Fig. 3. Binding of Bin and Cyt1A to midgut epithelial cells of Bin-resistant fourth instars of Culex quinquefasciatus five hours after treatment. (A) Phase-contrast micrograph of a posterior midgut epithelial cell of a Bin-resistant larva fed only Bin labeled with Oregon Green. (B) The same cell as in A viewed with an Oregon Green filter, showing that little or no Bin bound to the microvilli or entered the cell. (C-F) Micrographs of a single posterior midgut epithelial cell from a Bin-resistant larva fed a 1:1 mixture of Bin and Cyt1A. Bin was labeled with Oregon Green, and Cyt1A with Rhodamine Red-X. (C) Phase contrast without filter; (D) superimposed micrographs taken with Oregon Green and Rhodamine Red-X filters; (E) micrograph of the cell using Oregon Green filter showing Bin throughout the cytoplasm, with no Bin detectable bound to the microvilli. (F) Micrograph of the cell taken using Rhodamine Red-X filter showing Bin bound primarily to the microvilli. N, nucleus; bb, microvilli of the brush border.