MOSQUITOES
Mosquitoes are no more abundant now than they have been in the past, but
when Linnæus in 1758 made his list of all the animals known to exist at
that time he catalogued only six species of mosquitoes. Only a few years
ago, 1901, Dr. Theobald of the British Museum published a book on the
mosquitoes of the world in which he listed three hundred and forty-three
kinds. Soon other volumes appeared, adding more species, and
systematists everywhere have been describing new ones until now the
total number of described species is probably over five hundred, more
than sixty of which occur in the United States.
This shows only one phase of the great interest that has been taken in
the mosquitoes since the discovery of their importance as carriers of
disease. Not only have they been studied from a systematic standpoint
but an endless amount of work has been done and is being done in
studying their development, habits, and structure until now, if one
could gather together all that has been written about mosquitoes in the
last ten or twelve years he would have a considerable library.
[Illustration: FIG. 50--Pupa of house-fly with the end broken to allow
the fly to issue.]
[Illustration: FIG. 51--Head of stable-fly showing sharp piercing beak.]
[Illustration: FIG. 52--Mass of mosquito eggs (_Theobaldia incidens_).]
[Illustration: FIG. 53--Mosquito eggs and larvæ (_Theobaldia
incidens_); two larvæ feeding on bottom, others at surface to breathe.]
[Illustration: FIG. 54--Mosquito larvæ (_T. incidens_), dorsal view.]
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.
[Illustration: FIG. 55--Eggs, larvæ and pupæ of mosquitoes (_T.
incidens_).]
[Illustration: FIG. 56--Larva of mosquito (_T. incidens_).]
[Illustration: FIG. 57--Mosquito larvæ and pupæ (_T. incidens_) with
their breathing-tubes at the surface of the water.]
[Illustration: FIG. 58--Anopheles larvæ (_A. maculipennis_) resting at
the surface of the water.]
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.
[Illustration: FIG. 59--Mosquito pupæ (_T. incidens_) resting at the
surface of the water.]
[Illustration: FIG. 60--Mosquito pupa (_T. incidens_) with its
breathing-tubes in an air bubble below the surface of the water.]
[Illustration: FIG. 61--Mosquito larvæ and pupæ (_T. incidens_) resting
at the surface of the water.]
[Illustration: FIG. 62--A female mosquito (_T. incidens_); note the
thread-like antennæ.]
[Illustration: FIG. 63--A male mosquito (_T. incidens_); note the
feathery antennæ.]
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.
[Illustration: FIG. 64--Head and thorax of female mosquito
(_Ochlerotatus lativittatus_); the short maxillary palpi are just above
the proboscis and below the thread-like antennæ.]
[Illustration: FIG. 65--Head and thorax of male mosquito (_O.
lativittatus_); the maxillary palpi are as long as the proboscis.]
[Illustration: FIG. 66--Head of female mosquito (_Anopheles_), with
mouth-parts separated to show the needle-like parts: _a_, _a_ antennæ;
_b_, _b_, palpi; _c_, labrum; _d_, _d_, mandibles; _e_, hypopharynx;
_f_, _f_, maxillæ; _g_, labium; _h_, labella. (After Manson.)]
[Illustration: FIG. 67--Cross-section of proboscis of female (_a_) and
male (_b_) mosquito. _lxe_, labrum-epipharynx; _mn_, mandibles; _mx_,
maxillæ; _hp_, hypopharynx; _sal_, salivary duct; _li_, labium; _tr_,
trachea; _mus_, muscles. (After Nuttall and Shipley.)]
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.
[Illustration: FIG. 68--Wing of Mosquito (_O. lativittatus_).]
[Illustration: FIG. 69--End of mosquito wing highly magnified to show
the scales on the veins.]
[Illustration: FIG. 70--Diagram to show the alimentary canal and
salivary glands of a mosquito.]
[Illustration: FIG. 71--Salivary glands of _Culex_ at right. _Anopheles_
at left. (After Christophers.)]
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.
[Illustration: FIG. 72--Heads of Culicinæ mosquitoes; _a_, male; _b_,
female. (After Manson.)]
[Illustration: FIG. 73--Heads of Anophelinæ mosquitoes; _c_, male; _d_,
female. (After Manson.)]
[Illustration: FIG. 74--Wing of _Anopheles maculipennis_.]
[Illustration: FIG. 75--Wing of _Theobaldia incidens_.]
[Illustration: FIG. 76--A non-malarial mosquito (_T. incidens_), male,
standing on the wall.]
[Illustration: FIG. 77--Female of same.]
[Illustration: FIG. 78--A malarial mosquito (_A. maculipennis_), male,
standing on the wall.]
[Illustration: FIG. 79--Female of same.]
CLASSIFICATION
For our purpose it will not be necessary to try to give a system of
classification of all the mosquitoes. Those interested in this phase of
the subject will find several books and papers devoted wholly to it. It
is quite important, however, that we know something about a few of the
more familiar groups and kinds, especially those concerned in the
transmission of diseases.
THE ANOPHELES
In pointing out the differences between male and female mosquitoes we
noted that in one group, the genus _Anopheles_, both sexes have long
maxillary palpi (Figs. 72, 73). This is the most important character
separating this genus from the other common forms and as the
_Anopheles_ are the malaria carriers it is important that this
difference be remembered. Most of the members of this group have spotted
wings (Fig. 74), but as some other common kinds also have spotted wings
(Fig. 75) this character will not always be reliable. When an
_Anopheles_ mosquito is at rest the head and proboscis are held in one
line with the body and the body rests at a considerable angle to the
surface on which it is standing. Other kinds rest with the body almost
or quite parallel to the surface on which they are standing. So if you
find a female mosquito with long mouth-palpi and spotted wings resting
at an angle to the surface on which it stands you may be reasonably sure
that it is an _Anopheles_ and therefore may be dangerous (Figs. 76, 77,
78, 79).
In the United States there are three species of
_Anopheles_--_maculipennis_, _punctipennis_ and _crucians_--which are
common in various localities, and one or two other species that so far
as known are local or rare.
The _Anopheles_ eggs are not laid in masses as are the eggs of many
other mosquitoes, but are deposited singly on the surface of the water
where they may be found often floating close together.
[Illustration: FIG. 80--Egg of Anopheles, side view. (After Nuttall and
Shipley.)]
[Illustration: FIG. 81--Egg of Anopheles, dorsal view. (After Nuttall
and Shipley.)]
[Illustration: FIG. 82--Anopheles larvæ, the one to the right feeding.]
[Illustration: FIG. 83--Anopheles larvæ, the one to the right feeding,
the other just coming to the surface.]
[Illustration: FIG. 84--Anopheles larva, dorsal view.]
[Illustration: FIG. 85--Anopheles pupæ resting at surface of water.]
The eggs (Figs. 80, 81) are elliptical in outline and are provided with
a characteristic membranous expansion near the middle.
The larvæ may be found at the proper season and in the localities where
they are abundant in almost any kind of standing water, in clear little
pools beside running streams, in the overflow from springs, in swamps
and marshy lands, in rain-barrels or any other places or vessels where
the water is quiet. They do not breed in brackish water. As they feed
largely on the algæ or green scum on the surface of the water they are
especially apt to be found where this is present. We have already noted
that their positions in the water differ from that assumed by other
species (Fig. 82).
As the breathing-tube is very short the larvæ must come close to the
surface to breathe, and when they are feeding we find them lying just
under and parallel to the surface of the water with their curious round
heads turned entirely upside down as they feed on the particles that are
floating on the surface (Figs. 83, 84).
The pupæ do not differ very much from the pupæ of other species although
the breathing-tubes on the thorax are usually shorter and the creature
usually rests with its abdomen closer to the surface, that is, it does
not hang down from the surface quite as straight as do other forms (Fig.
85).
The adults may be found out of doors or in houses, barns or other
outbuildings. They do not seem to like a draft and consequently will be
more apt to frequent rooms or places where there is little circulation
of air. Although they are usually supposed to fly and bite only in the
evening or at night, they may occasionally bite in the daytime. One
hungry female took two short meals from my arm while we were trying to
get her to pose for a photograph one warm afternoon.
The female passes the winter in the adult condition, hibernating in any
convenient place about old trees or logs, in cracks or crevices in doors
or out of doors. In the house they hide in the closets, behind the
bureau, behind the head of the bed, or underneath it, or in any place
where they are not apt to be disturbed. During a warm spell in the
winter or if the room is kept warm they may come out for a meal almost
any time.
THE YELLOW FEVER MOSQUITO
Ranking next in importance to _Anopheles_ as a disseminator of disease
and in fact solely responsible for a more dreaded scourge, is the
species of mosquito now known as _Stegomyia calopus_. While this species
is usually restricted to tropical or semi-tropical regions it sometimes
makes its appearance in places farther north, especially in summer
time, where it may thrive for a time. The adult mosquito (Fig. 104) is
black, conspicuously marked with white. The legs and abdomen are banded
with white and on the thorax is a series of white lines which in
well-preserved specimens distinctly resembles a lyre. These mosquitoes
are essentially domestic insects, for they are very rarely found except
in houses or in their immediate vicinity. Once they enter a room they
will scarcely leave it except to lay their eggs in a near-by cistern,
water-pot, or some other convenient place.
Their habit of biting in the daytime has gained for them the name of
"day mosquitoes" to distinguish them from the night feeders. But they
will bite at night as well as by day and many other species are not at
all adverse to a daylight meal, if the opportunity offers, so this habit
is not distinctive. The recognition of these facts has a distinct
bearing in the methods adopted to prevent the spread of yellow fever.
There are no striking characters or habits in the larval or pupal stages
that would enable us to distinguish without careful examination this
species from other similar forms with which it might be associated. For
some time it was claimed that this species would breed only in clean
water, but it has been found that it is not nearly so particular, some
even claiming that it prefers foul water. I have seen them breeding in
countless thousands in company with _Stegomyia scutellaris_ and _Culex
fatigans_ in the sewer drains in Tahiti in the streets of Papeete. As
the larvæ feed largely on bacteria one would expect to find them in
exactly such places where the bacteria are of course abundant.
The fact that they are able to live in any kind of water and in a very
small amount of it well adapts them to their habits of living about
dwellings.
So far as known the members of these two genera are the only two that
are concerned in the transmission of disease in the United States. In
other countries other species are suspected or proven disseminators of
certain diseases, but these will be discussed in connection with the
particular diseases in later chapters.
OTHER SPECIES
The many other species of mosquitoes that we have may be conveniently
divided as to their breeding-habits into the fresh-water and the
brackish-water forms. Among the fresh-water kinds some are found
principally associated with man and his dwelling places, others live in
the woods or other places and so are far less troublesome. Most of
these do not fly far. Several of the species that breed in brackish
water are great travelers and may fly inland for several miles. Thus the
towns situated from one to three or four miles inland from the lower
reaches of San Francisco Bay are often annoyed more by the mosquitoes
that breed only in the brackish water on the salt marshes than they are
by any of the fresh-water forms (Figs. 86, 87). The worst mosquito pest
along the coast of the eastern United States and for some distance
inland is a species that breeds in the salt marshes.
NATURAL ENEMIES OF MOSQUITOES
In combating noxious insects we learned long ago that often the most
efficient, the easiest and cheapest way is to depend on their natural
enemies to hold them in check. Under normal or rather natural conditions
we find that they are usually kept within reasonable bounds by their
natural enemies, but under the artificial conditions brought about by
the settling and developing of any district great changes come about. It
very often happens that these changes are favorable to the development
of the noxious insects and unfavorable to the development of their
enemies.
A striking example and one to the point is afforded in the introduction
of mosquitoes into Hawaii. Up to 1826 there were no mosquitoes on these
islands. It is supposed that they were introduced about that time by
some ships that were trading at the islands. Indeed it is claimed that
the very ship is known that brought them over from Mexico.
Once introduced they found conditions there very favorable to their
development, plenty of standing water and few natural enemies to prey on
them, so they increased very rapidly and gradually spread over all the
islands of the group. This was the so-called night mosquito, _Culex
pipiens_. Much later another species, _Stegomyia calopus_, just as
annoying and much more dangerous was introduced and has also become very
troublesome. We have a few species of top-minnows (Fig. 88) occurring in
sluggish streams in the southern part of the United States that are
important enemies of the mosquitoes of that region. A few years ago some
of these were taken over to Hawaii and liberated in suitable places to
see if they would not help solve the mosquito problem there. The fishes
seem to be doing well. Already they are destroying many mosquito larvæ,
and there are indications that they are going to do an important work,
but of course can be depended on only as an aid.
[Illustration: FIG. 86--Salt-marsh mosquito (_Ochlerotatus
lativittatus_); male.]
[Illustration: FIG. 87--Salt-marsh mosquito (_O. lativittatus_);
female.]
[Illustration: FIG. 88--Top-minnow (_Mollienisia latipinna_). (From
Bull., 47 U.S. Fish Com.)]
[Illustration: FIG. 89--Dragon-flies. (From Kellogg's Amer. Insects.)]
On account of the various habits of both the larvæ and adults it will
never be possible for any natural enemy or group of natural enemies
effectively to control the mosquitoes of any region, but as certain of
them are important as helpers they deserve to be mentioned.
ENEMIES OF THE ADULTS
Birds devour a few mosquitoes, the night-flying forms being particularly
serviceable, but the number thus destroyed is probably so small as to be
of little practical importance.
The dragon-flies (Figs. 89, 90, 91) or mosquito hawks have long been
known as great enemies of mosquitoes, and they certainly do destroy many
of them as they are hawking about places where mosquitoes abound. Dr.
J.B. Smith of New Jersey very much doubts their efficiency, but
observations made by other scientific men would seem to indicate that
they often devour large numbers of mosquitoes during the course of the
day and evening.
Spiders and toads destroy a few mosquitoes each night. Certain external
and internal parasites destroy a few more, but the sum total of all of
these agencies is probably not very considerable, for while the adults
may have several natural enemies they are not of sufficient importance
to have any appreciable effect on the number of mosquitoes in a badly
infested region.
ENEMIES OF THE LARVÆ AND PUPÆ
The larvæ and pupæ on the other hand have many important enemies. Indeed
under favorable conditions these may keep small ponds or lakes quite
free from the pests. The predaceous aquatic larvæ of many insects feed
freely on wrigglers. The larvæ of the diving beetles which are known as
water-tigers are particularly ferocious and will soon destroy all the
wrigglers in ponds where they are present (Fig. 92). Dragon-fly larvæ
also feed freely on mosquito larvæ. Whirligig beetles are said to be
particularly destructive to _Anopheles_ larvæ and many other insects
such as water-boatmen, back-swimmers, etc., feed on the larvæ of various
species. A few of these introduced into a breeding-jar with _Anopheles_
larvæ will soon destroy all of them, even the very young bugs attacking
larvæ much larger than themselves.
It is interesting to note that the larvæ of some mosquitoes are
themselves predaceous and feed freely on the other wrigglers that may
chance to be in the same locality.
[Illustration: FIG. 90--The young (nymph) of a dragon-fly. (From
Kellogg's Amer. Insects.)]
[Illustration: FIG. 91--The cast skin (exuvæ) of a dragon-fly nymph.]
[Illustration: FIG. 92--Diving-beetles and back-swimmers. (From
Kellogg's Amer. Insects.)]
Various species of fish are, however, the most important enemies of the
mosquitoes. Great schools of tide-water minnows (Fig. 93) are often
carried over the low salt-marshes by the extreme high-tides and left in
the hundreds of tide pools as the tide recedes. No mosquitoes can breed
in a pool thus stocked with these fish. In the fresh-water streams and
lakes there are several species of the top-minnows, sticklebacks (Fig.
94), etc., that feed voraciously on mosquito larvæ and unless the grass
or reeds prevent the fish from getting to all parts of the ponds or
lakes very few mosquitoes can breed in places where they are present.
Minute red mites such as attack the house-flies and other insects
sometimes attack adult mosquitoes, but they are rarely very abundant.
Parasitic roundworms attack certain species. Others suffer more or less
from the attacks of various Sporozoan parasites.
FIGHTING MOSQUITOES
When mosquitoes are bothering us we usually begin by trying to kill the
individual pests that are nearest to us. We try to crush them if they
bite us; we screen the doors and windows to keep them from the house. In
warmer countries the people are a little more hospitable and do not
screen the mosquitoes out of the house entirely, but screen the beds for
protection at night, and if the mosquitoes get too insistent during the
day the bed makes a safe and comfortable retreat. All the mosquitoes in
a room may be killed by fumigating with sulphur at the rate of two
pounds to the thousand cubic feet of air-space. Pyrethrum is also used
largely, but it only stupefies the mosquitoes temporarily instead of
killing them. While in that condition they may be swept up and
destroyed.
Various substances are sometimes used as repellants by those who must be
in regions where the mosquitoes are abundant. With many of these,
however, "the cure is worse than the disease." Smudges are often built
to the windward of a house or barn-yard and the smoke from a good
smoldering fire will keep a considerable area quite free from
mosquitoes. The man who can keep himself enveloped in a cloud of tobacco
smoke will not be bothered by mosquitoes. Oil of pennyroyal, oil of tar
or a mixture of these with olive oil, and various other concoctions are
sometimes smeared over the face and hands. These will furnish protection
as long as they last. Dr. Smith says that he has found oil of citronella
quite effective and of course less objectionable than the other things
usually used. Care should be taken not to get it in the eyes. An
ointment made of cedar oil, one ounce; oil of citronella, two ounces;
spirits of camphor, two ounces, is said to make a good repellant and is
effective for a long time.
[Illustration: FIG. 93--Killifish (_Fundulus heteroliatus_). (From
Bull. 47, U.S. Fish Com.)]
[Illustration: FIG. 94--Stickleback (_Apeltes quadracus_). (From Bull.
47, U.S. Fish Com.)]
[Illustration: FIG. 95--An old watering trough, an excellent
breeding-place for mosquitoes.]
FIGHTING THE LARVÆ
All of the efforts directed against the adult mosquitoes are usually of
little avail in decreasing the number in any region. It is comparatively
easy, however, to fight them successfully in the larval stage. We have
seen that standing water is absolutely necessary for mosquitoes to breed
in. This makes the problem much simpler than if they could breed in any
moist places such as well-sprinkled lawns, a shady part of the garden,
etc. The whole problem of successful campaigns against the mosquitoes
resolves itself into the problem of finding and destroying or properly
treating their breeding-places. We have seen how certain kinds, such as
the yellow fever mosquito, are "domestic" species. They never go far
from their breeding-places. If a house is infected by one of these
species the immediate premises should be searched for the source.
Cisterns, rain-barrels, sewer-traps, cesspools, tubs or buckets of water
or old tin cans in out-of-the-way corners, are all suitable places for
them to breed in. Cisterns and rain-barrels should be thoroughly
screened so that no mosquitoes can get in or out, or the surface should
be covered with a film of kerosene which will kill all the larvæ in the
water when they come to the surface to breathe, and will also kill the
females when they come to deposit their eggs. The vent to open cesspools
should be thoroughly screened or the surface of the water kept well
covered with oil. Water standing in any vessels in the yards should be
emptied every week or ten days and the old tin cans destroyed or hauled
away. In fighting these domestic species you need be concerned only with
your own yard and that of your near-by neighbors. Other species, while
also rather local in their distribution, fly much farther than the
really domestic ones. In fighting these the region for a considerable
distance around must be taken into consideration. Watering-troughs (Fig.
95) that are left filled from week to week, the overflow from such
places, and the tracks made in the mud round about them (Fig. 96), small
sluggish streams, irrigating ditches, and small ponds or lakes not
supplied with fish are excellent breeding-places for several species of
mosquitoes including _Anopheles_ and others. The remedy at once suggests
itself. The watering-trough can be emptied and renewed every week during
the summer time, the overflow can be taken care of in a ditch that will
lead it away from the trough to where it will sink into the ground, the
banks of the streams or ponds or lakes can be cleared in such a way that
fish can get to all parts of the water; most of the small ponds can be
drained or their surface may be covered over with a thin film of
kerosene. This is best applied as a spray; one ounce to fifteen square
feet will suffice. If the oil is simply poured over the surface more
will be required.
The fighting of the species that breed on the extensive salt-marshes in
many regions is a larger and more difficult problem, but as it is a
matter that usually concerns large communities, sometimes whole states,
it can be dealt with on a larger scale. The very excellent results that
have been accomplished in New Jersey and on the San Francisco peninsula,
and in a smaller way in other places, show what may be done if the
community goes about the fight in an intelligent manner. In the fight in
New Jersey hundreds of acres of tide-lands have been drained so that
they no longer have tide pools standing where the mosquitoes may breed.
When it is impracticable to drain them the pools may be sprayed
occasionally with kerosene.
The value of the land that is reclaimed by a good system of draining is
often enough to pay many times over the cost of draining, thus the
mosquitoes are gotten rid of and the land enhanced in value by a single
operation.