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.