Fig. 15.—Eggs of M. domestica, × 40. (From Gordon Hewitt).
In our country house-flies usually begin to breed in June and July, continuing well on into October if the weather be but warm. Their greatest activity is, however, in the hotter month of August and the beginning of September. But in warm stables, restaurants, and kitchens flies are able to reproduce the whole year round. A single fly will deposit at one time 100 to 150 eggs, and in the course of her summer life may produce five, or even six, batches of ova of this size. The eggs are pearly white, elongated structures, with two converging lines, along which the egg-case will ultimately split to give exit to the larva. The eggs are laid, by means of a long ovipositor, a little way beneath the surface of the dung-heap in a position where they will not readily be dried up. In favourable conditions the eggs hatch in from eight to twenty-four hours.
The first larva is legless, tapering towards the head, which bears a pair of breathing-holes, or spiracles; the body is much stouter towards the hinder end. On the whole it is a white, unpleasant-looking maggot, called by freshwater-fishermen a ‘gentle.’ By contracting and expanding its body it pushes its way through the moist, semi-liquid surroundings. The skin is usually moulted some twenty-four hours after birth, but all these time-limits depend much upon the temperature and favourable conditions. With normally high temperatures—say, with 30° C. to 35° C.—the larva will become fully grown in five or six days. The third and final larval stage, after the second moult or ecdysis, lasts three days, and when fully grown the maggot is now about half an inch in length. Externally, twelve segments are visible, but the internal anatomy shows that thirteen are really present, though one is almost ‘masked.’
Fig. 16.—Abdomen of female house-fly, showing the extended ovipositor. (From Gordon Hewitt.)
It is only during these larval stages that the insect grows, and it is never more bulky than in the third larval stage. Now it leaves the moist situation, in which it has flourished, and, crawling through the manure, seeks some dry or sheltered corner. For a time it rests, and then after an hour or two’s quiescence it retracts its anterior end and assumes a barrel-shaped outline, its creamy white colour slowly changing to a mahogany brown. The larval skin forms the pupa-case, and within this pupa-case the body of the larva undergoes a wonderful change, far greater than even human beings undergo at the time of puberty. Many of its organs are disintegrated and re-formed, and in the course of three or four days the white, legless, repellent maggot, who ‘loves darkness rather than light,’ is changed into a lively, flying insect, seeking ‘a place in the sun’ and the companionship of man. As the Frenchman said of the pig which goes into one end of the machine in the Chicago meat-factory as live pig and comes out at the other end in the form of sausages, ‘Il est diablement changé en route.’
Fig. 17.—Mature larva of M. domestica. a.sp, Anterior spiracular process; an.l, anal lobe; sp, spiniferous pad. I-XIII, Body segments. (From Gordon Hewitt.)
In a very short time after leaving the pupa-case the adult fly has stretched her wings, the chitin of her body has hardened, and she flies away ‘on her several occasions.’