Electron microscopy and electrophoretic analysis of Isotomurus intestinal terminal web. (a,b) The terminal web (tw) is a discontinuous structure formed by filaments which contact the septate junction (sj). Microfilaments (mf) descending from microvilli insert onto the terminal web. Bars = 1 μm. (c) Electron microscopy analysis of cytoskeletal Triton-resistant midgut preparations obtained as described in Material and methods: the terminal web is not affected by treatment with detergent and high ionic strength solutions. Bar = 200 nm. (d) Electrophoretic analysis on an 8% SDS-polyacrylamide gel of the detergent-insoluble fraction from Isotomurus intestine: a major protein band (I) is present, which migrates immediately below actin (A).