CHAPTER II

THE IDENTIFICATION OF THE COMMON HOUSE-FLY

Although there are several other kinds of flies which occasionally visit the dwellings of mankind, there is one super-abundant species, _Musca domestica_, to which the name of "house-fly" pre-eminently belongs. In the scientist's discriminating judgment, when viewed microscopically, it differs substantially from others; but it differs very little in general appearance from certain outdoor flies and from one not uncommon indoor smaller companion, _Fannia canicularis_, which is not classified amongst the _Muscidæ_ but amongst the _Anthomyidæ_. This latter has been fitly termed the "lesser House-fly;" it has the same habit of delighting to pester man as much as or more than cattle outdoors. Both these flies join with several others in frequenting stables and cow-sheds.

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.

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.

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.

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.

The pattern of the second figure is rather similar, for the vein 4 likewise has a sudden bend upwards; it terminates practically contiguous with vein 3 at the margin. This pattern is characteristic of the "house-fly"; thus it will be easy for the reader to identify the common house-fly by the close resemblance of its wing pattern to that of the blue-bottle, with which it is classified in the family of the _Muscidæ_.

In the pattern of the next figure the vein 4 runs comparatively straight throughout and meets the margin at a spot intermediate between the third and fifth veins; here all the main nerve-lines diverge more evenly and terminate more equi-distantly apart; this latter plan is the wing pattern which will suffice to identify the lesser house-fly, but it is shared with all the _Anthomyidæ_, and more or less with some others, which are very common outdoor flies.

The pattern of the lowest figure illustrates the wing of the common blood-sucking stable-fly, _Stomoxys calcitrans_, which only occasionally invades the house. Here the vein 4 is deflected upwards towards the margin ending near the termination of the vein 3, but the bend is a smoothly rounded curve and not a curiously abrupt angle, as in the first and second figures.

If the reader will study the house-fly in captured specimens, he will be able to observe that they slightly differ in their inconspicuous colouration and markings.

The male of the lesser house-fly is sometimes more observable than the male of the commoner house-fly, by reason of his being a most indefatigable dancer with companions in mid air around any central ornament, and also by reason of his possessing pale patches, more or less yellowish grey, on the sides of the abdomen; but such markings are also in some degree observable in other male flies, being very conspicuously of a brighter yellow in the common small outdoor raven-fly, _M. corvina_. The back of the thorax of the house-fly is marked sometimes distinctly, sometimes indistinctly, with four dark lines on an ash-grey background; the lesser house-fly has three faintly darkish lines only. Quite a number of outdoor flies have similar markings, but these often look like closely adjacent or indistinct spots. The wing pattern is the readiest guide for distinguishing the lesser house-fly, both male and female. The males of the hairy (almost bristly) raven-fly also indulge in the dancing habit, but still more so do those of the latrine-fly, _Fannia scalaris_, which may be distinguished by its uniformly ashy-grey abdomen.

These common co-inhabitants of our dwellings vary in size according to their nourishment when in the larval stage (maggots); after the perfect insect emerges from the puparium, it swells out and fattens, but does not grow in the real sense of the word. If 1000 house-flies will weigh an ounce, then it may be calculated that 1600 average specimens of the other kind will likewise weigh an ounce.

In representing that the house-fly exceeds the lesser house-fly in numbers in the proportion of twenty or thirty to one, it must be borne in mind that the occurrence of the latter varies widely—casually according to the locality, and temporarily according to the time of the year, being more commonly observed when and where the other kind is scarce.

The lesser house-fly has summer broods at longer intervals than has the common house-fly. Towards the end of the summer its last brood hibernates in the puparium, and emerges as early as the end of March or early in April, whilst the common house-fly is not usually observable until a later date, although it is credited with more generally braving the dangers of attempting to hibernate in the imago stage. My attempts to test the capability of the house-fly by aiding October and November flies to hibernate invariably terminated in the creature's death long before springtime. However, it is very apparent that under the shelter and encouragement of warm winter environments in towns, amidst restaurants, bakeries, large hotels and certain factories, as well as and even more than in mews, adult flies of the latest autumn broods can, to some extent, survive mid-winter with very little or no prolonged hibernation.