O = Cells of the wing [_see_ the following Table]; the rib-like nervures are termed Veins; these divide the area of the wing into more or less twelve compartments called Cells or Areolets. V = Longitudinal Veins [_see_ the following Table]; two groupings within the margin can be recognised; the principal veins of the Fore Region are numbered, 1, 2, and 3, whilst those of the Hind Region are numbered, 4, 5, 6 and 7 respectively; some have branches, _a_, _b_, _c_, etc., when sections (separately referred to) may have index numerals attached. X = Cross-Veins [_see_ the following Table]; one very significant X-vein termed X, 4 (or the Discal X) in the middle of the wing connects the longitudinal V, 4 to the V, 3, which are thus to be identified even in very abnormal patterns. The cross-veins and cells are symbolised by having attached to the letters X and O respectively the indicative and qualifying figures and letters belonging to the V upon which they rest, that is to say, the V (or part of V) which is the lower boundary. TABLE OF WING CELLS AND VEINS O, 1.—Costal Cell (undivided), or Costal Areolet. O, 1_a_^1.—The Humeral Cell } O, 1_a_^2.—Second Costal Cell } Costal Cell, when divided. O, 1_b_.—Subcostal Cell } O, 2.—Marginal Cell (resting on V, 2). O, 3.—Submarginal Cell (resting on V, 3). O, 4^1.—Cell resting on 4^1 (the 1st part of V, 4), a "basal" cell. O, 4^2.—Cell resting on 4^2 (the 2nd part of V, 4). O, 4_b_.—Cell resting on 4_b_ (a lower branch of V, 4). O, 5^1.—Cell resting on 5^1 (the 1st part of V, 5), a "basal" cell. O, 5^2.—Cell resting on 5^2 (the 2nd part of V, 5), a "discal" cell. O, 5^3.—Cell resting on 5^3 (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, _c_1_a_.—Part of the Costa (often ciliated and spined) reaching to the end of V, 1_a_. V, 1_a_.—The upper branch of the V, 1. V, 1_b_.—The lower branch of the V, 1. V, 2.—Second (longitudinal) Vein. V, 2_b_.—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, 1_a_.—The Humeral Cross-vein, connecting V, 1_a_ to V, _c_. X, 1_b_.—The Subcostal X-vein, connecting V, 1_b_ to V, 1_a_. X, 2.—Cross-vein connecting V, 2 to V, 1_b_. X, 4.—Discal, Middle, or Central X-vein. X, 5^1.—Lower Cross-vein, connecting V, 5^1 to V, 4. X, 5^2.—Lower Marginal Cross-vein, connecting V, 5^2 to V, 4 (or to some branch of V, 4). X, 6.—Anal Cross-vein. GLOSSARY OF TERMS WITH INDEX REFERENCES