or man. The fruit fly, a much smaller species than any of those mentioned, hovers about fruit juices, cider barrels, and like situations, being often found in the dregs of wine. None of these species, however, is as important as the common house fly. Structure. The parts of the fly are the head, thorax, and abdomen. The head is connected with the thorax by a narrow neck which permits of rather wide movement. The greater part of the head is occupied by the eyes, some several thousand in number, described as compound. Between the compound eyes and near the top of the head is a triangular arrangement of three simple eyes. The upper two are much farther apart in the female than in the male, thus serving to easily differentiate the sexes. In spite of the arrangement of the eyes and the great mobility of the head, it is not believed that the vision of flies is especially acute, although the range of vision is wide. The sense of smell, however, is highly developed. The proboscis protruding from the under and back part of the head is the most interesting part of the fly. When the insect is at rest, the proboscis is folded against the head, but upon alighting it is protruded through the mechanical action of certain air sacs. Capping the end of the proboscis are two oval projections or lobes forming an opening leading into the mouth. The oral lobes in the house fly entirely prevent penetration of the skin by the proboscis, therefore this particular species is in no sense a biting fly. Biting invariably constitutes positive proof that the insect is not a house fly, however much it may resemble that species. On the under and inner side of the oral lobes are grooved channels which lead into the mouth. When these channel-like surfaces are placed in contact with liquids, suction is performed by the pharynx and the substance is drawn into the oesophagus, to be continued through the narrow neck into the thorax. If the food is solid it must first be dissolved through the action of saliva secreted by the salivary glands or reduced to very minute particles. The greater part of the thorax is occupied by the muscles used in flying, these being placed above the stomach. Connecting with the sophagus after it passes into the chest is a small duct leading to a dilatation within the abdomen known as the crop. From this receptacle food is frequently regurgitated, appearing at the mouth parts in the form of small globules, to be again devoured at the insect's leisure. It is this habit of regurgitation, or vomiting, which renders flies extremely objectionable from a sanitary standpoint, particularly as the stomach contents are obtained in most instances from filth and garbage. 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 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. M 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 |