of flies invariably seen about places of this character during the summer season is an indication that not only feeding but actual propagation is in process.

The breeding season varies with the climate, usually beginning during May in the North, but more often in early April or March in the South; propagation continues until late September. The fly population is usually greatest during August and September, after which it very rapidly diminishes.

The eggs of the common house fly are smooth, white, glistening bodies about a twentieth of an inch in length, oval in shape and slightly broader at one extremity than the other. They are usually found in irregularly massed batches, each female depositing several such aggregations during her lifetime. The long ovipositor enables the female to deposit the eggs in crevices or beneath the surface of filth where the desiccating action of the atmosphere is less pronounced. Ordinarily a period of but 12 hours is required from the time the eggs are deposited until they are transformed into larvæ, although if the temperature is not favorable, two, three, or more days may be necessary. Hatching merely consists of the splitting of one extremity of the egg sac and the emerging of the larva.

The larvæ, or maggots as they are commonly known, represent the second stage in the development of the fly. They are about twice the size of the eggs but of much the same color and hape, the body being somewhat indistinctly ented. There "ts and rudime y the larva Lance. Duri ults, const atained an xhibit a te Lis mass: rectly

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U.S.PUBLIC HEALTH SERVICE

Eggs of house fly, greatly enlarged. (After Hewitt.)

no legs; nevertheless, by aid of the mouth largements on the under surface of the ly motile and may travel a considerable rse of their growth they pass through two ng upon the substance in which they are maturity in from 3 to 6 days. The larvæ ngregate in a zone just beneath the surface of they are developing, seldom being found diace. After arriving at maturity they migrate ce where they commenced their growth, burroweven traveling a distance of several feet over the igration must always be considered and guarded -eradicative measures are instituted.

third or chrysalis stage of its developThis period is characterized by a cono a darker color, and a disintegration rresponding growth of the wings and

Several segments make up the abdomen or after portion of body, the number varying with the sex and species. The last f segments in the female form the ovipositor. Owing to its telesco character, this organ may be partially withdrawn within the domen or extended when in the act of depositing the eggs. In t manner eggs are laid in cracks and crevices or deposited beneath surface of filth, thus affording excellent harborage for the lar During the breeding season, which continues throughout the sum months, the abdominal cavity of the female is densely packed w eggs.

The wings are attached to the thorax and are characterized dark lines or veins extending through the wing membrane, the ma ings varying with the different species. There are three pairs of le all rather thickly covered with hair. Both the legs and wings admirably adapted to the mechanical transference of substances w which they come in contact. It is this interchange of material, rived in many instances from polluted and filthy sources, which o stantly exposes man to the danger of disease.

Reproduction.

Flies are extremely prolific. The stages in the life cycle occ at the most but a few days, and sexual maturity is reached wit three or four days from the date of birth. As the eggs depos by the female usually number a hundred or more, an enormous crease in the fly population is possible within a short period of ti Like many other insects, flies pass through several developme stages, the immature forms differing radically from the adult. T are four stages in the developmental cycle, namely, the egg, t the larva or maggot period, next the pupa, chrysalis or resting st and finally the adult fly. A thorough understanding of each these stages is essential for a proper appreciation of the rôle play in disease transmission.

Nearly all flies breed in organic filth. A favorite mediun horse manure, but decaying vegetables, fermenting kitchen ref human excreta, and putrefying animal matter offer sites which nearly as well adapted to the conditions necessary for propagat The barn manure pile may be the place of origin of thousand flies, the unprotected and unscreened privy serves as an excel nidus for their growth, while fermenting foodstuffs and other w food products scattered about the yard may also assist in t propagation. But three conditions are necessary for fly propaga in filth of this character, namely, proper temperature, moisture, food supply. The warm manure pile and the decaying and ferm ing garbage heap admirably fulfill these conditions. The sw

of flies invariably seen about places of this character during the summer season is an indication that not only feeding but actual propagation is in process.

The breeding season varies with the climate, usually beginning during May in the North, but more often in early April or March in the South; propagation continues until late September. The fly population is usually greatest during August and September, after which it very rapidly diminishes.

The eggs of the common house fly are smooth, white, glistening bodies about a twentieth of an inch in length, oval in shape and slightly broader at one extremity than the other. They are usually found in irregularly massed batches, each female depositing several such aggregations during her lifetime. The long ovipositor enables the female to deposit the eggs in crevices or beneath the surface of filth where the desiccating action of the atmosphere is less pronounced. Ordinarily a period of but 12 hours is required from the time the eggs are deposited until they are transformed into larvæ, although if the temperature is not favorable, two, three, or more days may be necessary. Hatching merely consists of the splitting of one extremity of the egg sac and the emerging of the larva.

U.S.PUBLICHEALTH SERVICE

Eggs of house fly, greatly enlarged. (After Hewitt.)

The larvæ, or maggots as they are commonly known, represent the second stage in the development of the fly. They are about twice the size of the eggs but of much the same color and shape, the body being somewhat indistinctly segmented. There are no legs; nevertheless, by aid of the mouth. parts and rudimentary enlargements on the under surface of the body the larvæ are actively motile and may travel a considerable distance. During the course of their growth they pass through two moults, constantly feeding upon the substance in which they are contained and reaching maturity in from 3 to 6 days. The larvæ exhibit a tendency to congregate in a zone just beneath the surface of the mass upon which they are developing, seldom being found directly upon the surface. After arriving at maturity they migrate or leave the substance where they commenced their growth, burrowing into the soil or even traveling a distance of several feet over the ground. This migration must always be considered and guarded against when fly-eradicative measures are instituted.

When the fly reaches the third or chrysalis stage of its development it is known as the pupa. This period is characterized by a contraction of the body, a change to a darker color, and a disintegration of the larval parts, with a corresponding growth of the wings and

Bearing a strong resemblance to the bluebottle fly is the greenbottle fly, which is slightly smaller and metallic green in color. It is commonly found near putrefying flesh, such as dead animals, excreta, and similar filth. A member of this family often breeds in the excrement on the backs of sheep, the larvæ or maggots developing and feeding thereon, resulting in a serious pest to flockowners. When the larvæ mature in either filth or flesh they are especially voracious and consume a large part of the substance on which they are developing. The fact that insects of this family alternate between human excreta and food products renders them especially dangerous, although their number is usually limited and they are seldom satisfied to remain within doors.

One of the most important species is the stable fly, or, as it is sometimes called, the "biting stable fly." It is less often found in filth than the other varieties, but owing to the fact that it is a bloodsucking fly, opportunity for the direct inoculation of persons and animals with the organisms of disease is presented. It is this insect which has been incriminated in the spread of anthrax. The stable fly is about the size of the common house fly and resembles it in appearance, being gray in color and somewhat more stoutly built. Its proboscis, however, is of an entirely different character, as campers and others can testify, being arranged for penetrating and sucking. It frequently torments horses and cattle and may even cause detriment to stock through its activities. The insect is widely distributed. Closely allied to the stable fly is an African species known as the tsetse fly, which is responsible for the spread of sleeping sickness, a fatal infection found in certain regions of the African Continent.

The lesser house fly" is the name given to a species which, next to the house fly, is the most common indoor resident. Probably everyone has observed the useless and apparently aimless, jerky flight of this insect beneath some suspended article, such as a chandelier. This fly is an early visitor, usually being found before the common house fly is present in large numbers. Its breeding habits are the same as those of the house fly, but as it feeds less diligently and seldom alights, it is somewhat less objectionable. It strongly resembles the house fly but is slightly smaller and more slender, being, perhaps, better adapted to flight. The larval form of this fly is easily distinguished from that of the common house fly, as it is covered with spines.

In addition to the species enumerated, many other bloodsucking and nonbloodsucking varieties are of interest. The cheese fly deposits its eggs in cheese or fatty material, producing the so-called cheese skippers. The dung and the yellow dung flies and the latrine fly are so named because they develop in the excrement of animals