Again, if we tried to group our flies with adequate regard to their very diverse habits of life, in the larval stage as well as to their subsequent metamorphoses, we should find that these are details which are obscure and in many cases unknown or imperfectly recorded. However, after much study and many revisions, a scientific classification has been contrived based upon the minutely differentiated characteristics which are technically explained in the Appendix to this booklet. 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.