O, 1.—Costal Cell (undivided), or Costal Areolet.
O, 1a1.—The Humeral Cell }
O, 1a2.—Second Costal Cell } Costal Cell, when divided.
O, 1b.—Subcostal Cell }
O, 2.—Marginal Cell (resting on V, 2).
O, 3.—Submarginal Cell (resting on V, 3).
O, 41.—Cell resting on 41 (the 1st part of V, 4), a "basal" cell.
O, 42.—Cell resting on 42 (the 2nd part of V, 4).
O, 4b.—Cell resting on 4b (a lower branch of V, 4).
O, 51.—Cell resting on 51 (the 1st part of V, 5), a "basal" cell.
O, 52.—Cell resting on 52 (the 2nd part of V, 5), a "discal" cell.
O, 53.—Cell resting on 53 (the 3rd part of V, 5), a "marginal" cell.
O, 6.—The Anal Cell.
O, 7.—The Axillary Cell.
O, 8.—The Subaxillary Cell.
O, 9.—The Alula, or Axillary Lobe.
V, c (or C).—The Costal Vein, or Costa, bounding the front Margin.
V, c1a.—Part of the Costa (often ciliated and spined) reaching to the end of V, 1a.
V, 1a.—The upper branch of the V, 1.
V, 1b.—The lower branch of the V, 1.
V, 2.—Second (longitudinal) Vein.
V, 2b.—Lower branch of V, 2.
V, 3.—Third or Cubital Vein.
V, s.—The "Vena Spuria," a thickening of the wing (characteristic of Syrphidæ), an imperfect V between V, 3 and V, 4 crossing X, 4.
V, h.—A "Hang" V (or Appendix) a more or less irregular incomplete V.
V, 4.—Fourth Vein, often much branching.
V, 5.—Fifth Vein.
V, 6.—The Anal Vein.
V, 7.—The Axillary Vein.
X, 1a.—The Humeral Cross-vein, connecting V, 1a to V, c.
X, 1b.—The Subcostal X-vein, connecting V, 1b to V, 1a.
X, 2.—Cross-vein connecting V, 2 to V, 1b.
X, 4.—Discal, Middle, or Central X-vein.
X, 51.—Lower Cross-vein, connecting V, 51 to V, 4.
X, 52.—Lower Marginal Cross-vein, connecting V, 52 to V, 4 (or to some branch of V, 4).
X, 6.—Anal Cross-vein.