Insects and Diseases / A Popular Account of the Way in Which Insects may Spread or Cause some of our Common Diseases

LIFE-HISTORY AND HABITS

The eggs of the house-fly may be laid on almost any kind of decaying or fermenting material. If this is kept moist and a proper temperature maintained the larvæ or maggots (Fig. 47) that hatch from the eggs may develop. As a rule, however, these requirements are found only under certain conditions and are ordinarily found only in manure heaps or in privy vaults or latrines. All observers agree that the female fly prefers to deposit her eggs in horse manure when this can be found and when this is piled in heaps in the barn-yard (Fig. 48) or in the field the heat caused by the decay and fermentation makes ideal conditions for the development of the larvæ. Cow manure may serve as a breeding-place to a limited extent. The flies are immediately attracted to human excrement and breed freely in it when opportunity offers. Decaying vegetables or fruit, fermenting kitchen refuse and other materials sometimes also serve as breeding-places.

In suitable places in warm weather the eggs will hatch in from eight to twelve hours and the larvæ will become fully developed in from eight to fourteen days. They then change to pupæ (Fig. 50) in which stage they may remain for another eight to twenty days when the adult flies will emerge. These figures must necessarily be indefinite because the weather and other conditions always vary. Under the most favorable conditions of moisture and temperature it is probably never less than eight days from egg to adult fly and under unfavorable conditions it may be as long as six weeks.

The larvæ thrive best when the manure is kept quite wet. I have often found them in almost incredible numbers in stables that had not been cleaned for some time. The horses standing there at night added fresh material and kept it just wet enough to make conditions almost ideal (Fig. 49).

The pupæ are usually found where the manure is a little dryer, but it must not be too dry. When the flies issue from the pupæ they push their way up to the surface where they remain for a short time and allow the body to harden and the wings to dry before they fly away to other manure or, as too often happens, to some near-by kitchen or restaurant or market place.

Of course it is impossible for them to issue from this filth without more or less of it clinging to their bodies. Now if these flies would breed only in barn-yard manure and fly directly from the stable to the house there would be comparatively little reason to complain, at least from a sanitary standpoint, for the amount of barn-yard filth that they carried to our food would be of little consequence. But when they breed in privy vaults or similar places, or visit such places before coming into the house or dairy or market place the results may be much more serious.

FLIES AND TYPHOID

It has been abundantly demonstrated that the excrement or the urine of a typhoid patient may contain virulent germs for some time before he is aware that he has the disease, and it has been shown that the germs may be present for weeks or months, and in some cases even years after the patient has recovered. If a fly breeds in such infected material, or feeds or walks on it, it is very apt to get some of the germs on its body where they may retain their virulence for some time, and should it visit our food while covered with these germs some of them would probably be left there where they might produce serious results. More than that. If the fly should feed on such infected material the typhoid germs would go on developing in the intestine of the fly and would be passed out with the feces in which they retain their virulence for some days. In other words, the too familiar "fly-specks" are not only disgusting, but may be a very grave source of danger. It will be seen that in this way several members of a community might become infected with the typhoid germs before anyone was aware that there was a case of typhoid or a "bacillus carrier" in the neighborhood.

One more example out of the scores that might be cited to show how the fly may carry typhoid germs. They may enter the sick chamber in the home or in the hospital and there gain access to the typhoid germs. These they may carry to other parts of the house or to near-by houses, or the flies may light on passing carriages or cars and be carried perhaps for miles before they enter another house and contaminate the food there.

These are hypothetical cases, but they illustrate what is taking place hundreds of times every season all over the world wherever typhoid fever and flies occur, and no country or race is known to be immune from typhoid, and the fly is found "wherever man is found."

In the summer of 1898 a commission was appointed to investigate the prevalence of typhoid fever in the United States Army Concentration Camps. The following are some of the conclusions as reported by Dr. Vaughan:

In Dr. Daniel D. Jackson's report to the Merchants' Association of New York on the "Pollution of New York Harbor as a Menace to the Health by the Dissemination of Intestinal Diseases Through the Agency of the Common House-fly," he shows graphically that the prevalence of typhoid and other intestinal diseases is coincident with the prevalence of flies, and that the greatest number of deaths from such diseases occurs near the river front where the open or poorly constructed sewers scatter the filth where the flies can feed on it, or along the wharves with their inadequate accommodations and the resulting accumulation of filth.

FLIES AND OTHER DISEASES

Not only is the house-fly an important factor in the dissemination of typhoid fever, but it has been definitely shown that it is capable of transmitting several other serious diseases.

The evidence that flies carry and spread the deadly germs of cholera is most conclusive. The germs may be carried on the body where they will live but a short time, or they may be carried in the alimentary canal where they will live for a much longer period and are finally deposited in the fly-specks where they retain their virulence for some time. Flies that had been allowed to contaminate themselves with cholera germs were allowed access to milk and meat. In both cases hundreds of colonies of the germs could later be recovered from the food. As with the typhoid germs milk seems to be a particularly good medium for the development of the cholera germs. In several of the experiments that have been made along this line the milk has been readily infected by the flies visiting it.

Of course an outbreak of cholera is of rare occurrence in our country, but unfortunately this is not so in regard to some other intestinal diseases such as diarrhea and enteritis which annually cause the death of many children, especially bottle-fed babies. Those who have made close studies of the way in which these diseases are disseminated are convinced that the flies are one of the most important factors in their spread.

It has long been observed that flies are particularly fond of sputum and will feed on it on the sidewalk, in the gutter, the cuspidor or wherever opportunity offers. It is well known, too, that the sputum of a consumptive contains myriads of virulent tubercular germs. A fly feeding and crawling over such material must necessarily get some of it on its body, and as it dries and the insect flies about the germs will be distributed through the air, possibly over our food. It has been shown that the excretion from a fly that has fed on tubercular sputum contains tubercular bacilli that may remain virulent for at least fifteen days. Thus we see again the danger that may lurk in the too familiar "fly-specks."

Although it is generally supposed that the flea is solely responsible for the spread of the bubonic plague and no doubt is the principal distributing agent, the fact must not be overlooked that the house-fly may also be of considerable importance in this connection. Carefully planned experiments have shown that flies that have become infected by being fed on plague-infected material may carry the germs for several days and that they may die of the disease. During plague epidemics flies may become infected by visiting the sores on human or rat victims or by feeding on dead rats or on the excreta of sick patients, and an infected fly is always a menace should it visit our food or open wounds or sores. Anthrax bacilli are carried about and deposited by flies showing the possibility of the disease being spread in this way.

Some believe that leprosy, smallpox and many other diseases are carried by the house-fly, so it is little wonder that it is fast losing its standing as a household companion and that we are beginning to regard it not only as a nuisance but as a source of danger which should no longer be tolerated in any community.

Of course only a small per cent of the flies that visit our food in the dairies or market places or kitchens actually carry dangerous diseases, but they are all bred in filth and it is not possible without careful experiments or laboratory analysis to determine whether any of the germs among the millions that are on their bodies are dangerous or not. The chances that they may be are too great. The only safe way is to banish them all or to see that all of our food is protected from them.

FIGHTING FLIES

Screens and sticky fly-paper have their places and give some little relief in a well-kept house. But of what use is it to protect your food after it has entered your home if in the stores, in the market place, in the dairy barn, or dairy wagon, in the grocers' and butchers' cart, it has been exposed to contamination by hundreds of flies that have visited it.

The problem is a larger one than keeping the house free from flies; larger but not more difficult, for the remedy is simple, effective, practicable and inexpensive. Destroy their breeding-places and you will have no flies. As the flies breed principally in manure the first remedial measure is to see that all manure is removed from the barn-yard at least once a week and spread over the fields to dry, for the flies cannot breed in the dry manure. If it is not practicable to remove it this often the manure should be kept in a bin that is closed so tight that no flies can get into it to lay their eggs. Sometimes the manure may be treated with some substance such as kerosene, crude oil, chlorid of lime, tobacco water or mixture of two or more of these and thus rendered unsuitable for the flies to breed in, but in general practice none of them has been found very satisfactory for the treatment is either not thorough enough or is too expensive of time and material.

Outdoor privies and cesspools must be carefully attended to. The latter can be easily covered so no flies can get in and if the filthy and in every way dangerous pit under the privy be filled and the dry-earth closet substituted one of the greatest sources of danger, especially in the country and in towns with inadequate sewerage facilities, will be done away with. After these things are done there remain only the garbage cans and the rubbish heaps to look after.

Of course your neighbor must keep his place clean too, for his flies are just as apt to come into your house as his, so the problem becomes one for the whole community.

Almost all cities and many of the smaller towns have ordinances which if enforced would afford adequate protection from flies, but they are seldom if ever rigidly enforced and it yet remains for some enterprising town to be able to advertise itself as a "speckless town" as well as a "spotless town."

AN EXPERT'S OPINION

In a recent important bulletin issued by the Bureau of Entomology, Dr. L.O. Howard discusses the economic importance of several of the insects that carry disease. I wish to quote two or three paragraphs from the pages in which he discusses the house-fly or typhoid fly to show the opinion of this excellent authority in regard to this pest.

On another page:

Again:

The whole bulletin (No. 78, Bureau of Entomology) should be read and studied by all who are interested in this subject.

OTHER FLIES

Occasionally other flies looking more or less like the house-fly are seen in houses. Some of these have the same type of sucking mouth-parts and have habits very similar to the house-fly, but as they are usually much less common and as nearly all that has been said in regard to the house-fly would apply equally well to them and as the same measures should be adopted in fighting them they need not be discussed further here.

I have already called attention to the fact that a fly which looks very much like the house-fly is sometimes found in the house and will often bite severely. It has quite a different style of beak, one that is fitted for piercing so it may suck the blood of its victim (Fig. 51). As these flies often seem to be more persistent before a rain the weather prophet will tell you that "It is surely going to rain for the house-flies are beginning to bite."

These stable-flies, as they are called, are great pests of cattle and horses in some sections. It is thought that they are important factors in the spread of some of the diseases of domestic animals, and their habit of sometimes attacking human beings makes it possible for them to carry certain disease germs from animals to man or from man to man.

CHAPTER VI

Those who are particularly interested in the group will find some of these books and papers easily accessible, so there may be given here only a brief summary of the more important facts in regard to the structure and habits of the mosquitoes in order that we may more readily understand the part that they play in the transmission of diseases and see the reasonableness of the recommendations in regard to fighting them.

THE EGGS

Mosquito eggs are laid in water or in places where water is apt to accumulate, otherwise they will not hatch. Some species lay their eggs in little masses (Fig. 52) that float on the surface of the water, looking like small particles of soot. Others lay their eggs singly, some floating about on the surface, others sinking to the bottom where they remain until the young issue. Some of the eggs may remain over winter, but usually those laid in the summer hatch in thirty-six to forty-eight hours or longer according to the temperature.

THE LARVÆ

When the larvæ are ready to issue they burst open the lower end of the eggs and the young wrigglers escape into the water. The larvæ are fitted for aquatic life only, so mosquitoes cannot breed in moist or damp places unless there is at least a small amount of standing water there. A very little will do, but there must be enough to cover the larvæ or they perish.

The head of the larvæ of most species is wide and flattened. The eyes are situated at the sides, and just in front of them is a pair of short antennæ which vary with the different species.

The mouth-parts too vary greatly according to the feeding habits. Some mosquito larvæ are predaceous, feeding on the young of other species or on other insects. These of course have their mouth-parts fitted for seizing and holding their prey. Most of the wrigglers, however, feed on algæ, diatoms, Protozoa and other minute plant or animal forms which are swept into the mouth by curious little brush-like organs whose movements keep a stream of water flowing toward the mouth.

Another group containing the Anopheles are intermediate between these two and have mouth-parts fitted for feeding on minute organisms as well as for attacking and holding other larger things.

A few kinds feed habitually some distance below the surface, others on the bottom, while still others feed always at the surface. With one or two exceptions, the larvæ must all come to the surface to breathe (Figs. 53–57). Most species have on the eighth abdominal segment a rather long breathing-tube the tip of which is thrust just above the surface of the water when they come up for air. In this tube are two large vessels or tracheæ which open just below the tip of the tube and extend forward through the whole length of the body, giving off branches here and there that divide into still smaller branches until every part of the body is reached by some of the small divisions of this tracheal system that carries the oxygen to all the tissues. The length of the breathing-tube is correlated with the feeding-habits of the larvæ. Anopheles larvæ which feed at the surface have very short tubes (Fig. 58), others that feed just below the surface have breathing-tubes as long or very much longer than the ninth abdominal segment. The last segment has at its tip four thin flat plates, the tracheal gills. These too are larger or smaller according to the habits of the larvæ. Those species that feed close to the surface and have the tip of the breathing-tube above the surface most of the time have very small tracheal gills, while those that feed mostly on the bottom have them well developed.

When first hatched the larvæ are of course very small. If the weather is warm and the food is abundant they grow very rapidly. In a few days the outer skin becomes rather firm and inelastic so it will not allow further growth. Then a new skin forms underneath and the old skin is cast off. This process of casting off the old skin is called molting, and is repeated four times during the one, two, three or more weeks of larval life.

PUPA

With the fourth molt the active feeding larva changes to the still active but non-feeding pupa (Fig. 59). The head and thorax are closely united and a close inspection will reveal the head, antennæ, wings and legs of the adult mosquito folded away beneath the pupal skin. Instead of the breathing-tube on the eighth segment of the abdomen as in the larva, the pupa has two trumpet-shaped tubes on the back of the thorax through which it now gets its air from above the surface. The pupal stage lasts from two to five or six days or more. When the adult is ready to issue the pupal skin splits along the back and the mosquito gradually and slowly issues. It usually takes several minutes for the adult to issue and for its wings to become hard enough so it can fly. In the meantime, it is resting on the old pupal skin or on the surface of the water, where it is entirely at the mercy of any of its enemies that might happen along and is in constant danger of being tumbled over should the water not be perfectly smooth.

THE ADULT

The adult mosquito is altogether too familiar an object to need description, but it is necessary that we keep in mind certain particular points in regard to its structure, in order that we may better understand how it is that it is capable of transmitting disease.

If we examine closely the antennæ of a number of mosquitoes that are bothering us with their too constant attentions we shall see that they all look very much alike (Fig. 62), small cylindrical joints bearing whorls of short fine hairs. But if we examine a number of mosquitoes that have been bred from a jar or aquarium we will find two types of antennæ, the one described above belonging to the female. The antennæ of the male (Fig. 63) are much more conspicuous on account of the whorl of dense, fine, long hairs on each segment. Another interesting difference in the antennæ is to be noted in the size of the first joint. In both sexes it is short and cup-shaped, but in the male it is somewhat larger. This basal segment contains a highly complex auditory organ which responds to the vibrations of the whorls of hairs on the other segments. Interesting experiments have shown that these hairs vibrate best to the pitch corresponding to middle C on the piano, the same pitch in which the female "sings." Of course mosquitoes and other insects have no voice as we ordinarily understand the word, but produce sound by the rapid vibration of the wings or by the passage of air through the openings of the tracheæ. The males and females are thus easily distinguished and, as we shall see later, this is of some importance for only the females can bite. The males and females differ in another way. Just below the antennæ and at the sides of the proboscis or beak is a pair of three-to five-jointed appendages, the maxillary palpi or mouth-feelers which in the females of most species are very short (Fig. 64) while in the males they are usually as long as the proboscis (Fig. 65). The females of Anopheles and related forms have palpi quite as long as the males, but they are slender throughout while the male palpi are usually somewhat enlarged toward the tip and bear more or less conspicuous patches of rather long hairs or scales.

THE MOUTH-PARTS

The mouth-parts of the mosquito are of course of particular interest to us. At first they appear to consist of a long slender beak or proboscis, but by dissecting and examining with a microscope we find this beak to be made up of several parts (Fig. 66). The labium, which is the largest and most conspicuous, is apparently cylindrical but is grooved above throughout its length. At the tip of the labium are the labellæ, two little lobes which serve to guide the piercing organs. Lying in this groove along the upper side of the labium are six very fine, sharp-pointed needles. The uppermost of these, the labrum-epipharynx, or labrum as we will call it, is the largest and is really a hollow tube very slightly open on its under side. Just below this is the hypopharynx, the lateral margins of which are very thin. Down through the median line of the hypopharynx runs a minute duct (Fig. 67, sal) which, though exceedingly small, is of very great importance, for through it is poured the saliva which may carry the malaria germs into the wound made when the mosquito bites. The other four needles consist of a pair of mandibles which are lance-shaped at the tip and a heavier pair of maxillæ, the tips of which are serrate on one edge.

HOW THE MOSQUITO BITES

When the female mosquito is feeding on man or any other animal the tip of the labium is placed against the surface and the six needles are thrust into the skin, the labellæ serving as guides. As they are thrust deeper and deeper the labium is bowed back to allow them to enter. As soon as the wound is made the insect pours out through the tube of the hypopharynx some of the secretion from the salivary glands and then begins to suck up the blood through the hollow labrum into the pharynx and on into the stomach.

The mouth-parts of the male differ in some important respects from those of the female. The hypopharynx is united to the labium, the mandibles are wanting and the maxillæ are very much reduced so that the insect is unable to pierce the tough skin of animals. The male feeds on the juices of plants as do the females when they cannot get blood. It is not at all necessary for mosquitoes to have the warm blood of man or other animals. Comparatively few of them ever taste blood. They have been seen feeding on blossoms, ripe fruit, watermelons, plant juices, etc. They are very fond of ripe bananas and are fed on them in the laboratory when we wish to keep mosquitoes for experimental purposes.

THE THORAX

The middle part of the body, called the thorax, is really a strong box with heavy walls for the attachment of the powerful wing and leg muscles. The three pairs of legs are covered with hairs and scales, and their tips are provided with a pair of claws which vary somewhat in the different species. The wings (Fig. 68) are long and narrow with a characteristic venation. Along the veins and the margin of the wings are the scales which readily enable one to distinguish mosquitoes from other insects that may look much like them. In some species these scales are long and narrow, almost hair-like, in others they are quite broad and flat (Fig. 69). Just back of the wings is a pair of balancers, short thread-like processes knobbed at the end. These probably represent the second pair of wings with which most insects are provided, and seem to serve as balancers or orienting organs when the insect is flying. On the sides of the thorax are two small slit-like openings, the breathing-pores. These are the openings into the tracheal or respiratory system.

THE ABDOMEN

The long cylindrical abdomen is composed of eight segments. These are rather strongly chitinized above and below, but a narrow strip along the side is unchitinized. In this strip are situated the abdominal breathing-pores. The tip of the abdomen is furnished with a pair of movable organs, which in the male are variously modified and serve as clasping organs at mating time.

THE DIGESTIVE SYSTEM

The mouth-parts of the mosquito have just been described. It will be remembered that the labrum is provided with a groove. Through this the blood or other food is sucked up by means of a strong-walled pumping organ, the pharynx, situated in the head (Fig. 70). Just back of the pharynx is the esophagus which leads to the beginning of the stomach. Close to its posterior end the esophagus gives off three food reservoirs, two above and a single larger one below. In dissections these will often be seen to be filled with minute bubbles. The stomach reaches from the middle of the thorax to beyond the middle of the abdomen. At its posterior end are given off five long slender processes, the Malpighian tubules which are organs of excretion, acting like the kidneys of higher animals. The hindgut is that portion of the intestine from the stomach to the end of the body.

THE SALIVARY GLANDS

Lying under the alimentary canal in the forward part of the thorax are the salivary glands. There are two sets of these, each having three lobes with a common duct which joins the duct from the other set a short distance before they enter the base of the hypopharynx. Each of these lobes is made up of a layer of secreting cells (Fig. 71) which produces the saliva that is poured into the wound as soon as the insect pierces the skin of the victim, and we shall see, too, that the malarial germs also collect in these glands to be carried by the saliva to the new host.

EFFECTS OF THE BITE

After a mosquito has bitten a person and withdrawn the stylets, a small area about the puncture whitens, then soon becomes pink and begins to swell, then to itch and burn. Some people suffer much more from the bites of mosquitoes than do others. For some such bites mean little or no inconvenience, indeed may pass wholly unnoticed, to others a single bite may mean much annoyance, and several bites may cause much suffering.

After an hour or so the itching usually ceases, but in some cases it continues longer. In some instances little or no irritation is felt until some hours, sometimes as much as a day, after the bite. In such cases the effect of the bite is apt to be severe and to last for several days. Sometimes a more or less serious sore will follow a bite, probably due to infection of the wound by scratching. It is doubtless the saliva that is poured into the wound that causes the irritation. It is frequently asserted that if the mosquito is allowed to drink its fill and withdraw its beak without being disturbed no evil results will follow. Those who hold this theory say that the saliva that is poured into the wound is all withdrawn again with the blood if the mosquito is allowed to feed long enough. There may be some truth in this, but for most of us a bite means a hurt anyway and few will be content to sit perfectly still and watch the little pest gradually fill up on blood.

It is not known just what the action of the saliva is, its composition or reaction on the tissues. It is generally supposed to prevent coagulation of the blood that is to be drawn through the narrow tube of the labrum. Others think that its presence causes a greater flow of blood to the wound. But the sad part of it is, for us at least, that it hurts and may cause malaria and possibly other diseases.

HOW MOSQUITOES BREATHE

Mosquitoes and other insects do not have any nostrils nor do they breathe through any openings on the head. Along the sides of the thorax and abdomen is a series of very minute openings known as the spiracles. Through these the air passes into a system of air-tubes, the tracheæ. There are two main trunks or divisions of the tracheæ just inside the body-wall and a number of shorter connecting trunks. From these larger vessels arise a great number of smaller ones which branch and subdivide again and again until all the tissues are supplied by these minute little air-tubes that carry the oxygen to all parts of the body and carry off the waste carbon dioxid. These air-tubes are emptied and filled by the contractions of the walls of the abdomen. When the body-wall contracts the air is forced out of the thin-walled trachea through the spiracles; when the pressure is removed they are refilled by the fresh air rushing in.

THE BLOOD

After a mosquito has been feeding on a man or some other animal it is often so distended that the blood shows rich and red through the thin sides of the walls of the abdomen. This, however, is the blood of the victim and not of the mosquito. The blood of insects is not red but pale yellowish or greenish. It is not confined in definite vessels, but fills all the space inside the body cavity that is not occupied by some of the tissues or organs. It bathes the walls of the alimentary canal and gathers there the nourishment which it carries to all parts of the body. It does not carry oxygen or collect the carbon dioxid as does the blood of higher animals. That work, as we have just seen, is done by the air-tubes. Above the alimentary canal, extending almost the whole length of the abdomen and thorax, is a thin-walled pulsating vessel, the heart. This consists of a series of chambers each communicating with the one in front of it by an opening which is guarded by a valve. When one of these chambers contracts it forces the blood that is in it forward into the next chamber which, in its turn, sends it on. As the walls relax the valves at the sides are opened and the blood that is in the body-cavity rushes in to fill the empty chamber. As these regular rythmical pulsations recur the blood is forced forward through the heart into the head where it bathes the organs there. We shall see in another chapter that the malarial parasite escapes from the walls of the stomach of the mosquito into the blood in the body-cavity and finally reaches the salivary glands. As the heart is constantly driving blood to this part of the body the parasites readily reach the glands from which they finally escape into the new host.