Handbook of Medical Entomology Part 11

Ol. Eucalypti 3 grs.

Ol. Terebenth 3 grs.

Unq. Acid Borac.

Of sprays he recommends as the least objectionable and at the same time one of the most effective, formalin. "The dark portions and angles of sleeping apartments should be sprayed with a one per cent. solution of this substance every day during the season in which the flies are prevalent. A fine spraying apparatus is necessary for its application and an excessive amount must not be applied. It is considered an excellent plan also to spray the mosquito curtains regularly every day towards sunset; nets thus treated are claimed to repel the attacks of these insects." This effectiveness of formalin is very surprising for, as we have seen, it is almost wholly ineffective against bed-bugs, mosquitoes, house flies and other insects, where it has been tried.

A measure which promises to be very effective, where it can be adopted, is the use of electric fans so placed as to produce a current of air in the direction of the windows of sleeping apartments. On account of the inability of the Phlebotomus flies to withstand even slight breezes, it seems very probable that they would be unable to enter a room so protected.

Culicidae or Mosquitoes

From the medical viewpoint, probably the most interesting and important of the blood-sucking insects are the mosquitoes. Certainly this is true of temperate zones, such as those of the United States. The result is that no other group of insects has aroused such widespread interest, or has been subjected to more detailed study than have the mosquitoes, since their role as carriers of disease was made known. There is an enormous literature dealing with the group, but fortunately for the general student, this has been well summarized by a number of workers.

The most important and helpful of the general works are those of Howard (1901), Smith (1904), Blanchard (1905), Mitch.e.l.l (1907), and especially of Howard, Dyar, and Knab, whose magnificent monograph is still in course of publication.

Aside from their importance as carriers of disease, mosquitoes are notorious as pests of man, and the earlier literature on the group is largely devoted to references to their enormous numbers and their blood-thirstiness in certain regions. They are to be found in all parts of the world, from the equator to the Arctic and Antarctic regions.

Linnaeus, in the "Flora Lapponica," according to Howard, Dyar and Knab, "dwells at some length upon the great abundance of mosquitoes in Lapland and the torments they inflicted upon man and beast. He states that he believes that nowhere else on earth are they found in such abundance and he compares their numbers to the dust of the earth. Even in the open, you cannot draw your breath without having your mouth and nostrils filled with them; and ointments of tar and cream or of fish grease are scarcely sufficient to protect even the case-hardened cuticle of the Laplander from their bite. Even in their cabins, the natives cannot take a mouthful of food or lie down to sleep unless they are fumigated almost to suffocation." In some parts of the Northwestern and Southwestern United States it is necessary to protect horses working in the fields by the use of sheets or burlaps, against the ferocious attacks of these insects. It is a surprising fact that even in the dry deserts of the western United States they sometimes occur in enormous numbers.

Until comparatively recent years, but few species of mosquitoes were known and most of the statements regarding their life-history were based upon the cla.s.sic work of Reaumur (1738) on the biology of the rain barrel mosquito, _Culex pipiens_. In 1896, Dr. Howard refers to twenty-one species in the United States, now over fifty are known; Giles, in 1900, gives a total of two hundred and forty-two for the world fauna, now over seven hundred species are known. We have found eighteen species at Ithaca, N. Y.

All of the known species of mosquitoes are aquatic in the larval stage, but in their life-histories and habits such great differences occur that we now know that it is not possible to select any one species as typical of the group. For our present purpose we shall first discuss the general characteristics and structure of mosquitoes, and shall then give the life-history of a common species, following this by a brief consideration of some of the more striking departures from what have been supposed to be the typical condition.

The CULICIDae are slender, nematocerous Diptera with narrow wings, antennae plumose in the males, and usually with the proboscis much longer than the head, slender, firm and adapted for piercing in the female.

The most characteristic feature is that the margins of the wings and, in most cases, the wing veins possess a fringe of scale-like hairs. These may also cover in part, or entirely, the head, thorax, abdomen and legs.

The females, only, suck blood.

On account of the importance of the group in this country and the desirability of the student being able to determine material in various stages, we show in the accompanying figures the characters most used in cla.s.sification.

The larvae (fig. 73) are elongate, with the head and thorax sharply distinct. The larval antennae are prominent, consisting of a single cylindrical and sometimes curved segment. The outer third is often narrower and bears at its base a fan-shaped tuft of hairs, the arrangement and abundance of which is of systematic importance. About the mouth are the so-called rotary mouth brushes, dense ma.s.ses of long hairs borne by the labrum and having the function of sweeping food into the mouth. The form and arrangement of thoracic, abdominal, and a.n.a.l tufts of hair vary in different species and present characteristics of value. On either side of the eighth abdominal segment is a patch of scales varying greatly in arrangement and number and of much value in separating species. Respiration is by means of tracheae which open at the apex of the so-called a.n.a.l siphon, when it is present. In addition, there are also one or two pairs of tracheal gills which vary much in appearance in different species. On the ventral side of the a.n.a.l siphon is a double row of flattened, toothed spines whose number and shape are likewise of some value in separating species. They const.i.tute the comb or pecten.

[Ill.u.s.tration: 73. Culex larva showing details of external structure.]

The pupa (fig. 139, b) unlike that of most insects, is active, though it takes no food. The head and thorax are not distinctly separated, but the slender flexible abdomen in sharply marked off. The antennae, mouth-parts, legs, and wings of the future adult are now external, but enclosed in chitinous cases. On the upper surface, near the base of the wings are two trumpets, or breathing tubes, for the pupal spiracles are towards the anterior end instead of at the caudal end, as in the larva.

At the tip of the abdomen is a pair of large chitinous swimming paddles.

As ill.u.s.trative of the life cycle of a mosquito we shall discuss the development of a common house mosquito, _Culex pipiens_, often referred to in the Northern United States as the rain barrel mosquito. Its life cycle is often given as typical for the entire group, but, as we have already emphasized, no one species can serve this purpose.

The adults of _Culex pipiens_ hibernate throughout the winter in cellars, buildings, hollow trees, or similar dark shelters. Early in the spring they emerge and deposit their eggs in a raft-like ma.s.s. The number of eggs in a single ma.s.s is in the neighborhood of two hundred, recorded counts varying considerably. A single female may deposit several ma.s.ses during her life time. The duration of the egg stage is dependent upon temperature. In the warm summer time the larvae may emerge within a day. The larvae undergo four molts and under optimum conditions may transform into pupae in about a week's time. Under the same favorable conditions, the pupal stage may be completed in a day's time. The total life cycle of _Culex pipiens_, under optimum conditions, may thus be completed in a week to ten days. This period may be considerably extended under less favorable conditions of temperature and food supply.

_Culex pipiens_ breeds continuously throughout the summer, developing in rain barrels, horse troughs, tin cans, or indeed in any standing water about houses, which lasts for a week or more. The catch basins of sewers furnish an abundant supply of the pests under some conditions. Such places, the tin gutters on residences, and all possible breeding places must be considered in attempts to exterminate this species.

Other species of mosquitoes may exhibit radical departures from _Culex pipiens_ in life-history and habits. To control them it is essential that the biological details be thoroughly worked out for, as Howard, Dyar, and Knab have emphasized, "much useless labor and expense can be avoided by an accurate knowledge of the habits of the species." For a critical discussion of the known facts the reader is referred to their monograph. We shall confine ourselves to a few ill.u.s.trations.

The majority of mosquitoes in temperate climates hibernate in the egg stage, hatching in the spring or even mild winter days in water from melting snow. It is such single-brooded species which appear in astounding numbers in the far North. Similarly, in dry regions the eggs may stand thorough dessication, and yet hatch out with great promptness when submerged by the rains. "Another provision to insure the species against destruction in such a case, exists in the fact * * * that not all the eggs hatch, a part of them lying over until again submerged by subsequent rains." In temperate North America, a few species pa.s.s the winter in the larval state. An interesting ill.u.s.tration of this is afforded by _Wyeomia smithii_, whose larvae live in pitcher plants and are to be found on the coldest winter days imbedded in the solid ice.

Late in the spring, the adults emerge and produce several broods during the summer.

In the United States, one of the most important facts which has been brought out by the intensive studies of recent years is that certain species are migratory and that they can travel long distances and become an intolerable pest many miles from their breeding places. This was forcibly emphasized in Dr. Smith's work in New Jersey, when he found that migratory mosquitoes, developing in the salt marshes along the coast, are the dominant species largely responsible for the fame of the New Jersey mosquito. The species concerned are _Aedes sollicitans_, _A.

cantator_ and _A. taeniorhynchus_. Dr. Smith decided that the first of these might migrate at least forty miles inland. It is obvious that where such species are the dominant pest, local control measures are a useless waste of time and money. Such migratory habits are rare, however, and it is probable that the majority of mosquitoes do not fly any great distance from their breeding places.

While mosquitoes are thought of primarily as a pest of man, there are many species which have never been known to feed upon human or mammalian blood, no matter how favorable the opportunity. According to Howard, Dyar, and Knab, this is true of _Culex territans_, one of the common mosquitoes in the summer months in the Northern United States. There are some species, probably many, in which the females, like the males, are plant feeders. In experimental work, both s.e.xes are often kept alive for long periods by feeding them upon ripe banana, dried fig, raisins, and the like, and in spite of sweeping a.s.sertions that mosquitoes must have a meal of blood in order to stimulate the ovaries to development, some of the common blood-sucking species, notably _Culex pipiens_, have been bred repeatedly without opportunity to feed upon blood.

The effect of the bite varies greatly with different species and depends upon the susceptibility of the individual bitten. Some persons are driven almost frantic by the attacks of the pests when their companions seem almost unconscious of any inconvenience. Usually, irritation and some degree of inflammation appear shortly following the bite. Not infrequently a hardened wheal or even a nodule forms, and sometimes scratching leads to secondary infection and serious results.

The source of the poison is usually supposed to be the salivary glands of the insect. As we have already pointed out, (p. 34), Macloskie believed that one lobe of the gland, on each side, was specialized for forming the poison, while a radically different view is that of Schaudinn, who believed that the irritation is due to the expelled contents of the sophageal diverticula, which contain a gas and a peculiar type of fungi or bacteria. In numerous attempts, Schaudinn was unable to produce any irritation by applying the triturated salivary glands to a wound, but obtained the typical result when he used the isolated diverticula.

The irritation of the bite may be relieved to some extent by using ammonia water, a one per cent. alcoholic solution of menthol, or preparations of cresol, or carbolic acid. Dr. Howard recommends rubbing the bite gently with a piece of moist toilet soap. Castellani and Chalmers recommend cleansing inflamed bites with one in forty carbolic lotion, followed by dressing with boracic ointment. Of course, scratching should be avoided as much as possible.

Repellents of various kinds are used, for warding off the attacks of the insects. We have often used a mixture of equal parts of oil of pennyroyal and kerosene, applied to the hands and face. Oil of citronella is much used and is less objectionable to some persons. A recommended formula is, oil of citronella one ounce, spirits of camphor one ounce, oil of cedar one-half ounce. A last resort would seem to be the following mixture recommended by Howard, Dyar, and Knab for use by hunters and fishermen in badly infested regions, against mosquitoes and blackflies.

Take 2 lbs. of mutton tallow and strain it. While still hot add lb. black tar (Canadian tar). Stir thoroughly and pour into the receptacle in which it is to be contained. When nearly cool stir in three ounces of oil of citronella and 1 oz. of pennyroyal.

At night the surest protection is a good bed net. There are many types of these in use, but in order to be serviceable and at the same time comfortable it should be roomy and hung in such a way as to be stretched tightly in every direction. We prefer one suspended from a broad, square frame, supported by a right-angled standard which is fastened to the head of the bed. It must be absolutely free from rents or holes and tucked in securely under the mattress or it will serve merely as a convenient cage to retain mosquitoes which gain an entrance. While such nets are a convenience in any mosquito ridden community, they are essential in regions where disease-carrying species abound. Screening of doors, windows and porches, against the pests is so commonly practiced in this country that its importance and convenience need hardly be urged.

Destruction of mosquitoes and prevention of breeding are of fundamental importance. Such measures demand first, as we have seen, the correct determination of the species which is to be dealt with, and a knowledge of its life-history and habits. If it prove to be one of the migratory forms, it is beyond mere local effort and becomes a problem demanding careful organization and state control. An excellent ill.u.s.tration of the importance and effectiveness of work along these lines is afforded by that in New Jersey, begun by the late Dr. John B. Smith and being pushed with vigor by his successor, Dr. Headlee.

In any case, there is necessity for community action. Even near the coast, where the migratory species are dominant, there are the local species which demand attention and which cannot be reached by any measures directed against the species of the salt marshes. The most important of local measures consist in the destruction of breeding places by filling or draining ponds and pools, clearing up of more temporary breeding places, such as cans, pails, water barrels and the like. Under conditions where complete drainage of swamps is impracticable or undesirable, judicious dredging may result in a pool or series of steep-sided pools deep enough to maintain a supply of fish, which will keep down the mosquito larvae. Where water receptacles are needed for storage of rain water, they should be protected by careful screening or a film of kerosene over the top of the water, renewed every two weeks or so, so as to prevent mosquitoes from depositing their eggs.

When kerosene is used, Water drawn from the bottom of the receptacle will not be contaminated by it to any injurious extent. Where ponds cannot be drained much good will be accomplished by spraying kerosene oil on the surface of the water, or by the introduction of fish which will feed on the larvae.

Detailed consideration of the most efficient measures for controlling mosquitoes is to be found in Dr. Howard's Bulletin No. 88 of the Bureau of Entomology, "Preventive and remedial work against mosquitoes" or, in more summarized form, in Farmers' Bulletin No. 444. One of these should be obtained by any person interested in the problems of mosquito control and public health.

[Ill.u.s.tration: 74. Mouth parts of Simulium. After Grunberg.]

The Simuliidae, or Black Flies

The SIMULIIDae, or black flies, are small, dark, or black flies, with a stout body and a hump-back appearance. The antennae are short but eleven-segmented, the wings broad, without scales or hairs, and with the anterior veins stout but the others very weak. The mouth-parts (fig. 74) are fitted for biting.

The larvae of the Simuliidae (fig. 75) are aquatic and, unlike those of mosquitoes, require a well aerated, or swiftly running water. Here they attach to stones, logs, or vegetation and feed upon various micro-organisms. They pupate in silken coc.o.o.ns open at the top. Detailed life-histories have not been worked out for most of the species. We shall consider as typical that of _Simulium pictipes_, an inoffensive species widely distributed in the Eastern United States, which has been studied especially at Ithaca, N.Y. (Johannsen, 1903).

[Ill.u.s.tration: 75. Larva of Simulium, (8). After Garman.]

The eggs are deposited in a compact yellowish layer on the surface of rock, on the brinks of falls and rapids where the water is flowing swiftly. They are elongate ellipsoidal in shape, about .4 by .18 mm. As myriads of females deposit in the same place the egg patches may be conspicuous coatings of a foot or much more in diameter. When first laid they are enveloped in a yellowish white slime, which becomes darker, until finally it becomes black just before the emerging of the larvae.

The egg stage lasts a week.

The larvae (fig. 75) are black, soft skinned, somewhat cylindrical in shape, enlarged at both ends and attenuated in the middle. The posterior half is much stouter than the anterior part and almost club-shaped. The head bears two large fan-shaped organs which aid in procuring food.

Respiration is accomplished by means of three so-called blood gills which are pushed out from the dorsal part of the r.e.c.t.u.m. The larvae occur in enormous numbers, in moss-like patches. If removed from their natural habitat and placed in quiet water they die within three or four hours.

Fastened to the rock by means of a disk-like sucker at the caudal end of the body, they ordinarily a.s.sume an erect position. They move about on the surface of the rocks, to a limited extent, with a looping gait similar to that of a measuring worm, and a web is secreted which prevents their being washed away by the swiftly flowing water. They feed chiefly upon algae and diatoms.

The complete larval stage during the summer months occupies about four weeks, varying somewhat with the temperature and velocity of the water.

At the end of this period they spin from cephalic glands, boot-shaped silken coc.o.o.ns within which they pupate. The coc.o.o.n when spun is firmly attached to the rock and also to adjacent coc.o.o.ns. Cl.u.s.tered continuously over a large area and sometimes one above another, they form a compact, carpet-like covering on the rocks, the reddish-brown color of which is easily distinguishable from the jet-black appearance of the larvae. The pupal stage lasts about three weeks. The adult fly, surrounded by a bubble of air, quickly rises to the surface of the water and escapes. The adults (fig. 76) are apparently short lived and thus the entire life cycle, from egg to egg is completed in approximately eight weeks.

[Ill.u.s.tration: 76. Simulium venustum, (8). After Garman.]

In the case of _Simulium pictipes_ at Ithaca, N. Y., the first brood of adults emerges early in May and successive generations are produced throughout the summer and early autumn. This species winters in the larval condition. Most of the other species of _Simulium_ which have been studied seem to be single brooded.

While _Simulium pictipes_ does not attack man, there are a number of the species which are blood-sucking and in some regions they are a veritable scourge. In recent years the greatest interest in the group has been aroused by Sambon's hypothesis that they transmit pellagra from man to man. This has not been established, and, indeed, seems very doubtful, but the importance of these insects as pests and the possibility that they may carry disease make it urgent that detailed life-histories of the hominoxious species be worked out.

As pests a vivid account of their attacks is in Aga.s.siz's "Lake Superior" (p. 61), quoted by Forbes (1912).

"Neither the love of the picturesque, however, nor the interests of science, could tempt us into the woods, so terrible were the black flies. This pest of flies which all the way hither had confined our ramblings on sh.o.r.e pretty closely to the rocks and the beach, and had been growing constantly worse, here reached its climax. Although detained nearly two days, * * * we could only sit with folded hands, or employ ourselves in arranging specimens, and such other operations as could be pursued in camp, and under the protection of a 'smudge.' One, whom scientific ardor tempted a little way up the river in a canoe, after water plants, came back a frightful spectacle, with blood-red rings round his eyes, his face b.l.o.o.d.y, and covered with punctures. The next morning his head and neck were swollen as if from an attack of erysipelas."