(Plautus, Mercator.)
‘The common house-fly [says Ruskin] is the most perfectly free and republican of creatures. There is no courtesy in him; he does not care whether it is a king or clown whom he teases, and in every step of his swift mechanical march and in every pause of his resolute observation there is one and the same perfect expression of perfect egotism, perfect independence and self-confidence and conviction of the world having been made for flies. Your fly free in the air, free in the chamber, a black incarnation of caprice, wandering, investigating, fleeting, flitting, feasting at his will with rich variety of feast from the heaped sweets in the grocer’s window to those of the butcher’s back yard, and from the galled place on your horse’s neck to the brown spot on the road from which, as the hoof disturbs him, he rises with angry republican buzz; what freedom is like his?’
The house-fly is all that Ruskin describes it to be, but it is more. It is the most cosmopolitan of insects. Wherever man is there is the fly. It is found—
From Greenland’s icy mountains
To India’s coral strand.
But it is naturally more frequent in warm climates than in cold, as the rate of its development depends very largely upon an average high temperature.
Unlike the lice and the bed-bug, the fly like the flea, passes through a complete metamorphosis—egg, larva, pupa, and imago. It will breed in almost any rotten matter, whether vegetable or animal, and it breeds most successfully, as Gordon Hewitt has pointed out, when certain processes of organic fermentation are taking place in its breeding-place. Probably the fermentation has a favourable effect upon the food of the larvae. Undoubtedly the place most readily selected by the female for laying her eggs is stable-manure. A few years ago there was a remarkable reduction in the number of house-flies in London, and Lord Montagu of Beaulieu attributed this reduction to the refreshing and insecticidal petrol vapour with which the streets of that town were then bathed.
Fig. 14.—Mass of eggs of M. domestica. (From Gordon Hewitt.)
I do not know what experiments Lord Montagu had made on the subject of the insecticidal value of petrol vapour, but the ordinary man in the street attributed—and I think more correctly—the diminution of the plague of flies to the absence of the nidus in which the female fly lays her eggs. Stable-yards had been turned into garages. But flies will, indeed, breed in almost any kind of dejecta—including the human—and in rotten straw, rotten wool, cotton garments, decaying vegetables and fruits, bad meat, rotten grain, and even in spittoons, but they prefer horse-manure.