Control measures, in the case of the body louse, consist in boiling or steaming the clothes or in some cases, sterilizing by dry heat. The dermatitis may be relieved by the use of zinc-oxide ointment, to which Pusey recommends that there be added, on account of their parasiticidal properties, sulphur and balsam of Peru, equal parts, 15 to 30 grains to the ounce.
Phthirius pubis (= P. inguinalis), the pubic louse, or so-called "crab louse," differs greatly from the preceding in appearance. It is characterized by its relatively short head which fits into a broad depression in the thorax. The latter is broad and flat and merges into the abdomen. The first pair of legs is slender and terminated by a straight claw. The second and third pairs of legs are thicker and are provided with powerful claws fitted for clinging to hairs. The females (fig. 69) measure 1.5 to 2 mm. in length by 1.5 mm. in breadth. The male averages a little over half as large. The eggs, or nits, are fixed at the base of the hairs. Only a few, ten to fifteen are deposited by a single female, and they hatch in about a week's time. The young lice mature in two weeks.
The pubic louse usually infests the hairs of the pubis and the perineal region. It may pass to the arm pits or even to the beard or moustache. Rarely, it occurs on the eyelids, and it has even been found, in a very few instances, occurring in all stages, on the scalp. Infestation may be contracted from beds or even from badly infested persons in a crowd. We have seen several cases which undoubtedly were due to the use of public water closets. It produces papular eruption and an intense pruritis. When abundant, there occurs a grayish discoloration of the skin which Duguet has shown is due to a poisonous saliva injected by the louse, as is the melanoderma caused by the body louse.
The pubic louse may be exterminated by the measures recommended for the head louse, or by the use of officinal mercurial ointment.
Several species of Hemiptera-Heteroptera are habitual parasites of man, and others occur as occasional or accidental parasites. Of all these, the most important and widespread are the bed-bugs, belonging to the genus Cimex (= Acanthia).
The Bed-bugs—The bed-bugs are characterized by a much flattened oval body, with the short, broad head unconstricted behind, and fitting into the strongly excavated anterior margin of the thorax. The compound eyes are prominent, simple eyes lacking. Antennæ four-jointed, the first segment short, the second long and thick, and the third and fourth slender. The tarsi are short and three segmented.
It is often assumed in the literature of the subject that there is but a single species of Cimex attacking man, but several such species are to be recognized. These are distinguishable by the characters given in Chapter XII. We shall consider especially Cimex lectularius, the most common and widespread species.
Cimex lectularius (= Acanthia lectularia, Clinocoris lectularius), is one of the most cosmopolitan of human parasites but, like the lice, it has been comparatively little studied until recent years, when the possibility that it may be concerned with the transmission of various diseases has awakened interest in the details of its life-history and habits.
The adult insect (fig. 70) is 4-5 mm. long by 3 mm. broad, reddish brown in color, with the beak and body appendages lighter in color. The short, broad and somewhat rectangular head has no neck-like constriction but fits into the broadly semilunar prothorax. The four segmented labium or proboscis encloses the lancet-like maxillæ and mandibles. The distal of the four antennal segments is slightly club-shaped. The prothorax is characteristic of the species, being deeply incised anteriorly and with its thin lateral margins somewhat turned up. The mesothorax is triangular, with the apex posteriorly, and bears the greatly atrophied first pair of wings. There is no trace of the metathoracic pair. The greatly flattened abdomen has eight visible segments, though in reality the first is greatly reduced and has been disregarded by most writers. The body is densely covered with short bristles and hairs, the former being peculiarly saber-shaped structures sharply toothed at the apex and along the convex side (fig. 159b).
The peculiar disagreeable odor of the adult bed-bug is due to the secretion of the stink glands which lie on the inner surface of the mesosternum and open by a pair of orifices in front of the metacoxæ, near the middle line. In the nymphs, the thoracic glands are not developed but in the abdomen there are to be found three unpaired dorsal stink glands, which persist until the fifth molt, when they become atrophied and replaced by the thoracic glands. The nymphal glands occupy the median dorsal portion of the abdomen, opening by paired pores at the anterior margin of the fourth, fifth and sixth segments. The secretion is a clear, oily, volatile fluid, strongly acid in reaction. Similar glands are to be found in most of the Hemiptera-Heteroptera and their secretion is doubtless protective, through being disagreeable to the birds. In the bed-bug, as Marlatt points out, "it is probably an illustration of a very common phenomenon among animals, i.e., the persistence of a characteristic which is no longer of any special value to the possessor." In fact, its possession is a distinct disadvantage to the bed-bug, as the odor frequently reveals the presence of the bugs, before they are seen.
The eggs of the bed-bug (fig. 70) are pearly white, oval in outline, about a millimeter long, and possess a small operculum or cap at one end, which is pushed off when the young hatches. They are laid intermittently, for a long period, in cracks and crevices of beds and furniture, under seams of mattresses, under loose wall paper, and similar places of concealment of the adult bugs. Girault (1905) observed a well-fed female deposit one hundred and eleven eggs during the sixty-one days that she was kept in captivity. She had apparently deposited some of her eggs before being captured.
The eggs hatch in six to ten days, the newly emerged nymphs being about 1.5 mm. in length and of a pale yellowish white color. They grow slowly, molting five times. At the last molt the mesathoracic wing pads appear, characteristic of the adult. The total length of the nymphal stage varies greatly, depending upon conditions of food supply, temperature and possibly other factors. Marlatt (1907) found under most favorable conditions a period averaging eight days between molting which, added to an equal egg period, gave a total of about seven weeks from egg to adult insect. Girault (1912) found the postembryonic period as low as twenty-nine days and as high as seventy days under apparently similar and normal conditions of food supply. Under optimum and normal conditions of food supply, beginning August 27, the average nymphal life was 69.9 days; average number of meals 8.75 and the molts 5. Under conditions allowing about half the normal food supply the average nymphal life was from 116.9 to 139 days. Nymphs starved from birth lived up to 42 days. We have kept unfed nymphs, of the first stage, alive in a bottle for 75 days. The interesting fact was brought out that under these conditions of minimum food supply there were sometimes six molts instead of the normal number.
The adults are remarkable for their longevity, a factor which is of importance in considering the spread of the insect and methods of control. Dufour (1833) (not De Geer, as often stated) kept specimens for a year, in a closed vial, without food. This ability, coupled with their willingness to feed upon mice, bats, and other small mammals, and even upon birds, accounts for the long periods that deserted houses and camps may remain infested. There is no evidence that under such conditions they are able to subsist on the starch of the wall paper, juices of moistened wood, or the moisture in the accumulations of dust, as is often stated.
There are three or four generations a year, as Girault's breeding experiments have conclusively shown. He found that the bed-bug does not hibernate where the conditions are such as to allow it to breed and that breeding is continuous unless interrupted by the lack of food or, during the winter, by low temperature.
Bed-bugs ordinarily crawl from their hiding places and attack the face and neck or uncovered parts of the legs and arms of their victims. If undisturbed, they will feed to repletion. We have found that the young nymph would glut itself in about six minutes, though some individuals fed continuously for nine minutes, while the adult required ten to fifteen minutes for a full meal. When gorged, it quickly retreats to a crack or crevice to digest its meal, a process which requires two or three days. The effect of the bite depends very greatly on the susceptibility of the individual attacked. Some persons are so little affected that they may be wholly ignorant of the presence of a large number of bugs. Usually the bite produces a small hard swelling, or wheal, whitish in color. It may even be accompanied by an edema and a disagreeable inflammation, and in such susceptible individuals the restlessness and loss of sleep due to the presence of the insects may be a matter of considerable importance. Stiles (1907) records the case of a young man who underwent treatment for neurasthenia, the diagnosis being agreed upon by several prominent physicians; all symptoms promptly disappeared, however, immediately following a thorough fumigation of his rooms, where nearly a pint of bed-bugs were collected.
It is natural to suppose that an insect which throughout its whole life is in such intimate relationship with man should play an important rôle in the transmission of disease. Yet comparatively little is definitely known regarding the importance of the bed-bug in this respect. It has been shown that it is capable of transmitting the bubonic plague, and South American trypanosomiasis. Nuttall succeeded in transmitting European relapsing fever from mouse to mouse by its bite. It has been claimed that Oriental sore, tuberculosis, and even syphilis may be so carried. These phases of the subject will be considered later.
The sources of infestation are many, and the invasion of a house is not necessarily due to neglect, though the continued presence of the pests is quite another matter. In apartments and closely placed houses they are known to invade new quarters by migration. They are frequently to be met with in boat and sleeper berths, and even the plush seats of day coaches, whence a nucleus may be carried in baggage to residences. They may be brought in the laundry or in clothes of servants.
Usually they are a great scourge in frontier settlements and it is generally believed that they live in nature under the bark of trees, in lumber, and under similar conditions. This belief is founded upon the common occurrence of bugs resembling the bed-bug, in such places. As a matter of fact, they are no relation to bed-bugs but belong to plant-feeding forms alone (fig. 19 c, d).
It is also often stated that bed-bugs live in poultry houses, in swallows nests, and on bats, and that it is from these sources that they gain access to dwellings. These bugs are specifically distinct from the true bed-bug, but any of them may, rarely, invade houses. Moreover, chicken houses are sometimes thoroughly infested with the true Cimex lectularius.
Control measures consist in the use of iron bedsteads and the reduction of hiding places for the bugs. If the infestation is slight they may be exterminated by a vigilant and systematic hunt, and by squirting gasoline or alcohol into cracks and crevices of the beds, and furniture. Fumigation must be resorted to in more general infestations.
The simplest and safest method of fumigation is by the use of flowers of sulphur at the rate of two pounds to each one thousand cubic feet of room space. The sulphur should be placed in a pan, a well made in the top of the pile and a little alcohol poured in, to facilitate burning. The whole should be placed in a larger pan and surrounded by water so as to avoid all danger of fire. Windows should be tightly closed, beds, closets and drawers opened, and bedding spread out over chairs in order to expose them fully to the fumes. As metal is tarnished by the sulphur fumes, ornaments, clocks, instruments, and the like should be removed. When all is ready the sulphur should be fired, the room tightly closed and left for twelve to twenty-four hours. Still more efficient in large houses, or where many hiding places favor the bugs, is fumigation with hydrocyanic acid gas. This is a deadly poison and must be used under rigid precautions. Through the courtesy of Professor Herrick, who has had much experience with this method, we give in the Appendix, the clear and detailed directions taken from his bulletin on "Household Insects."
Fumigation with formaldehyde gas, either from the liquid or "solid" formalin, so efficient in the case of contagious diseases, is useless against bed-bugs and most other insects.
Other Bed-bugs—Cimex hemipterus (= C. rotundatus) is a tropical and subtropical species, occurring in both the old and new world. Patton and Cragg state that it is distributed throughout India, Burma, Assam, the Malay Peninsula, Aden, the Island of Mauritius, Reunion, St. Vincent and Porto Rico. "It is widely distributed in Africa, and is probably the common species associated there with man." Brumpt also records it for Cuba, the Antilles, Brazil, and Venezuela.
This species, which is sometimes called the Indian bed-bug, differs from C. lectularius in being darker and in having a more elongate abdomen. The head also is shorter and narrower, and the prothorax has rounded borders.
It has the same habits and practically the same life cycle as Cimex lectularius. Mackie, in India, has found that it is capable of transmitting the Asiatic type of recurrent fever. Roger suggested that it was also capable of transmitting Kala-azar and Patton has described in detail the developmental stages of Leishmania, the causative organism of Kala-azar, in the stomach of this bug, but Brumpt declares that the forms described are those of a common, non-pathogenic flagellate to be found in the bug, and have nothing to do with the human disease. Brumpt has shown experimentally that Cimex hemipterus may transmit Trypanosoma cruzi in its excrement.
Cimex boueti, occurring in French Guinea, is another species attacking man. Its habits and general life history are the same as for the above species. It is 3 to 4.5 mm. in length, has vestigial elytra, and much elongated antennæ and legs. The extended hind legs are about as long as the body.
Cimex columbarius, a widely distributed species normally living in poultry houses and dove cotes, C. inodorus, infesting poultry in Mexico, C. hirundinis, occurring in the nests of swallows in Europe and Oeciacus vicarius (fig. 19i) occurring in swallows' nests in this country, are species which occasionally infest houses and attack man.
Conorhinus sanguisugus, the cone-nosed bed-bug. We have seen in our consideration of poisonous insects, that various species of Reduviid bugs readily attack man. Certain of these are nocturnal and are so commonly found in houses that they have gained the name, of "big bed-bugs." The most noted of these, in the United States, is Conorhinus sangiusugus (fig. 71), which is widely distributed in our Southern States.
Like its near relatives, Conorhinus sangiusugus is carnivorous in habit and feeds upon insects as well as upon mammalian and human blood. It is reported as often occurring in poultry houses and as attacking horses in barns. The life history has been worked out in considerable detail by Marlatt, (1902), from whose account we extract the following.
The eggs are white, changing to yellow and pink before hatching. The young hatch within twenty days and there are four nymphal stages. In all these stages the insect is active and predaceous, the mouth-parts (fig. 72) being powerfully developed. The eggs are normally deposited, and the early stages are undoubtedly passed, out of doors, the food of the immature forms being other insects. Immature specimens are rarely found indoors. It winters both in the partly grown and adult stage, often under the bark of trees or in any similar protection, and only in its nocturnal spring and early summer flights does it attack men. Marlatt states that this insect seems to be decidedly on the increase in the region which it particularly infests,—the plains region from Texas northward and westward. In California a closely related species of similar habits is known locally as the "monitor bug."
The effect of the bite of the giant bed-bug on man is often very severe, a poisonous saliva apparently being injected into the wound. We have discussed this phase of the subject more fully under the head of poisonous insects.
Conorhinus megistus is a Brazilian species very commonly attacking man, and of special interest since Chagas has shown that it is the carrier of a trypanosomiasis of man. Its habits and life history have been studied in detail by Neiva, (1910).
This species is now pre-eminently a household insect, depositing its eggs in cracks and crevices in houses, though this is a relatively recent adaptation. The nymphs emerge in from twenty to forty days, depending upon the temperature. There are five nymphal stages, and as in the case of true bed-bugs, the duration of these is very greatly influenced by the availability of food and by temperature. Neiva reckons the entire life cycle, from egg to egg, as requiring a minimum of three hundred and twenty-four days.
The nymphs begin to suck blood in three to five days after hatching. They usually feed at night and in the dark, attacking especially the face of sleeping individuals. The bite occasions but little pain. The immature insects live in cracks and crevices in houses and invade the beds which are in contact with walls, but the adults are active flyers and attack people sleeping in hammocks. The males as well as the females are blood suckers.
Like many blood-sucking forms, Conorhinus megistus can endure for long periods without food. Neiva received a female specimen which had been for fifty-seven days alive in a tightly closed box. They rarely feed on two consecutive days, even on small quantities of blood, and were never seen to feed on three consecutive days.
Methods of control consist in screening against the adult bugs, and the elimination of crevices and such hiding places of the nymphs. Where the infestation is considerable, fumigation with sulphur is advisable.
Of the Diptera or two-winged flies, many species occasionally attack man. Of these, a few are outstanding pests, many of them may also serve to disseminate disease, a phase of our subject which will be considered later. We shall now consider the most important of the group from the viewpoint of their direct attacks on man.
The Psychodidæ or Moth-flies, include a few species which attack man, and at least one species, Phlebotomus papatasii, is known to transmit the so-called "three-day fever" of man. Another species is supposed to be the vector of Peruvian verruga.
The family is made up of small, sometimes very small, nematocerous Diptera, which are densely covered with hairs, giving them a moth-like appearance. The wings are relatively large, oval or lanceolate in shape, and when at rest are held in a sloping manner over the abdomen, or are held horizontally in such a way as to give the insect a triangular outline. Not only is the moth-like appearance characteristic, but the venation of the wings (fig. 163, d) is very peculiar and, according to Comstock, presents an extremely generalized form. All of the longitudinal veins separate near the base of the wing except veins R2 and R3 and veins M1 and M2. Cross veins are wanting in most cases.
Comparatively little is known regarding the life-history and habits of the Psychodidæ, but one genus, Phlebotomus, contains minute, blood-sucking species, commonly known as sand-flies. The family is divided into two subfamilies, the Psychodinæ and the Phlebotominæ. The second of these, the Phlebotominæ, is of interest to us.
The Phlebotominæ—The Phlebotominæ differ from the Psychodinæ in that the radical sector branches well out into the wing rather than at the base of the wing. They are usually less hairy than the Psychodinæ. The ovipositor is hidden and less strongly chitinized. The species attacking man belong to the genus Phlebotomus, small forms with relatively large, hairy wings which are held upright, and with elongate proboscis. The mandibles and maxillæ are serrated and fitted for biting.
According to Miss Summers (1913) there are twenty-nine known species of the genus Phlebotomus, five European, eleven Asiatic, seven African and six American. One species only, Phlebotomus vexator, has been reported for the United States. This was described by Coquillett, (1907), from species taken on Plummer's Island, Maryland. It measures only 1.5 mm. in length. As it is very probable that this species is much more widely distributed, and that other species of these minute flies will be found to occur in our fauna, we quote Coquillett's description.
Phlebotomus vexator, Coq.: Yellow, the mesonotum brown, hairs chiefly brown; legs in certain lights appear brown, but are covered with a white tomentum; wings hyaline, unmarked; the first vein (R1) terminates opposite one-fifth of the length of the first submarginal cell (cell R2); this cell is slightly over twice as long as its petiole; terminal, horny portion of male claspers slender, bearing many long hairs; the apex terminated by two curved spines which are more than one-half as long as the preceding part, and just in front of these are two similar spines, while near the middle of the length of this portion is a fifth spine similar to the others. Length 1.5 mm.
The life-history of the Phlebotomus flies has been best worked out for the European Phlebotomus papatasii and we shall briefly summarize the account of Dœrr and Russ (1913) based primarily on work on this species. The European Phlebotomus flies appear at the beginning of the warm season, a few weeks after the cessation of the heavy rains and storms of springtime. They gradually become more abundant until they reach their first maximum, which in Italy is near the end of July (Grassi). They then become scarcer but reach a second maximum in September. At the beginning of winter they vanish completely, hibernating individuals not being found.
After fertilization there is a period of eight to ten days before oviposition. The eggs are then deposited, the majority in a single mass covered by a slimy secretion from the sebaceous glands. The larvæ emerge in fourteen to twenty days. There is uncertainty as to the length of larval life, specimens kept in captivity remaining fifty or more days without transforming. Growth may be much more rapid in nature. The larvæ do not live in fluid media but in moist detritus in dark places. Marett believes that they live chiefly on the excrement of pill-bugs (Oniscidæ) and lizards. Pupation always occurs during the night. The remnants of the larval skin remain attached to the last two segments of the quiescent pupa and serve to attach it to the stone on which it lives. The pupal stage lasts eleven to sixteen days, the adult escaping at night.
Only the females suck blood. They attack not only man but all warm-blooded animals and, according to recent workers, also cold-blooded forms, such as frogs, lizards, and larvæ. Indeed, Townsend (1914) believes that there is an intimate relation between Phlebotomus and lizards, or other reptiles the world over. The Phlebotomus passes the daylight hours within the darkened recesses of the loose stone walls and piles of rock in order to escape wind and strong light. Lizards inhabit the same places, and the flies, always ready to suck blood in the absence of light and wind, have been found more prone to suck reptilian than mammalian blood.
On hot summer nights, when the wind is not stirring, the Phlebotomus flies, or sand-flies, as they are popularly called, invade houses and sleeping rooms in swarms and attack the inmates. As soon as light begins to break the flies either escape to the breeding places, or cool, dark places protected from the wind, or a part of them remain in the rooms, hiding behind pictures, under garments, and in similar places. Wherever the Phlebotomus flies occur they are an intolerable nuisance. On account of their small size they can easily pass through the meshes of ordinary screens and mosquito curtains. They attack silently and inflict a very painful, stinging bite, followed by itching. The ankles, dorsum of the feet, wrists, inner elbow, knee joint and similar places are favorite places of attack, possibly on account of their more delicate skin.
Special interest has been attracted to these little pests in recent years, since it has been shown that they transmit the European "pappatici fever" or "three day fever." More recently yet, it appears that they are the carriers of the virus of the Peruvian "verruga." This phase of the subject will be discussed later.
Control measures have not been worked out. As Newstead says, "In consideration of the facts which have so far been brought to light regarding the economy of Phlebotomus, it is clearly evident that the task of suppressing these insects is an almost insurmountable one. Had we to deal with insects as large and as accessible as mosquitoes, the adoption of prophylactic measures would be comparatively easy, but owing to the extremely minute size and almost flea-like habits of the adult insects, and the enormous area over which the breeding-places may occur, we are faced with a problem which is most difficult of solution." For these reasons, Newstead considers that the only really prophylactic measures which can at present be taken, are those which are considered as precautionary against the bites of the insects.
Of repellents, he cites as one of the best a salve composed of the following:
| Ol. Anisi | 3 grs. |
| 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.
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 rôle 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), Mitchell (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. Linnæus, 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 classic 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 Culicidæ are slender, nematocerous Diptera with narrow wings, antennæ 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 classification.
The larvæ (fig. 73) are elongate, with the head and thorax sharply distinct. The larval antennæ 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 masses 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 anal 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 tracheæ which open at the apex of the so-called anal 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 anal siphon is a double row of flattened, toothed spines whose number and shape are likewise of some value in separating species. They constitute the comb or pecten.
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 antennæ, 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 illustrative 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 mass. The number of eggs in a single mass is in the neighborhood of two hundred, recorded counts varying considerably. A single female may deposit several masses during her life time. The duration of the egg stage is dependent upon temperature. In the warm summer time the larvæ may emerge within a day. The larvæ undergo four molts and under optimum conditions may transform into pupæ 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 illustrations.
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 pass the winter in the larval state. An interesting illustration of this is afforded by Wyeomia smithii, whose larvæ 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. tæniorhynchus. 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 sexes are often kept alive for long periods by feeding them upon ripe banana, dried fig, raisins, and the like, and in spite of sweeping assertions 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 œsophageal 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 2¼ lbs. of mutton tallow and strain it. While still hot add ½ 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 1¼ 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 illustration 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 larvæ. 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 larvæ.