum* and S. callidum are confirmed as natural vectors of the disease in Guatemala in descending order of importance, whereas S. gonzalezi, S. haematopotum, S. veracruzanum and S. horacioi are potential vectors. Third, the migration and fate of O. volvulus microfilariae ingested by female blackflies are highlighted on the basis of the findings of the cibarial armature of S. ochraceum and physiological incompatibility of S. metallicum as main barriers against microfilariae, both of which greatly decrease the number of ingested O. volvulus microfilariae developing to the third-stage larvae per female. Fourth, among many ecological factors of female blackfly populations, geographical and altitudinal distributions, habits of blood feeding, host preferences for blood feeding, preference for human body parts, parous rates, daily and seasonal fluctuations of biting activities, in particular, of parous females, gonotrophic cycle, longevity, flight range, and annual transmission potential are reviewed, and their influences on the transmission dynamics of the disease agents are considered. Fifth, effects of air temperatures on the O. volvulus–S. ochraceum complex are examined, with a special reference to the characteristic altitudinal distributions of the disease. The importance of reliable identification of both the vector blackfly species and filarial larvae found in female blackflies is emphasized to understand the transmission of the disease. [*It is now known that these two species are actually species complexes.]

Two new black-fly species of the subgenus Simulium (Simulium) are described and illustrated, based on female specimens captured while approaching a human near a mountain summit in Doi Inthanon National Park, Chiang Mai Province, northern Thailand. The first species, Simulium suchariti sp. nov., is assigned to the griseifrons species-group and is distinguished from most other related species by the cibarium with tubercles near the base of its medial projection; the second, S. setsukoae sp. nov., is very similar to S. rufibasis of the tuberosum species-group but is distinguished from the latter by the shorter length of clustered stout hairs on the seventh abdominal segment and the ovipositor valve with its posteromedial corner widely bare.

Forty-five known species of Simulium Latreille s. l. in Thailand are listed, and keys to subgenera and species within each subgenus are provided for adults, pupae and mature larvae.

Two known and two new species of the griseifrons species-group of the subgenus Simulium (Simulium) in Northern Thailand are reported. The male, pupa and mature larva of S. (S.) rudnicki Takaoka and Davies, and the pupa and mature larva of S. (S.) suchariti Takaoka and Choochote, are described for the first time. Furthermore, two new simuliid species, S. (S.) mediocoloratum sp. nov. and S. (S.) crocinum sp. nov., both of which are very similar to S. (S.) rudnicki, are also described.

Simulium (Simulium) phukaense sp. nov. is described on the basis of the observation of females and males (both sexes of adults reared from pupae) and pupae collected in Nan Province, northern Thailand. This new species is assigned to the griseifrons species-group of the subgenus Simulium (Simulium) and is easily distinguished from other known species of this species-group by the simple shoe-shaped cocoon, as well as the arrangement of the six gill filaments.

Two new species of black flies, Simulium (Nevermannia) fruticosum sp. nov. and S. (N.) chiangklangense sp. nov. are described on the basis of samples collected in northern Thailand. These new species are assigned to the feuerborni species-group of the subgenus Simulium (Nevermannia), and are easily distinguished from S. (N.) feuerborni Edwards, the only species of the same group known thus far in Thailand, by their simple cocoon without any anterodorsal projection.

Two more new black-fly species of the rare subgenus Simulium (Montisimulium) were discovered in Doi Inthanon National Park, Chiang Mai, Thailand, where S. (M.) merga Takaoka and Choochote was known as the only named species. These two new species, S. (M.) angkaense sp. nov. and S. (M.) laoleense sp. nov., are described on the basis of the pupal and⁄or mature larvae. Both new species are easily distinguished from S. (M.) merga by the pupal gill with 12 slender filaments, and from all the 16 known species with 12 pupal gill filaments in other countries by the long and very long common basal stalk of the gill, respectively.

Simulium (Nevermannia) satakei sp. nov. is described on the basis of the pupa and mature larvae collected from the Ogasawara (Bonin) Islands in Japan. This new species, tentatively (due to lack of the adult stage) assigned to the vernum species-group of the subgenus Nevermannia, is characterized in the pupa by four gill filaments lacking transverse ridges, and in the larva by a small, M-shaped postgenal cleft, antenna without hyaline bands, and simple rectal papilla. The morphological differences among this new species and the two known species, S. (N.) uemotoi from Japan and S. (N.) karzhantacum from Uzbekistan and Turkmenistan, are noted. This is the second species of the family Simuliidae from the Islands.

Simulium (Simulium) chanyae sp. nov. is described based on two female adult specimens collected in Northern Thailand. This new species is assigned to the multistriatum species-group of the subgenus Simulium (Simulium), and is easily distinguished from most of the other known species of this species-group by the yellowish legs.

Recent studies on the epidemiology and control of Guatemalan onchocerciasis, chiefly made by the Guatemala–Japan Cooperative Project on Onchocerciasis Research and Control, are reviewed. Epidemiological features of Guatemalan onchocerciasis are summarized as to characteristic altitudinal distribution of endemic areas, disease manifestation, vector taxonomy, biology and transmission dynamic of the disease. Extensive insecticide studies in the field and laboratory demonstrate that the characteristic situations of Guatemalan streams where Simulium ochraceum, the main vector of onchocerciasis, breeds require ingenious methods of larviciding. Finally, the feasibility of an area vector control is indicated by the successful control operation in the San Vicente Pacaya Pilot Area, in which a new fixed-dose larviciding method was applied.