Adults intercepted by Malaise traps of the Gressitt and Gressitt style were used to determine the seasonal flight activity of Atylotus duplex, A. thoracicus, and Stonemyia rasa at three sites in Connecticut over one or two years. In Guilford, adults of A. duplex entered two traps placed between an apple orchard and a mesic forest during four weeks in July 1996 and in four between late June and late July 1997. In an acidic bog in Norfolk, adults of A. thoracicus flew into a trap over five weeks between mid-July and mid-August 2011. Stonemyia rasa entered two traps in Guilford over four weeks between mid-August and early September 1996 and between late August and mid-September 1997. In Meriden, S. rasa was captured in two traps between an apple orchard and a field with scattered woody vegetation between late July and early September 1996. A significant female bias occurred in trap samples of A. duplex from Guilford in 1997 and of S. rasa from Guilford and Meriden in 1996. Flight periods in Connecticut are compared with others reported for northern areas. In New England, adults of Stonemyia rasa visited flowers of six plant species (four new floral hosts) in five families, and those of S. tranquilla were on seven species (six new) in six families. The most important results of this study were the first systematic sampling of A. duplex and S. rasa, the first state records of A. duplex, and the discovery of many new floral hosts for S. rasa and S. tranquilla. New information on flight periods of A. duplex and S. rasa was the direct result of examining the bycatch of traps operated for other purposes.