Whilst the notorious house-frequenting flies above-mentioned and the blue-bottle are remarkably omnivorous in their feeding, the great majority of outdoor flies are quite otherwise inclined, and do not find much attraction in anything but their own individual preferences. Indeed, the breeze-flies, and many others, avoid human habitations; even the grey blow-fly, unlike the blue-bottle, rather seems shy of the house. In the above grouping, according to feeding habits, the house-fly must be preferably consorted with (2) sweat-flies, but the blue-bottle with (6) carrion flies; however, the house-fly and the blue-bottle are very near akin, and by reason of similarity of wing-pattern both are included in the family of the Muscidæ.
In the entomological systematist's classification the primary separation of flies into two sub-divisions starts with a difficulty, for it is based upon circumstances often obscure and in some cases at least imperfectly known.
The first sub-division, Diptera Orthorrhapha, comprises those flies which in the stage of the pupa or chrysalid disclose the outline of the perfect insect; in the other sub-division, Diptera Cyclorrhapha, there are grouped together all those flies of which the larvæ make for themselves a puparium or barrel-like case out of their larval skin.
The first mentioned sub-division comprises all the gnats, midges, and most of the slender flies which are outside the scope of the present work, but it also includes a few kinds of more stoutly built flies, to which some allusion will be made in the following pages, as for example, the breeze-flies, Tabanidæ.
The second sub-division comprises many families, including the muscid-like flies, of which the house-fly is the type. The flies of this type are to be found in the families of Muscida, Anthomyida, Tachinida, and Cordylurida, comprising nearly 700 British species, of which many rather closely resemble one another when superficially observed.
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, 42," will suffice.
Fig. 3. Wing patterns contrasted.