these articles scatter the dust as badly as corn brooms when sweeping the floor. Wool dusters with short handles. (fig. 31) or cloths treated with kerosene or gasoline in the same manner as the wool floor dusters are prepared are the best for this purpose.

TRANSMISSION OF DISEASE BY INSECTS.

Flies.

There are many kinds of flies, the most important in this country being the house fly. Among others may be mentioned the blue-bottle fly, the green-bottle fly, the stable fly (which is a biting or bloodsucking fly), the cheese fly, the lesser or small house fly, the fruit fly, the sand fly, and the tse tse fly (also a biting fly, a native of South Africa, which causes the disease known as sleeping sickness).

The house fly is found in nearly all parts of the world, but seldom in places where there are no human habitations. It consists of a head, thorax, and abdo

FIG. 33.—Side view of blow-fly (Calliphora crythrocephala) (× 5). A, Cheek (jowl); B, squama;

C, halter.

men.

In the head are several thousand eyes. It is believed, however, that it depends more on its sense of smell than its vision in finding its food. It sucks its food up through a tube, called a proboscis, attached to the under portion of the head. The wings are fastened

to the thorax. There are three pairs of legs covered rather thickly with coarse hair. The breeding season in the North is from May to October, while in the South it begins as early as March. The eggs are deposited in batches in fresh horse manure, kitchen refuse, decayed vegetables, human excreta, putrifying animal matter, or any other kind of organic filth. In such substances the proper temperature, moisture, and food for fly propagation is found. Their number increases very rapidly, as a fly is fully developed in 8 or 10 days after birth, and each female is capable of laying 100 or more eggs. The eggs are smooth, white, glistening bodies. Under favorable circumstances they are transformed into larvæ or maggots within 12 hours. These reach maturity in from three to six days and leave the substance in which they are hatched and burrow into the ground or travel several feet; along its surface. A contraction of the maggot now occurs; it changes to a dark color, and is known as a pupa. In this stage the

wings and other structures of the full grown insect attain their full development, and in about three days the adult fly breaks through the sack and escapes.

Substances intended for human consumption should be protected from house flies, as these insects may carry the parasites or microorganisms of disease on their bodies, especially on their wings and hairy legs, from privies and other places where such agents abound. Solid food is contaminated by their crawling over it, and liquids by their drinking it or falling into it. Flies also have a habit of regurgitating their food after it is swallowed and smearing it on articles over which they crawl. (Fig. 35.) As this material may have come from privies, cuspidors, or other places where disease germs may be present, it is evident that these germs may be left on food or in drink with which the fly comes in contact and cause disease in the person swallowing it. The same germs may also pass through the fly's body and be deposited as part of its excreta on food, and thus gain an entrance into the human body.

Among the diseases which may be transmitted by the house fly are typhoid fever, diarrhea, dysentery, cholera, and probably tuberculosis, diphtheria, and scarlet fever. It is also believed that the eggs of intestinal worms, such as tape worms, hook worms, and round worms, may be carried to human beings by flies.

Eradicative measures.-Lizards, toads, spiders, and wasps are the natural enemies of the fly. Beetles, ants, and birds feed on both larvæ and pupæ. These animals, however, make but slight inroad upon the fly population. There is a disease which attacks and kills great numbers of flies late in summer; it is caused by a fungus which invades the body and destroys the vital organs. It is more prevalent from August to October, and accounts to some extent for the decrease in the number of flies at this time of year.

In order to eradicate the fly it is necessary to do away with its breeding places. Privies must be screened so that flies can not get into them. Garbage must be kept in covered containers until it can be burned or buried, and fresh horse manure must be removed from the premises at least every four days, as but four days are required from the time the eggs are deposited until the maggots begin to migrate from the manure heap into the ground to continue their development into full-grown flies. M. E. Roubaud states:

Fresh manure alone plays a part in the production of flies. The laying of eggs even takes place in the stable on the dung impregnated with urine. Oviposition may continue for 24 hours, but never later. Fermentation, after barely 24 hours, definitely protects the manure from the laying of eggs. Antiseptic substances and larvicides (borax, cresol, ferrous and ferric salts), by delaying fermentation, may prolong deposition one or two days. Employed as larvacides, these substances, by prolonging the period of infestation, often

produce a result the very opposite of that intended. From the sixth day manure when placed in a heap does not contain larvæ, these having migrated to the base for nymphosis. Antifly measures, therefore, ought to be taken within five days of the removal of the manure from the stable. Manure 24 hours old at the time of the removal does not contain visible larvæ. The eggs which are disseminated throughout the manure heap then open and the larvæ come to the surface, leaving the central parts as fermentation develops and the temperature rises. On the following day a temperature of 70° C. to 90° C. (158° F. to 194° F.) may be found in the center of the heap. The heat arising from fermentation in a manure heap may be used as a means of destroying the larvæ which it contains. The larva of the domestic fly, protected from the gases of fermentation, dies in three minutes when exposed to a temperature of 50° C. (122° F.). In contact with the gases it dies in one minute at 51° C. (123.8° F.), in five to seven seconds at 59° C. (138.2° F.), and four to five seconds at 60° C. (140° F.).

When a manure heap is turned over the larvae which come in contact with the hot parts in the interior are killed at once. A complete stirring up of the manure on the day after the deposition, and repeated on the two following days, causes a disappearance of 90 per cent of the larvæ. This operation is more easily and quickly done if, instead of waiting until the infected heap has itself produced the necessary temperature, it is exposed to the heat of a heap previously fermented. For this purpose, instead of placing the new manure on the surface of the heap as is usually done, it should be buried in the hot parts by covering all its surfaces with a layer of hot manure 20 centimeters thick. In four or five hours the new manure may be considered as entirely free from eggs and larvæ, which would otherwise have developed in thousands. This biological method of delarvization by heat is equivalent to the heating of the whole of the fresh manure to a temperature of 50° C. (122° F.) to 60° C. (140° F.), and is effected without apparatus and without fuel. It is within the reach of all, and only a simple training of the personnel is required. In practice it is found that the mass of fermented manure required to furnish the necessary temperature is about eight times that of the fresh manure to be treated. Next day this may be used in its turn as a source of heat. The biothermic method of treating fresh manure can alone destroy quickly and cheaply the eggs and larvæ in a manure heap.

"Swat the fly" campaigns will not eliminate flies unless other measures are taken. They do more good in the early part of the season. Dwellings, markets, and bakeries should always be carefully screened, and where it is not possible to screen the buildings food supplies should not be exposed unless they are protected by netting. Large flytraps, placed in localities where flies are abundant, have been found to be useful in diminishing their number. Various forms of these traps are used, one of which is shown in figure 38. Howard states that

Manure boxes with flytraps attached (see fig. 39) should be used by all stock owners in towns and cities, and they are also adaptable to farms. The size of the manure bin should be governed by the individual needs, but for use on the farm it is desirable to make it large enough to hold all of the manure produced during the busiest season of the year. A box 14 feet long, 10 feet