The legs are rather thickly covered with coarse hairs or bristles and with a mat of fine, short hairs. On some of the segments the larger hairs are arranged in rows and are used as a sort of comb with which the fly combs the dirt from the rest of its body. The last segment (Fig. 45) of the leg bears at its tip a pair of large curved claws and a pair of membranous pads known as the pulvillæ. On the under side of the pulvillæ are innumerable minute secreting hairs (Fig. 46) by means of which the fly is able to walk on the wall or ceiling or in any position on highly-polished surfaces. HOW THEY CARRY BACTERIA These same little pads, with their covering of secreting hairs, are perhaps the most dangerous part of the insect for they cannot help but carry much of the filth over or through which the fly walks, and as this may be well stocked with germs the danger is at once apparent. As the result of a series of carefully planned experiments it has been demonstrated that the number of bacteria on a single fly may range all the way from 550 to 6,600,000 with an average for the lot experimented with of about one and one-fourth million bacteria to each fly. Now where do all these bacteria come from? Necessarily from the place where the fly breeds or where it feeds. [Illustration: FIG. 43--Wing of Stable-fly (_Stomoxys calcitrans_).] [Illustration: FIG. 44--Wing of house-fly showing particles of dirt adhering to it.] [Illustration: FIG. 45--Last three segments of leg of house-fly showing the claws, the pulvillæ and the hairs on the legs.] [Illustration: FIG. 46--Foot of house-fly showing claws, hairs, pulvillæ and the minute clinging hairs on the pulvillæ.] [Illustration: FIG. 47--Larva of house-fly.] LIFE-HISTORY AND HABITS The eggs of the house-fly may be laid on almost any kind of decaying or fermenting material. If this is kept moist and a proper temperature maintained the larvæ or maggots (Fig. 47) that hatch from the eggs may develop. As a rule, however, these requirements are found only under certain conditions and are ordinarily found only in manure heaps or in privy vaults or latrines. All observers agree that the female fly prefers to deposit her eggs in horse manure when this can be found and when this is piled in heaps in the barn-yard (Fig. 48) or in the field the heat caused by the decay and fermentation makes ideal conditions for the development of the larvæ. Cow manure may serve as a breeding-place to a limited extent. The flies are immediately attracted to human excrement and breed freely in it when opportunity offers. Decaying vegetables or fruit, fermenting kitchen refuse and other materials sometimes also serve as breeding-places.