CHAPTER V
GENERAL LIFE HISTORY
Whereas the blue-bottle rarely enters the dwellings of mankind, except gravid females led by the sense of smell in search of fish, or flesh meat, and (less eagerly) sweets, both species of house-fly and both sexes seem to delight in the mere odour of humanity; breeding females will seek the larder and the dust-bin, but others will very provokingly pervade all quarters. Although avoiding a dark or deeply shaded room, the house-fly seems to like partial shade; it will be content to remain indoors and to rejoice in a warm kitchen, even on a hot summer's day, whilst all the other kinds of flies are enjoying the outdoor sunshine. It may be said of nearly a dozen other species, occasionally observable crawling on window panes, that they are "outdoor" flies, and that their occurrence indoors is accidental. In fact, they are mostly observed when trying to escape.
Next after human habitations, stables, cow-sheds and pig-sties are the delight of the breeding female house-fly. Round about and in these latter resorts she associates with an immense host of rather small sized flies, and amongst a few others of equal size with the skin-piercing and blood-sucking stable-fly; but many stablemen are ignorant of the difference of the two kinds of flies and of the serious suffering of their horses from the bites of the stable-fly. This lamentable ignorance was shared by the joint authors of "Humble Creatures," published in 1858, when Neo-Darwinism was in vogue, and many books were published for popularising a knowledge of common things and spreading an interest in nature-study; this publication, which is still (1914) in print and very little revised, has probably led some later would-be nature-study teachers to follow suit in confusing the characteristics of the two species. Very often the fly most numerously breeding in the manure heaps of the mews will be Borborus equinus, or some other of the same family, which are characterised by a very simple pattern of wing nervures and by the absence of squamæ or scales behind the wings; also the ankle joints of the feet are most peculiarly short and broad. B. equinus, and a great host of other dung breeding flies of a still smaller size, may be considered beneficial insects; they do not pester cattle, and their larvæ make food more scarce for injurious flies.
The breeding habits of the blue-bottle are very conspicuous by reason of its haste and boldness in taking possession of dead animals. It is incapable of breeding in horse or cow dung, to which latter the green-bottle fly often resorts.
The blue-bottle deposits her eggs, 500 or 600, preferably on dead fish, or flesh, and sometimes on the sores or the flesh of wounded animals, but both the house-flies preferably affect dung, carrion, garbage, and all kinds of fermenting vegetable matter. It has been commonly but not truly said that the principal breeding places of the house-fly are the mews and the farmyards where manure is allowed to accumulate; the house-fly has a preference for horse dung before cow dung, which is preferred by some other kinds of flies; however, near towns, the domestic dust-bins, heaps of market garbage, and deposits of town refuse give rise to a worse plague of house-flies than stables. All these flies deposit batches of white eggs, and are careful to place them as much as possible in crevices and shielded from exposure to strong light, or from draughts.
The two house-flies and the blue-bottle have similar larval stages, but their larvæ, called maggots, differ. The larvæ avoid daylight and cannot withstand dryness. As the larvæ feed, they have the power of ejecting or excreting a juice, which dissolves the food before they imbibe the material; their mouths are suctorial and are destitute of teeth or biting jaws.
The larva of the house-fly is an eyeless and legless maggot, one half inch long when full grown and extended; twelve cylindrical segments may be counted in its body, or even thirteen if we separately distinguish the small head segment, which may be withdrawn, and but little observable; five or six rear segments are of nearly equal stoutness when only half grown; afterwards counting from the three stoutish rear segments, the others taper towards the very small head. The middle and rear segments have pad-like bristly processes underneath, which aid the maggots in creeping, in which action they also make much use of the head segment's grappling hook. The maggots feed voraciously, but they seem, like the larvæ of the honey bee, to pass out very little anal excreta; some have thought that, like what is said of bee larvæ, no excrement is discharged until after the imago has emerged from the puparium; but such conduct seems altogether incredible. In the bee-hive doubtless the assiduous workers ever wash their babies clean and lick up all matter, just like domestic cats and dogs, when nursing their young.
The larva of the blue-bottle, called a gentle, is proportionately larger but very similar, except that the rear segment possesses a ring of tubercles, which may have some useful function in connection with two breathing tracks, which have their orifices at that part of the body.
The larva of the lesser house-fly is very peculiar; all its segments have projecting tubercles; its whole body is rather louse-shaped, having not cylindrical but somewhat flattened segments, of which the middle are the broader, and those near the head and tail the narrower.