Flies may be easily prevented from entering the open windows of any room, which has windows only on one side, by the use of venetian or louvre blinds or shutters, or of many kinds of screens, although the apertures thereof may allow of ample room for flies to pass to and fro. If, however, there be windows on two sides of a room, then venetian blinds and the like will be useless, and window screens must have very close meshes to be effective. The house-fly will pass through netting only when there is light shining on the further side. A knowledge of this fact is very important in the planning of hospital wards. In a sick room, if there be windows on two sides, one in summer time should be darkened when the other is open for ventilation. The protection of the larder and the screening of food should never be neglected, but what is of even greater importance is the prevention of access by flies to fæcal matter, or to purulent and all unhealthy discharges from the sick room. CHAPTER XII THE SERVICE AND UTILITY OF FLIES It is often asked—have not house-flies some use in Nature? The only true answer is that they are warning signals. They certainly do join with a multitude of other flies in promiscuous scavenging services, and they can be very active agents therein; but this work only aggravates the fact of their dangerous partiality to mankind, together with all his belongings and surroundings. These creatures may well be imagined to have developed out of some primæval species by reason of the increase of mankind upon this planet. The mere presence of the house-fly denotes some nuisance more or less remote; the local density of the brood indicates the degree and the proximity of unsanitary conditions. Under present circumstances the visitation of the house-fly is Nature's intimation that peril of a very insidious character is about. Very properly, Nature's messenger will not be denied, and pertinaciously manifests herself to us _indoors_!