These two flies and the familiar "blue-bottle" (again it seems that we are liable to confuse two species) are the special subject of our present study; but it will be as well to take passing notice of some few other members of the tribe classified by scientists as belonging to the order _Diptera_. The species of this order native to Great Britain are said to number nearly three thousand, of which quite two hundred of largish sizes are exceedingly common and widely distributed. This order is characterised by the fact that all the species are furnished with one pair of wings only:— dis = double, pteron = wing; they all undergo a metamorphosis analogous to that of four-winged insects. The dipterid flies are apt to be popularly recognised as flies (with fat bodies) and gnats (with slim bodies); but they may be more intelligently classified (with a few anomalous exceptions) as flies (_a_) having a trunk-like mouth or proboscis (miscalled a tongue), terminating with bilobed suctorial lips, and as flies (_b_) having a bayonet-like trunk, or a sheaf-like tubular spike with skin-piercing lancets. No two-winged flies have stings; the tail of the female, which terminates with the ovipositor and is retractile in a telescopic manner, is very soft and quite unlike the sting of the ichneumon or the ovipositor of the "saw-fly," both of which possess two pairs of wings like bees and wasps, and therefore are classified with the insect race called _Hymenopteræ_. Omitting _Aphides_ (green-flies, plant-lice, and the like) which are an "order" by themselves, and excluding gnats of slim form, mosquitoes, and midges, which are mainly crepuscular, nocturnal, or shade frequenting, we might try unscientifically to sub-divide the more conspicuously sunshine-loving and day-flying flies into:—(1) flower and honey seeking flies; (2) cattle pestering sweat-flies; (3) skin-piercing, blood-sucking flies; (4) insectivorous flies; (5) fungus flies; (6) carrion and filth flies; and to these must be added another small group (7) which comprises those of the wondrous family of the _Œstridæ_, the most horrible though not the most injurious of the animal persecuting and torturing flies; this last group, strange to say, are absolutely destitute of any mouth and feed only in the maggot stage. In many cases, however, it happens that the males and the females differ in feeding habits as well as in colours and markings, whilst only their patterns of wing-veins and some less prominently apparent features are constant in the two sexes. These circumstances discountenance the above grouping.