UNIVERSITY OF CALIFORNIA PUBLICATIONS COLLEGE OF AGRICULTURE AGRICULTURAL EXPERIMENT STATION BERKELEY. CALIFORNIA THE HOUSE ELY IN ITS RELATION TO PUBLIC HEALTH By WILLIAM B. HERMS BULLETIN No. 215 (Berkeley, Cal., May, 1911) SACRAMENTO W. W. SHANNON - - - - SUPERINTENDENT STATE PRINTING 1911 EXPERIMENT STATION STAFF. E. J. Wickson, M.A., Director and Horticulturist. E. W. Hilgard, Ph.D., LL.D., Chemist (Emeritus). W. A. Setchbll, Ph.D., Botanist. Lerot Anderson, Ph.D., Dairy Industry and Superintendent University Farm Schools. M. E. Jaffa, M.S., Nutrition Expert, in charge of the Poultry Station. R. H. Loughridge, Ph.D., Soil Chemist and Physicist (Emeritus). C. W. Wood worth, M.S., Entomologist. Ralph E. Smith, B.S., Plant Pathologist and Superintendent of Southern California Pathological Laboratory and Experiment Station. G. W. Shaw, M.A., Ph.D., Experimental Agronomist and Agricultural Technologist, in charge of Cereal Stations. E. W. Major, B.Agr., Animal Industry, Farm Manager, University Farm, Davis. F. T. Bioletti, B.S., Vitlculturist. B. A. Etcheverry, B.S., Irrigation Expert. George E. Colby, M.S., Chemist (Fruits, Waters, and Insecticides), in charge of Chemical Laboratory. H. J. Quayle, M.S., Assistant Entomologist, Plant Disease Laboratory, Whittier. W. T. Clarke, B.S., Assistant Horticulturist and Superintendent of University Exten- sion in Agriculture. H. M. Hall, Ph.D., Assistant Botanist. C. M. Haring, D.V.M., Assistant Veterinarian and Bacteriologist. John S. Burd, B.S., Chemist, in charge of Fertilizer Control. E. B. Babcock, B.S., Assistant Agricultural Education. W. B. Herms, M.A., Assistant Entomologist. J. H. Norton, M.S., Assistant Chemist, in charge of Citrus Experiment Station, River- side. W. T. Horne^-B.S., Assistant Plant Pathologist. J. E. Corr, Ph.D., Assistant Pomologist, Plant Disease Laboratory, Whittier. C. B. Lipman, Ph.D., Soil Chemist and Bacteriologist. R. E. Mansell, Assistant in Horticulture, in charge of Central Station grounds. A. J. Gaumnitz, M.S., Assistant in Cereal Investigations, University Farm, Davis. E. H. Hagemann, Assistant in Dairying, Davis. B. S. Brown, B.S.A., Assistant in Horticulture, University Farm, Davis. F. D. Hawk, B.S.A., Assistant in Animal Industry. J. I. Thompson, B.S., Assistant in Animal Industry, Davis. R. M. Roberts, B.S.A., Field Assistant in Viticulture, University Farm, Davis. J. C. Bridwell, B.S., Assistant Entomologist. C. H McCharles, B.S., Assistant in Agricultural Chemical Laboratory. N. D. Ingham, B.S., Assistant in Sylviculture, Santa Monica. E. H. Smith, M.S., Assistant Plant Pathologist. T. F. Hunt, B.S., Assistant Plant Pathologist. C. O. Smith, M.S., Assistant Plant Pathologist, Plant Disease Laboratory, Whittier. F. L. Yeaw, B.S., Assistant Plant Pathologist, Vacaville. F. E. Johnson, B.L., M.S., Assistant in Soil Laboratory. Charles Fuchs, Curator Entomological Museum. P. L. Hibbard, B.S., Assistant Fertilizer Control Laboratory. L. M. Davis, B.S., Assistant in Dairy Husbandry, University Farm, Davis. L. Bonnet, LA., Assistant in Viticulture. S. S. Rogers, B.S., Assistant Plant Pathologist, Plant Disease Laboratory, Whittier. B. A. Madson, B.S.A., Assistant in Cereal Laboratory. Walter E. Packard, M.S., Field Assistant Imperial Valley Investigation, El Centre M. E. Stover, B.S., Assistant in Agricultural Chemical Laboratory. P. L. McCreary, B.S., Laboratory Assistant in Fertilizer Control. E. E. Thomas, B.S., Assistant Chemist, Plant Disease Laboratory, Whittier. Anna Hamilton, Assistant in Entomology. Mrs. D. L. Bunnell, Secretary to Director. W. H. Volck, Field Assistant in Entomology, Watsonville. E. L. Morris, B.S., Field Assistant in Entomology, San Jose. J. S. Hunter, Field Assistant in Entomology, San Mateo. J. C. Roper, Patron University Forestry Station, Chico. J. T. Bearss, Foreman Kearney Park Station, Fresno. E. C. Miller, Foreman Forestry Station, Chico. CONTENTS. Page. Introduction. Insects and disease transmission ; medical entomology 513 Economic Importance. Losses occasioned by mosquitoes ; malaria and real estate ; typhoid fever losses; cost of temporary preventive measures 513 Methods of Disease Transmission. Direct and indirect methods ; dependent upon structure of mouthparts ; dis- tinctions in habit and ecology of pathogenic bacteria and protozoa 514 What is the House Fly? Relation to other flies 517 Life History and Development. Number of eggs deposited ; suitable situation for egg deposition ; estimate of number of larvae developing in a manure pile ; larval growth ; prepupal period ; pupal period ; time required for life cycle ; size of the adult fly ; distribution of sexes ; relative number of house flies in dwellings 518 Relation of House Fly to Disease Transmission. Probably not a necessary intermediate host of pathogenic organisms ; im- portance of structure and habit ; not a valuable scavenger ; experimental evidence of transmission 523 Typhoid Fever. Channels of infection; evidence of transmission by house fly 528 Dysentery. Varieties of; transmission 529 Summer Diarrhea in Infants. Infection 530 Tuberculosis. Channels of infection ; infection by way of the alimentary tract ; pre- cautions : 530 Asiatic Cholera. Methods of transmission 532 Other Diseases Probably Transmitted. Certain forms of Opthalmia ; Leprosy, Erysipelas; Smallpox 532 Objections Met. Opposition ; parasitism, abstracts from letter 532 Essentials of Control. Prevention planned along scientific lines; house fly can be controlled; co- operation needed ; open manure piles must be abolished -, fly-tight recep- tacles ; permanent preventive measures ; insecticides ; larvae ; adult flies ; carrying out preventive measures 539 Indoor Work — the Adult Fly 539' Community- Wide Campaigns. How begun ; newspaper agitation ; work of Department of Public Health and Safety 541 Literature Cited. THE HOUSE ELY IN ITS RELATION TO PUBLIC HEALTH i. INTRODUCTION. The importance of the study of insects in many departments of human interest is being the more fully recognized as science reveals the facts of interrelationships, both advantageous and destructive. Mosquitoes and flies have for centuries past been looked upon as a source of extreme annoyance to the human family, but that these insects might be trans- mitters of disease was hardly even suspected until the latter part of the last century. (King, '83.) That insects and arachnids of a given species might be the sole transmitters of a specific disease, and, what is more, a necessary factor, inasmuch as these forms serve as intermediate host, was not considered seriously until the latter five years of the last and the beginning of this, the twentieth century. (See sundry papers by A. Laveran, R. Ross, R. Koch, P. Manson, B. Grassi et al. ; all on malaria and its causative organism. ) There have now appeared literally hundreds of isolated papers relating to the transmission of disease by insects, and we are at this time obliged to recognize the new subject which we term Medical Entomology. (Herms, '09<1) This field embraces phases of the study of medicine, mainly the etiology and pathology of such diseases as malaria, yellow fever, sleeping sickness, filariasis, etc., of bacteriology, inasmuch as the causative germ must be studied, and of entomology, inasmuch as the mouthparts and other structures of the insect must be known as well as its systematic relationships. There need be no question as to the justification of our investigations in this ever broadening field when the economic loss to mankind is con- sidered. One need but consult the timely and valuable paper by Dr. L. 0. Howard ('09) entitled "Economic loss to the people of the United States through insects that carry disease," to be impressed with this enormous loss. Doctor Howard states, "Entirely aside from the loss occasioned by mosquitoes as carriers of specific diseases, their abundance brings about a great monetary loss in other ways. Possibly the greatest of these losses is in the reduced value of real estate in mosquito-infested regions, since these insects render absolutely uninhabitable large areas of land available for suburban homes, for summer resorts, for manufac- turing purposes, and for agricultural pursuits." (See, also, Herri ck, '03.) The expense to the United States incurred in the purchase of fly traps, sticky fly paper, fly poison and the like, must certainly exceed 514 UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION. two millions of dollars annually, while Howard ('09) estimates the cost of screening at over ten millions of dollars per annum. The commercial value of a human life is estimated on the average at three thousand dollars. The decrease in the vital assets of this country through typhoid fever alone (much of which is transmitted by the house fly or typhoid fly) amounts to $350,000,000. (Felt, '09.) The California State Board of Health in "The California Sanitation Exhibit, 1909," writes as follows: "California loses annually 5,000 citizens from tuberculosis, 500 from typhoid fever, and 500 more from diseases caused by infected milk and food supply. This means that approximately one out of every five residents eventually dies from one of these diseases contracted through personal or public failure to provide the essentials of sanitary environment. * '* * The six thousand deaths which could be prevented by the enforcement of public health laws represent an annual loss of $18,000,000 per year to the State, in addition to the personal and social losses which can not be estimated in terms of money. ' ' Malaria, typhoid fever, yellow fever, bubonic plague, sleeping sick- ness, cholera, are all preventable diseases, carried wholly or in part by insects. The enormous sums of money spent in the temporary control of these diseases might well be spent in a more effective manner, i. e., directed toward the root of the evil — at the cause. "Eliminate the cause, you elim- inate the effect." This is the service that medical entomology is to afford — its aim is the control of disease transmitting insects. The most vulnerable point in the life history of the insect is sought, and the most effective methods of control are then applied. METHODS OF DISEASE TRANSMISSION. Broadly speaking, there are two methods of disease transmission in which insects are concerned, namely, a direct and an indirect method, based on the structure of mouthparts. The direct method depends upon piercing mouth struc- tures (Fig. 1) capable of penetrating the animal skin and introducing Fig. 1. — Head of the stable fly, Stomoxys calcitrans, illustrating the type of piercing mouthparts which relate to the direct transmission of pathogenic organisms. The sheath or labium encloses slender, piercing bristles. Bulletin 215] THE HOUSE FLY AND HEALTH. 515 into the circulation a pathogenic organism. The indirect method is based on the accidental accumulation of pathogenic organisms upon foot or mouth structures and introducing these on the food of the human being, relating mainly, therefore, to intestinal diseases, such as typhoid fever, Asiatic cholera, and dysentery. Other than this, insects may act as parasites, both external (lice, etc.) and internal (bot-flies, etc.) causing irritations and disease, or they may produce wounds by the introduction of a specific poison through the bite, as does the bedbug, the kissing bug, and the like. Two common insects will serve to illustrate the two principal methods, namely, the stable fly (Fig. 3) on the one hand— the direct method ; and the house fly (Fig. 4) on the other — the indi- rect method. The former pos- sesses mouthparts which are adapted to penetrate the skin (Fig. 1), introducing into the blood pathogenic organisms which attack the red corpuscles! or other liquid portions of the body, such as the cerebro-spinal fluid. The stable fly is known to transmit a Trypanosome disease (Surra) of the Philippine Isl- ands (Laveran et Mesnil, '04) ; a closely related genus the Glossina or Tsetse fly transmits other Trypanosome diseases, such as sleeping sick- ness. (Laveran et Mesnil, '04.) The second type (indirect) is represented by the house fly (Fig. 4), an important transmitter of intestinal diseases, because it is readily attracted to excrementous matter, vomit and sputum, collecting there the "germs" upon its mouthparts (Fig. 2) and feet and then carrying them to the food of human beings, thus readily causing infection. The house fly, notwithstanding public opinion, can not pierce the skin since its proboscis is quite fleshy and not equipped with piercing bristles. It is the stable fly which inflicts the wound, as may be seen by a careful examination of the mouthparts as shown in the illustration (Fig. 1), but because of the mutual resemblance the house fly is blamed. Thus we see that the usual method of insect classification, that of Fig. 2. — Head of the house fly, Musca clomestica, illustrating the type of suc- torial mouthparts not adapted to piercing the human skin ; but because of the presence of numerous bristles and hairs a good collector of filth and germs relating to the indirect trans- mission of disease. 516 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION. Fig. 3. — The stable fly, Stomoxys calcitrans, enlarged. biting and sucking insects, does not apply in this work, since the two forms mentioned, namely, the house fly and the stable fly are both suctorial, and, indeed, very closely related systematically, yet they relate very differently to disease transmission. It is wise for the student of medical entomology to note the distinction between the vege- table pathogenic (disease producing) organisms, such as the bacteria, and the animal pathogenic organisms, such as the Protozoans, since the two classes vary considerably in their longevity and viru- lence when outside the human body, and behave differently within the bodies of different insects, e. g., typhoid fever is a bacterial disease, the causative germs of which outside the body are present in the excrement and urine; malaria, on the other hand, is a Protozoan dis- ease which can not live outside the human body, except in the mosquito of the genus Anopheles. Similar contradistinctions might be made between tuberculosis and African sleeping sickness, the pathogenic bacterial or- ganism of the former present outside the body largely in the sputum, and the protozoan patho- genic organism of the latter present in the body of the Glossina (Tsetse) fly, having previously been sucked up with the blood by this sucking fly. Again, bubonic plague is a bacterial disease, while Texas fever and the African tick fever are protozoan types. Fig. 4. — The house fly, Musca domestica, enlarged. Bulletin 215] THE HOUSE FLY AND HEALTH. 51' WHAT IS THE HOUSE FLY? Properly speaking, only one species of fly (Musca domestica Linn, Fig. 4) is rightly called the house fly, though there are several species which invade the house, either regularly or at times. A brief account of these latter species may be useful here by way of com- parison. The blow fly or blue bottle fly (Calliphora vom- it oria, Fig. 5) is the large, noisy fly seen frequently on the window and about meat. This is typically a flesh fly, depositing its eggs on the meat in the pantry or else- where, finding the proper food for its larvae often in the most Fig. 5. — The blow fly or blue bottle fly, Calliphora vomitoria, enlarged. protected situations. The female often deposits its eggs in proximity to meat and the larvae on hatching crawl to this food. While the blow fly is conspicuous it is not as plentiful as the house fly. and is not as liable to be found walking about on the prepared foods of man. Another flesh fly conspicuous because of its bright metallic green color is the green bottle fly (Lucilia Ccesar, Fig. 6). This insect rarely comes into the house, and seldom remains long, owing to its rapid response to differences in light in- tensities. (Herms, '09 d and e.) It is typically a fly of the out-of-doors, and a very good scavenger. (Herms, '07. ) The stable fly (Stomoxys calcitrans, Fig. 3), as has already been mentioned, is often confused with the house fly because of its close Fig. 6. — The flesh fly or green bottle fly, Lucilia Caesar, en- larged. 518 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION. resemblance, but the structure of the mouthparts will serve to readily distinguish the two. (Figs. 1 and 2.) Several other flies are commonly found indoors, but are not readily distinguished from the common house fly; only careful examination of the wing venation and other minute characters can serve to correctly iden- tify the species. Several species belonging to the following genera, Pollenia, Morellia, and Muscina belong to the same family as the house fly, namely, Muscidce, while others, such a s Homalomyia and Anthomyia (Fig. 7), belongs to another family, Fig. 8. — Wing of the house fly, Musca do- mestical illustrating the typical wing venation of the Muscidae. Fig. 7. — Anthomya fly (enlarged), whose larvae are known as root maggots. This fly is often found indoors and closely resembles the house fly. namely Anthomyidce. The wing venation of the two families is quite characteristic, as illus- trated by the two figures. (Figs. 8 and 9.) The cell, marked with an x, is more or less completely closed in the Muscidce, and is open in the Anthomyidce. Fig. 9. — Wing of the Anthomya fly, Antho- myia radicum, illustrating the typical wing venation of the Anthomyidae. LIFE HISTORY OR DEVELOPMENT. By life history is meant the development of the organism from the egg to the adult. The house fly belongs to that group of insects which passes through a complex metamorphosis unlike that of the grasshopper, for example, which gradually grows up to the adult without changing much in general form. The house fly, on the other hand, passes through several stages, each unlike the other, namely, the egg, the larva (mag- Bulletin 215] THE HOUSE FLY AND HEALTH. 519, got), the pupa (resting stage), and the imago or full grown winged insect. (Fig. 10.) From 75 to 125 eggs are deposited singly in one mass, and there are usually several (2 to 4) such layings. Excrementous material, especially of the horse, is the favorite place upon which the eggs are deposited. Other suitable situations are kitchen refuse, unused brewer 's grain, and other decaying vegetable matter. Where the city garbage is carefully disposed of with only ordinary attention to horse manure it seems quite safe to say that ninety-five per cent of the house flies are bred in the latter situation. (An effort made to breed these flies in the laboratory in cow manures proved unsuccessful, but adults were reared from full grown larvae and pupae collected in and near the manure pile from the dairy, showing that the house fly may breed in this material, though Fig. 10. — Illustrating the life history of the house fly: a egg stage; b larval stage or maggot, full grown ; c resting stage or pupa ; d the imago or adult. probably only to a limited extent.) The eggs hatch in from twelve to twenty-four hours and the newly hatched larvae begin feeding at once. To gain an estimate of the number of larvae developing in an average horse manure pile, samples were taken from such a pile after an expo- sure of four days with the following results: First sample (4 lbs.) con- tained 6,873 larvae; second sample (4 lbs.), 1,142; third sample (4 lbs.), 1,585; fourth sample (3 lbs.), 682; total, 10,282 larvae in 15 pounds. (Herms, '09.) All these larvae were quite or nearly full grown. This gives an average of 685 larvae per pound. The weight of the entire pile was estimated at not less than 1,000 pounds, of which certainly two thirds was infested. A little arithmetic gives us the astonishing estimate of 455,525 larvae (685 x 665), or in round numbers, 450,000. This par- ticular manure pile (not from a livery stable, either), was only one of many known to exist in various parts of the city. No wonder flies fairly 520 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION. swarm in the vicinity of these choice ornaments! Five samples of an ounce and a half of manure each furnished the following numbers, viz. : (1) 58 larva?, (2) 64, (3) 70, (4) 228, (5) 49. Total, 469 larva? to seven and one half ounces, an average of nearly one thousand per pound. The larval stage is the growing period of the fly and the size of the adult will depend entirely upon the. size that the larva attains. An underfed larva will result in an undersized adult, which fact is well illustrated by Fig. 11, based on experiments tried on the flesh fly, Lucilia Ccesar. (Herms, '07.) This growing stage requires from four to six days, after which the maggots often crawl away from their breed- ing place, many of them burrowing into the loose ground just under- neath the manure pile, or crawling under boards or stones, or into dry manure collected under platforms and the like. (One and three fourths pounds of dry manure, taken from a situation last men- tioned, contained 2,561 pupaa.) The larvae often pass three or four days in the prepupal or migrated stage before actually pupating; but in a given set of individuals under simi- lar conditions the various stages are remarkably similar in duration, when one pupates the rest will cer- tainly follow in short order, and when one emerges as an adult others quickly appear. The aver- age time required for development 1. 2. 3. 6. Larva overfed, pupation re- tarded. Optimum, 60-72 hours. 60 hours. 54 hours. 4 8 hours. 42 hours. 36 hours. Fig derL^ng^ela'rva'hLtn^e^L^r is differently estimated by various observers, inasmuch as temperature the adult fly (Lticilia Caesar). Over- feeding, if it does not result fatally, fhfo^um^m'S'mVy'^be ^^ %°& greatly influences the time required. uppermost individual, which is the same size as the next lower individual or Optimum. Each of the next lower in- dividuals is the result of decreasing the time of feeding by six hours. These results are based on a large number of individuals in each case. Packard (74) gives the time at from ten to fourteen days, Howard ('06) at Washington, D. C, as ten days. In Berkeley, where the weather is uniformly cooler (rarely above 80° and a mean of 48° during the winter months) the life cycle is completed usually in from fourteen to eighteen days, less often in twelve days. Prolonged cool weather or Bulletin 215] THE HOUSE FLY AND HEALTH. 521 artificially cooled environment results in greater retardation. Even allowing for such retardation, the number of generations produced during the summer is quite large and in California (Berkeley) I have seen house flies emerging from their breeding places during every month of the winter season. This latter fact lends even greater importance to a house fly campaign. In early March a veritable pest of flies was encountered while on a trip through the Imperial Valley (California). When the fly emerges from the pupa case with fully developed wings, it is as large as it ever will be, except in expansion of tissue and addition in weight, due to stomach contents or development of eggs in the female. This explains why no young house flies are seen (young in the sense of being small). The little flies upon the windows are not "baby" flies, but belong to another species, also adult. One can easily influence the size of a fly by underfeeding it in the larval stage, as illustrated in Fig. 10 (see Herms, '07). The question has been asked, "Why are all house flies so nearly of one size?" This is not altogether true. There are some undersized house flies, but the greater majority of the larvae or maggots find ample food for optimum development. Furthermore, experiments show that the house fly is not as plastic in respect to food conditions as the flesh fly, for instance : in other words, larvae which are underfed perish easily. In order to determine the distribution of the sexes, observations were made under two different conditions, viz., first, six sweepings with an insect net were made over a horse manure pile on which many flies had gathered (the results are shown in Table I) ; second, all but half a dozen flies were collected in one house, giving a fairly representative lot for indoors, even under screened conditions. (See Table II.) TABLE I. Showing results with regard to sexes and species in six sweepings from a horse manure pile on May 18 and 19, 1909. First. Second. Third. Fourth. Fifth. Sixth. Total. M F M F M F M ' F M F M F M F House fly ( Musca domestica) Muscina stabulans Blow fly (Calliphora vo'mitoria) Lucilia Cwsar Other species* 7 2 2 1 153 6 2 1 4 4 81 7 1 1 4 3 1 2 64 5 1 1 9 2 4 77 5 1 2 4 3 4 210 10 2 5 1 1 2 112 4 32 8 3 13 697 37 3 4 13 Totals 12 166 4 94 6 71 15 85 11 222 9 116 56 754 ♦Including Anthomyids and Scatophagids, but excluding many tiny Diptera, prob- ably Sepsis. 522 UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION. TABLE II. Showing number of individuals collected in a screened dwelling June 1, M House fly, Musca domestica. Muscina sp. Homalomyia sp. Calliphora Totals 95 116 1 2 119 Explanation and comparison of Tables I and II. — These two tables give us some information as to the relative abundance of the house fly, and the distribution of the sexes. Table I shows clearly that of those flies which frequent both the manure pile and the home, the house flies compose 90 per cent, and that of the total collected, over 95 per cent (95.4 per cent) were females. Thus, it is clear that it is the " instinct' ' to oviposit (to lay eggs) that has mainly attracted these insects to this situation. In fact, fresher parts of the manure pile are often literally white with house fly eggs in countless numbers. Observations made in the near vicinity of the manure pile proved that certainly the same per- centage (over 95 per cent) of the flies clinging to the walls of the stable, boxes, and so on, were males. Of the total number of house flies (202) collected indoors (June, '09), representing all but perhaps six of the total number in that particular house, 57 per cent were females, showing nearly equal distribution for the sexes. This would, it seems, indicate that the sexes of this insect are equally attracted to the house by odors issuing therefrom. Another interesting point is that of the total number of flies thus collected indoors (214), 94.4 per cent were the common house fly. This estimate can not, of course, be taken as a standard. The number is entirely too small, but it does come very near the percentage determined by Dr. L. 0. Howard (Howard, '00), who "made collections in the dining-rooms in different parts of this country, and out of a total of 23,087 flies, 22,808 were Musca domestica, that is 98 per cent of the whole number cap- tured." That the sexes in the housefly are normally about equal in number is apparent, inasmuch as of a total of 264 pupse collected indiscriminately and allowed to emerge in the laboratory, 129 were males and 135 were females. The author has, however, made observations on the flesh flies, Lucilia Ccesar, and Calliphora vomitoria, which indicate that the factor of underfeeding must be considered in this connection. From a large amount of unpublished data, it seems evident that underfeeding results Bulletin 215] THE HOUSE FLY AND HEALTH. 523 in the emergence of a greater percentage of males ; this does not mean, however, that sex is influenced by feeding, it only means that cutting short on food supply destroys the larval females first. Feeding experi- ments, not yet complete, on the house fly indicate that the same holds true here, but also that this insect is not so plastic as the flesh fly, hence does not vary so greatly in size and dies more easily when underfed. Anatomical considerations are here omitted, inasmuch as that phase of the subject is admirably treated in a work by Hewitt ( '07). RELATION TO DISEASE TRANSMISSION. We should be familiar with the actual method of disease transmission by the house fly. Some insects act as intermediate host for pathogenic organisms, which latter can not exist sexually and be transmitted with- out the insect, e. g., the malarial fever parasite (Plasmodium malarice and other species), which passes part of its life history in the body of the Anopheles mosquito. The house fly, as far as known, is not an Fig. 12. — Foot of the house fly greatly enlarged. Note the many fine hairs with which the foot-pads are provided. intermediate host necessary to the life history of a pathogenic organism, but is by accident of habit and structure one of the most important and dangerous of disease transmitting insects. In habit the house fly is revoltingly filthy, feeding indiscriminately on excrement of all kinds, on vomit and sputum, and is, on the other hand, equally attracted to the daintiest foods of man, and will, if unhindered, pass back and forth between the two extremes. The house fly's proboscis (Fig. 2) is pro- vided with a profusion of fine hairs which serve as collectors of germs and filth; the foot of the fly when examined under the microscope 524 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION. presents an astonishing complexity of structure, illustrated in Fig. 12. Each of the six feet is equally fitted with bristly structures and pads, which secrete a sticky material, adding thus to their collecting powers. This structural condition, added to the natural vile habits of the house fly. completes its requirements as a transmitter of infectious diseases. This creature has long been known to contaminate food, but has, nevertheless, been regarded as a scavenger, and thus as a real servant of man, but if there remains any doubt in the mind of the reader, after reading what follows, as to the necessity of getting rid of this wolf in sheep 's clothing, let him take the time to make a few careful observations for himself. Circumstantial evidence against the house fly as a transmitter of such infectious diseases as typhoid fever, tuberculosis, dysenteries, and cholera is complete as summed up thus : First, it possesses the best pos- sible structures for the conveyance of " germs" and filth; second, it possesses the habit of feeding on excrementous matter of all kinds, vomit and sputum; third, the causative organisms ("germs") of the above named diseases are present in the matter mentioned in the second clause ; fourth, the house fly is the principal fly found in dwellings, alighting upon the prepared food of man, or on food products in grocery stores, fruit stands and meat markets. Experimental evidence that the house fly actually does carry bacteria upon its mouthparts and feet or in its intestinal tract is not wanting. To illustrate, the following simple experiment may be cited. In order to show that the house fly (Musca domestica) can carry "germ" of a known kind, a partly sterilized fly was placed in a test tube containing a culture of Micrococcus aureus. After walking about in this tube and becoming contaminated with the Micrococci, the fly was transferred to a sterile agar-agar plate upon which it was allowed to crawl about for three minutes. The plate was then incubated for twenty-four hours, after which it was examined and photographed, as shown in Fig. 13. The photograph shows the trail of the fly as it had walked about. Every place that the foot touched is plainly marked by a vigorous bacterial growth. That the fly can not easily get rid of all the bacteria on its feet is also illustrated by this photograph, inasmuch as three minutes spent crawling about on the agar plate did not apparently lessen the growth-vigor of bacteria deposited, and a second plate of agar-agar contaminated by the same fly immediately after exposure of the first plate gave equally striking results. The same experiment was performed, using the Bacillus prodigeosus with even more pronounced results, as shown in Fig. 14. These experiments were repeated several times with like effect. Bulletin 215] THE HOUSE FLY AND HEALTH. 525 A second series of experiments was carried on as follows : During the midde of May (1909) house flies were captured in various parts of Berkeley, placed at once in sterilized vials, and in the laboratory placed under bell jars with agar-agar plates, all under sterilized conditions. After the flies had crawled about on the culture media, the latter was incubated for twenty-four hours. In every case, but one, a strong growth of bacteria appeared. This one was incubated longer and after forty hours four centers of infection appeared. This fly had been taken on a sunny wall on one of the main streets, and having been under Fig. 13. — Cultures of Micrococcus aureus transferred bv a house fly to a sterile agar-agar plate upon which it was allowed to crawl for three minutes. Incubation period 24 hours. observation in this position for a long time (as reported by the assistant) it was first supposed that the action of the sunlight had sterilized it. This series of experiments included flies taken from a number of situa- tions, namely, principal thoroughfares, sunny walls, street corners, manure piles, and the dining-room. Without exception these flies were laden with bacteria, and in all cases the greatest care was exercised not to introduce any accidental infection on to the culture plates. 2— Bul. 215 526 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION Probably the most accurate study of these factors was carried on by Est en and Mason (Esten and Mason, '08) on the " Sources of Bacteria Fig. 14. — Cultures of Bacillus prodigeosus transferred by a house fly to a sterile agar-agar plate upon which it was allowed to crawl for only a few moments. Incubation period 24 hours. in Milk, ' ' and certainly most striking facts were revealed. The follow- ing table and attached remarks are taken from that publication, and need no further comments or explanations : Bulletin 2151 THE HOUSE FLY AND HEALTH. 527 o M -h H e H £ J <4> pq O < . 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