The fly emerges after bursting apart the first four segments of the puparium; this it does by a curious provision, whereby it can inflate a chamber in its head in a queer, balloon-like fashion, making a bag-like extrusion, which it uses as a punching and pushing machine. After emerging from the ground, the fly withdraws the bag-like extrusion and cleans itself. Its body soon grows fit and it becomes very active, as long as daylight and warm weather favour it; otherwise it seeks shelter and becomes quiescent; however, artificial light and heat will awaken it to nocturnal activity. Sweets, carrion, and filth are all attractive to the blue-bottle, but the house-fly and the lesser house-fly also find extraordinary attraction in both man and his dwelling. Considering the superfluity of other flies, and the multitude of other insects ever ready to do duty as devourers of carrion, garbage, and filth, the scavenging services of the larvæ of the house-fly can be well dispensed with. In civilised communities cremation in a refuse destructor is the only sanitary method of treating town refuse. The economic value of the fly is very little, and consists merely in its food value for certain birds. In warm weather the scavenging capabilities of all the carrion and filth feeding maggots are very remarkable, and there appears no exaggeration in the statement by Linnæus, that the progeny of three flesh flies can eat up the carcase of a horse sooner than it could be devoured by a lion. When a batch of eggs has matured in the abdomen of the female, she is most careful in the location and manner of their disposal. Guided by the sense of smell, she will not lay her eggs except in contact with food, or in places securing her progeny access to their intended food. By the use of her soft, slender ovipositor, which is telescopically extensile and flexible, the eggs are deposited in shaded and concealed situations. The house-fly is credited with laying batches of eggs at intervals, perhaps four or more times, and about 150 on the first occasion, then 100, and less on subsequent occasions. Under favourable circumstances the eggs may hatch within a few hours of their being laid. The maggots of midsummer broods may be full grown and pupate in six days, and the perfect insect may emerge from the puparium in another ten days of warm weather, but in cold weather the pupæ of autumnal broods may remain dormant for several weeks, or even months. When nine or ten days old the mature fly may begin to lay eggs; hence, with such a life-cycle, in a month of very favourable weather the progeny of a single pair may number, say, 500; in two months' time the number may become 250 times 500; and in three months' time many millions! CHAPTER VI THE STRUCTURE OF THE HOUSE-FLY