The horseflies in a patch of gallery-forest in a savannah of French Guiana were collected by 1°) for 24 hr with a Malaise trap, and 2° for 2 hr at dawn and at dusk, with a Malaise trap baited with carbon dioxide. The crepuscular horseflies, Cryptotylus unicolor, Chlorotabanus mexicanus and Chlorotabanus inanis, were more abundant in the baited trap (average: 55% of the flies collected) than in the unbaited trap (3%). Seasonal changes in relative abundance of the crepuscular species were evident in the baited trap only. The baited trap caught more flies and more species (averages: 315 flies and 7.5 species per 2hr catch) than the unbaited trap (averages 64 flies and 5.1 species per 24 hr catch). However, the overall number of species collected in each trap during the study did not differ greatly (16 species for 126 catches of 24 hr for the unbaited trap vs. 14 species for 29 catches of 2 hr for the baited trap).
