tip. The
stable-fly has this vein slightly curved forward but not nearly so
conspicuously (Fig. 43).

Nearly all the other flies that are apt to be mistaken for the house-fly
do not have this vein curved forward. The wings, although apparently
bare, are covered with a fine microscopic pubescence. Among these fine
hairs on the wing as well as among similar fine ones and coarser ones
all over the body, particles of dust and dirt or filth (Fig. 44) or,
what interests us more just now, thousands of germs may find a temporary
lodgment and later be scattered through the air as the insect flies. Or
they may get on our food as the fly feeds or while it rests and combs
its body with the rows of coarse hairs on its legs.

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 pulvillae. On the under side of the pulvillae
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 pulvillae and the hairs on the legs.]

[Illustration: FIG. 46--Foot of house-fly showing claws, hairs,
pulvillae and the minute clinging hairs on the pulvillae.]

[Illustration: FIG. 47--Larva of house-f