The pupal stage lasts from one day to a few
weeks, no species being known to pass the
winter as pupae. At the end of the pupal
stage, the pupal skin opens and the adult
works its way out, crawls onto the surface of
the water, and is soon ready to fly away.

Adults

The adult mosquito (Fig. 6) is a small
fragile insect with a slender abdomen, one pair
of narrow wings, and three pairs of long,
slender legs. It varies in length from slightly
over 1/16 inch to about 1/2 inch (2-15 mm).
The three body regions, head, thorax, and ab-
domen are well-defined.

The head of a mosquito (Fig. 6) is almost
spherical and is joined to the thorax by a
narrow membranous connection. It bears a
pair of large-compound eyes, a pair of
antennae, a pair of palpi and a proboscis. The
antennae arise on the front of the head
between the eyes. They are long slender
structures consisting of 15 segments only 14 of
which are ordinarily visible. Each of the last
13 segments bears a whorl of hairs which are
short and sparse in the females, but long and
bushy in the males. The antennae are believed
to serve as organs of hearing and smell. The
palpi are five-segmented structures originating
at the lower front margin of the head near the
proboscis. In anophelines the palpi of the
female are about as long as the proboscis,
while those of the male are enlarged at the tip
(Fig. 4). In culicines the palpi of the females
are short, while those of the male are usually
long, densely haired, and pointed. The
proboscis projects downward and forward
from the lower front margin of the head. It
consists of a labium or sheath-like structure
enclosing a group of six stylets. The labium
serves as a protective sheath for the stylets but
does not pierce the skin when the mosquito is
biting. The stylets penetrate the skin of the
host animal and form a small duct through

18

which saliva is injected into the wound and a
canal through which liquid food is ingested.
The mouth-parts of the male are incapable of
piercing the skin of humans or animals.

The thorax, or middle region of the body,
bears the wings and legs. The upper surface of
the thorax or scutum is covered with coarse
hairs or scales which are variously colored.
These color patterns are often useful in
identification of species. The sides of the
thorax may be clothed with scale patches and
bear several groups of hairs or bristles, which
are useful for identification. The long, slender
legs arise from the lower sides of the thorax.
Each leg consists of a short conical coxa, a
small hinge-like trochanter, a stout femur, a
long slender tibia, and a 5-segmented tarsus.
The first segment of the tarsus is the longest
and is often equal in length to the tibia. The
fifth tarsal segment bears a pair of small
claws. The legs are covered with dark or
colored scales that form patterns which are
often useful in separation of species. The
wings are long and narrow with characteristic
venation. The veins are clothed with scales,
often of varying colors which may be
distributed to form definite patterns,
sometimes useful in identification. The hind
margin of the wing also bears a close-set row
of long, slender, fringe scales. Two small
knobbed structures known as halteres are
located behind and slightly below the wings.
The halteres vibrate rapidly when the mosquito
is in flight and serve as organs of equilibrium.

The elongate, nearly cylindrical abdomen
consists of ten segments, only eight of which
are readily visible. The ninth and tenth
segments are greatly modified for sexual
functions. North American species of
Anopheles usually have no scales on the upper
surface of the abdomen. In the culicines, the
abdomen is covered with scales which often
form characteristic markings. In Aedes and
Psorophora, the female abdomen is tapered



single brood, particularly Aedes in northern
areas, while many species of Aedes and Psoro-
phora produce multiple broods in a given
year. They overwinter in the egg stage. These
species are particularly troublesome as pests
and some characteristically fly long distances
from larval habitats, e.g., 5-20 miles in the
case of Aedes sollicitans, Ae. dorsalis, and Ps.
columbiae; the woodland species such as Ae.
atlanticus/tormentor and Ps. ferox tend to
remain near the larval habitat. Some
important vectors and pest species in this
ecologic group are Aedes sollicitans, Ae.
taeniorhynchus, Ae. atlanticus/tormentor, Ae.
thelcter, Ae. dorsalis, Ae. nigromaculis, Ae.
vexans, Psorophora ferox and Ps. columbiae.

The Artificial Container and Tree-Hole
Group

Characteristic of this group are members of
the genus Aedes, with specialized habits, the
more important being Ae. aegypti, Ae.
triseriatus, and Ae. sierrensis. Eggs are laid
singly on the inside wall of the container, at or
above the waterline, and hatch when inundated
after a period of desiccation. Overwintering is
in the egg stage and there are multiple broods.

Aedes aegypti and Ae. albopictus normally
lay their eggs in artificial containers, whereas
Ae. triseriatus and Ae. sierrensis females
usually oviposit in natural cavities containing
water, such as tree holes, but their larvae are
frequently found in artificial containers with
heavy sediment or decaying leaves.

This group includes not only the important
Aedes species mentioned, but species of
Toxorhynchites and Orthopodomyia as well,
none of which are troublesome to man.

Although not included in this group on the
basis of ecology and bionomics, females of the

21

Culex pipiens complex often oviposit in
artificial containers of all types, particularly
those rich in organic matter.

Because of the association of artificial
containers with human habitations and disease
implications, Aedes albopictus, Ae. aegypti,
and Culex pipiens quinquefasciatus all may
become locally abundant and annoying, and
are important targets of urban mosquito
control campaigns.

This manual will concentrate on
characterization of species which have known
potential importance as disease vectors, as well
as some of the more commonly found non-
vector species which may occupy the same
habitats and are important because of their
abundance or noxious habits. However, it is
not feasible to describe all mosquito species
which will be encountered in vector
surveillance. Identification of mosquitoes to
species requires considerable knowledge of
morphologic characteristics which are needed
for separation of genera and species. Simple
keys are appended at the end of the Manual
which will assist in separating the genera
found in the United States and in identifying
some of the more common species. For
mosquito survey and surveillance activities in
any given region or locality, state or regional
keys should be used, and simple keys can be
devised to facilitate identification of both com-
monly found and target vector species. Virus
associations of the mosquito species discussed
have been taken from numerous sources
particularly the 5 volumes, "The Arboviruses:
epidemiology and ecology" edited by T.
Monath (1988), Reeves (1990), Sudia er al.
(1971), Calisher and Thompson (1983), and
Tsai (1991).


Pupae

The mosquito pupa also lives in water and
is very active. It does not feed, but must come
to the surface for air except in the case of
Mansonia ot Coquillettdia spp. The pupa
differs greatly from the larva in shape and
appearance.

The head and thorax are greatly enlarged
and enclosed in a sheath. On the upper
surface is a pair of respiratory trumpets. The
abdomen consists of eight freely movable

HEAD
antenna-. -

THORAX scutum --.-
scuteflum -
Postnotum-~ +
halter -”
ABDOMEN
a cercus. --

maxillary palpus. \

eye~__
occiput. -~
anterior pronotal lobe --:

segments with a pair of paddles at the tip.

Mosquito pupae are probably the most
active of all insect pupae. Most species are
lighter than water, their buoyancy being due to
an air space between the wing cases on the
underside of the combined head and thorax.
By vigorous movement of the abdomen the
pupae move about with considerable speed,
rising directly to the surface when movement
stops.

proboscis.

--- 7* “I - tibia

LEG

tarsus --"

Figure 6. Diagram of Female Mosquito

17


apically, with the eighth segment withdrawn
into the seventh (Fig. 7). In other genera in
the United States the abdomen is bluntly
rounded at the apex. The terminal segments of
the male abdomen are greatly modified for
mating and are of value in identification of the
species.

HABITS OF THE ADULT
MOSQUITOES

About equal numbers of male and female
mosquitoes are produced. The males ordinarily
emerge first, remaining near the larval habitats
and mating with the females soon after their
emergence. Only the females bite and most
(but not all) species require a blood meal
before they can lay fertile eggs. The female
tends to travel greater distances and appears to
live longer than the male.

Flight habits vary considerably. Aedes
aegypti, probably the most domesticated of all
mosquitoes, breeds primarily in and around
human habitations and flies short distances,
usually a "block" or about 200 yards. Most
anophelines have a maximum flight Tange of
about 1 mile. However, other species such as
Ae. vexans and Ae. sollicitans can fly 10 to 20
miles or more.

Figure 7. A. Pointed Abdomen of Aedes.
B. Blunt Abdomen of Culex.

Mosquitoes also show considerable variation
as to their preferred sources of blood meals.

19

Some species by preference feed on birds or
domestic animals such as cattle or horses,
while others seem to prefer man. Because
these preferences are not exclusive, a species
will accept an alternate available host, which
accounts for the transmission of encephalitis
viruses from birds to humans or horses and of
the yellow fever virus from monkeys to
humans. A few species, which are not in-
volved in human disease transmission, feed
only on cold-blooded animals or subsist
entirely on nectar or plant juices. Some
Mosquitoes feed entirely during the daytime
while others may be active only during the
evening or at night. The female mosquito
requires two or more days to digest a blood
meal, lay a batch of eggs, and then seek
another blood meal. This cycle of feeding,
laying eggs, and feeding again can be repeated
many times in the life of a single female. Only
one mating by the female is generally required
to fertilize her lifetime egg production. The
life span of adult mosquitoes is not well
known. Some species apparently live one or
two months during the summer. Adults that
hibernate can live for six months or more.

A knowledge of the Mosquito life cycle
helps in understanding the epidemiology of
mosquito-bome diseases. The transmission, for
example, of arboviruses or the malaria parasite
depends not only on an initial blood meal from
an infected host and a subsequent feeding on
the victim, but also requires a period of
maturation of the infection in the mosquito,
perhaps a week or more for the viruses and
about 10 to 20 days for malaria. Once the
mosquito becomes infective, she may often
remain so for her lifetime, with the potential
for transmitting the infection each time she
feeds on a susceptible host. Thus, a knowledge
of the frequency of feeding, the selection of
hosts, life span, flight range, seasonal
occurrence, and many other factors are
important in disease epidemiology.


CULEX

Adak

PALP
SHORT

PALP SHORT

MALE ny MALE.

Figure 4, Characteristics of Anophelines and Culicines
(Kent S. Littig and Chester J. Stojanovich)

14



ECOLOGIC AND BIONOMIC TYPES
OF MOSQUITO LIFE HISTORIES

For practical purposes, the numerous
species of mosquitoes can be grouped on the
basis of similarities in their larval habitat
preferences which generally reflect other
important aspects of their bionomics and
ecology, ¢€.g., oviposition habits,
developmental patterns, brood patterns,
seasonal density and dispersal. A convenient
grouping of species includes: (1) The per-
manent pool group, (2) the transient water
group, (3) the floodwater group, and (4) the
artificial container and tree-hole group.

The Permanent Pool Group

Mosquito species belonging to this group
are generally found in fresh bodies of quiet
water exposed to sunlight and containing an
abundance of surface vegetation and flotage at
the air-water-plant interface. Typical habitats
are shallow margins of ponds, lakes and
smaller impoundments, the main characteristic
being a degree of permanency. Such habitats
are particularly suitable for Anopheles spp.,
Culex (Melanoconion) spp., Cx. salinarius,
Cx. territans, Coquillettidia spp., and Man-
sonia spp.

The mosquito species inhabiting permanent
waters deposit their eggs on the water surface,
singly in the case of Anopheles, and in rafts in
the case of Culex, Coquillettidia, and
Mansonia, with some variation in place of
oviposition in the latter genus. Such sites are
sheltered from wave action and are always in
the presence of vegetation suitable for larval
attachment or protection. Larval production is
generally continuous in these species with
several generations per year. Most Anopheles
and Culex overwinter as adult females, but
some, €.g., Ae. walkeri, may overwinter in the
egg stage, while Mansonia and Coquillettidia

20

overwinter as larvae attached to roots of
plants. Except for Coquillettidia and
Mansonia, the effective flight ranges of
permanent pool species are relatively short,
usually within a mile or so of the breeding
area.

The Transient Water Group

The members of this group are mostly
Culiseta spp., some Culex and occasionally
Anopheles. Their habits are similar to those in
the permanent pool group, but they seem to be
more often associated with pools of a
temporary nature as opposed to species with
an affinity for ponds and lakes. Typical sites
are roadside ditches, borrow pits, canals,
ground pools, clogged
Streams, irrigated lands, etc. Some species
associated with such habitats include Culex
pipiens quinquefasciatus, Cx. tarsalis, Cx.
restuans, Culiseta inornata and Culiseta
melanura. If water remains sufficiently long,
anophelines frequently become established in
late season, particularly such species as An.
albimanus, An. punctipennis and An.
pseudopunctipennis.

The Floodwater Group

The floodwater species of mosquitoes of the
genera Aedes and Psorophora generally
deposit their eggs singly on damp soil in
grassy or woodland depressions or along
vegetated shorelines that are intermittently
flooded. Salt-marsh mosquitoes are also
included in this group. When flooded after a
period of desiccation, the eggs hatch if
conditions (temperature, pH, oxygen
concentration, etc.) are favorable; otherwise,
the eggs may remain dormant and viable on
the soil until a more favorable inundation.
Typically, large numbers (broods) are
produced at a hatching; larval development is
uniform; and adults may appear as early as six
days after flooding. Some species produce a



hatching occurs, usually after only a few days.

Eggs that are not laid on water must be
placed so that the larvae can readily reach
water or they must be able to survive long
periods of drying until they are flooded. The
eggs of Aedes aegypti, Ae. triseriatus, and
Orthopodomyia are laid on the sides of
containers or tree holes just above the water
level. With a rise in the water, the eggs
hatch. Other species of Aedes and all species
of Psorophora \ay their eggs on the ground
where they remain until flooded. The eggs of
some species can survive for 3 to 5 years if
flooding does not occur. In some cases eggs
hatch as soon as they are flooded; thus, there
can be several generations a year. This is
typical of the Psorophora group and of Ae.
vexans and Ae. sollicitans. Eggs of some
other species must be subjected to freezing
before they will hatch; thus, there can be only
a single generation per year. Many species of
Aedes belong in this group; examples are Ae.
stimulans and Ae. abserratus.

Larvae

The larvae of all mosquitoes live in water,
permanent ponds and marshes, temporary
flood waters or woodland pools, water
contained in tree holes, leaves of plants, or
artificial containers. Mosquitoes have adapted
themselves to almost all kinds of aquatic situ-
ations except flowing streams and the open
waters of large streams, lakes, and seas.
Although mosquito larvae get their food from
the water in which they live, they must come
to the surface for air or, as in the case of
Mansonia, or Coquillettidia, obtain air from
the underwater portions of plants.

The larval period includes four
developmental instars, or stages, which
collectively require at least 4 days for
completion, often one to two weeks, the time
of development being controlled by such
factors as water temperature and food supply.

15

At the end of each instar the larva
molts--sheds its skin. The fourth instar is the
last larval stage, and the fourth molt produces
the pupa.

Mosquito larvae move about in two ways,
by jerks of the body and by propulsion with
the mouth brushes. Movements of anopheline
larvae at the surface are generally of the first
type. The “crawling” movements of culicine
larvae over the bottom and the slow movement
at the water surface are probably due to
propulsive action of the mouth brushes.
Mosquito larvae assume characteristic
positions in the water. Anopheline larvae lie
parallel to the surface, while most other
groups hang head down with only the tip of
the air tube penetrating the surface film.
Although larvae are heavier than water, they
can rest just beneath the surface without
muscular effort. Certain nonwetting
structures, such as the air tube in the culicines
and the spiracular plate and the palmate hairs
in the anophelines, suspend the larvae from the
water surface film.

Mosquito larvae are affected by light and by
water conditions including temperature and
movement, as well as by the dissolved gases
and salts, and other living organisms present
in their water habitat. Vegetation is important
as protection for the larvae. Predators such as
fish and insects destroy great numbers of
mosquito larvae.

The three body regions, head, thorax, and
abdomen are distinct (Fig. 5).

The head bears the antennae, eyes, and
mouthparts. The antennae are located on each
side toward the front. Behind the antennae
near the hind margin of the head are the eyes.
The mouthparts are at the underside of the
head near the front. They consist of a series
of brushes in addition to the grinding and
grasping structures. Thus, the larva is able to
strain out small aquatic organisms and
particles of plant and animal material present


in the water. A few predaceous species have
mouthparts adapted for grasping and
swallowing their prey.

The thorax is broader than head or
abdomen and somewhat flattened. It has
several groups of hairs which are useful in
identification of species.

The abdomen is long and subcylindrical,
consisting of ten segments. The first seven
segments are similar, put the eighth, ninth,
and tenth are considerably modified.
Anopheline larvae have float hairs, called
palmate hairs, on some of the abdominal

inner clypeal - - 44- ___.- - -outer clypeat

= -antenna

~ = frontals

- occipitals

ANOPHELES LARVA

HEAD

THORAX

ABDOMEN

spiracular plate

segments; culicine larvae do not have palmate
hairs. The eighth segment bears the
respiratory apparatus. In anophelines this
consists of a spiracular plate with two
spiracular openings. In culicines these
openings are at the tip of a siphon, or air tube.
The tenth segment is out of line with the other
segments and bears two or four membranous,
tapering appendages known as anal gills.
These anal gills serve more for the regulation
of osmotic pressure than for respiration (Fig.
5).

antenns. - ~

preantennal hair--

Jower head hair -

upper head hair- - ~~

lateral hair~ >

comb scales .

saddle. _

ventral
tufts

CULEX LARVA

Figure 5. Fourth Stage Anopheles and Culex Larvae

16