Fig. 4. (A,B) Drawings of three file teeth of Teleogryllus oceanicus near the middle of the right file. (A) Optical section near the middle of the teeth; the arrow shows the direction of movement of the plectrum during the wing-closing movement. (B) Plan view of the teeth from the underside of the wing showing the sclerotised edge and the angle of the teeth relative to the axis of the file. (C) Diagram showing the engagement of the plectrum with the file, based on a geometric model in which the fore-wings are separated by the 5 mm distance between the wing bases and travel through complementary arcs: the arrow shows the direction of movement of the left plectrum. The right file is drawn as if it were stationary but in the model both wings closed in opposite directions through the same angle, which is shown by the degree scale above the drawings of the left plectrum. Note that the centre of the plectrum engages with the centre of the file over most of the length of the file and that the angle made by the edge of the plectrum to the axis of the file is slightly greater than the angle of the teeth to the file axis shown in B.

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