The approved classification of flies is to some extent dependent upon the formation of the antennæ, but the unique feature of the systematic differentiation is based upon a very intricate method of scrutinising, identifying, and numbering the vein-like strengthening ribs called veins, nervures, or nerve-lines, which, starting from the shoulder, mark with characteristic patterns the transparent tissue of the wing. We are rather compelled to follow something like this plan (simplified) for the purpose of clearly distinguishing the "lesser house-fly" from the common "house-fly." In the accompanying illustrations rather similar patterns of wings are shown; these are typical of the _Muscidæ_ and _Anthomyidæ_, which, taken together, comprise amongst others all the cattle and human pestering "sweat-flies"; only a few really blood-sucking flies are included amongst the _Muscidæ_. In critically comparing these four patterns, the chief feature to be observed is the small rib-like nervure called the "discal" "cross-vein," which is situate in the very middle of the wing, and which connects the lowest of a group of longitudinal nerve-lines or veins in the front (or upper) half of the wing to the uppermost of the other group of longitudinal nerve-lines in the hind (or lower) half of the wing. Three "main" longitudinal lines, technically termed "veins," are theoretically recognised as constituting the upper group and four "main" longitudinal lines the lower group; but these "veins" (numbered 1, 2, 3, 4, 5, 6 and 7) will be found to be varied in different families and genera, each often with characteristic diverging branches, whilst some veins may be rather inconspicuous or quite absent. We will here devote our attention only to two such "veins," those respectively termed "vein 3" and "vein 4" which are connected in the very middle of the wing, as above mentioned, by the small but always distinct "discal" "cross-vein." The illustrated patterns herewith show wings divided into about twelve compartments or cells, to all of which learned entomological writers give troublesome technical names, not nearly so intelligible as the nomenclature symbols of the late Rev. W. J. Wingate, explained in the Appendix herewith. For present purposes a simple observation of the (externomedial) vein "V, 4," where it is the lower boundary of the (subapical) cell "O, 4^2," will suffice. [Illustration: Fig. 3. Wing patterns contrasted.] The pattern of the first figure illustrates the wing of the common blue-bottle; here "vein 4" does not run at all straight in the last part of its course, but curiously bends very suddenly upwards at an angle and meets the margin very near to "vein 3." In the wing of a large blue-bottle it will be easy to recognise this plan.