In order that we may better understand why it is that the house-fly is capable of so much mischief, let us consider briefly a few points in regard to its structure, its methods of feeding and its life-history.
The large compound eyes are the most conspicuous part of the head (Fig. 39). In front, between the eyes, are the three-jointed antennæ, the last joint bearing a short, feathery bristle. From the under side of the head arises the long, fleshy proboscis (Fig. 40). When this is fully extended it is somewhat longer than the head; when not distended and in use it is doubled back in the cavity on the under side of the head. About half-way between the base and the middle is a pair of unjointed mouth-feelers (maxillary palpi). At the tip are two membranous lobes (Fig. 41) closely united along their middle line. These are covered with many fine corrugated ridges, which under the microscope look like fine spirals and are known as pseudotracheæ. Thus it will be seen that the house-fly's mouth-parts are fitted for sucking and not for biting. Its food must be in a liquid or semi-liquid state before it can be sucked through the tube leading from the lobes at the tip up through the proboscis and on into the stomach. If the fly wishes to feed on any substance such as sugar, that is not liquid, it first pours out some saliva on it and then begins to rasp it with the rough terminal lobes of the proboscis, thus reducing the food to a consistency that will enable the fly to suck it up. Many people think that house-flies can bite and will tell you that they have been bitten by them. But a careful examination of the offender, in such instances, will show that it was not a house-fly but probably a stable-fly, which does have mouth-parts fitted for piercing.
[Illustration: FIG. 39--Head of house-fly showing eyes, antennæ and mouth-parts.]
[Illustration: FIG. 40--Proboscis of house-fly, side view.]
[Illustration: FIG. 41--Lobes at end of proboscis of house-fly showing corrugated ridges.]
[Illustration: FIG. 42--Wing of house-fly.]
The thorax bears the two rather broad, membranous wings (Fig. 42) which have characteristic venation. Three of these veins end rather close together just before the tip of the wing, the posterior one of the group being bent forward rather sharply a short distance from the tip. The stable-fly has this vein slightly curved forward but not nearly so conspicuously (Fig. 43).