1) Mosquitoes use olfactory cues like carbon dioxide and lactic acid to detect humans for blood feeding. Their antennae, maxillary palp, and labellum contain sensory neurons that detect these odorants. 2) Carbon dioxide acts as a long-range attractant while lactic acid and other odorants help mosquitoes track humans at close range. Heat from the human body also attracts mosquitoes. 3) Odorant-binding proteins and sensory neuron membrane proteins help mosquitoes detect odorants and direct them toward hosts. Disrupting these olfactory systems could help control mosquito-borne diseases.

1. BARASAT GOVT. COLLEGE
WEST BENGAL STATE UNIVERSITY
MSC. 1ST SEM
Reg no.-002559 of 2013
ROLL : Bgc/zool NO: 14105
Year-2014

2. INTRODUCTION
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Mosquitoes are small, long-legged, two-winged
insect
have been around for more than 30 million years.
Mosquitoes that act as disease vectors rely upon
olfactory cues to detect humans for blood feeding.
Female mosquitoes feed on vertebtrate(eg.
human,cow,bird) blood for egg
laying.

3. PURPOSE
 Mosquitoes transmit pathogens that cause diseases
such as dengue, yellow fever, filariasis and most
significantly human malaria.
 The dual receptor(designated as cpA(Gr 1,2,3) of
maxillary palp of mosquito of CO2 and skin-odorants
as a key target that could be useful to disrupt hostseeking behavior and thus aid in the control of disease
transmission.

4. HOST FINDING PROCEDURE:
 Female mosquitoes are attracted to us by smelling the
carbon dioxide(CO2) we exhale, being capable of
tracking us down even from a distance.
 But once they get close to us,
they often steer away toward
exposed areas such as
ankles and feet, being drawn
there by skin odors.

5. Olfactory Parts and Tissues of
Mosquito
 The antenna,maxillary palp and the labellum(tip of
proboscis) of the
mosquito are the three sides of mosquito sensing.Sensory
organs
called sensilla cover
these tissues and
house
Odoront Sensory
Neurons
(ORNs).

6.  The
graph
clearly
shows
that a
mosquito
es ability
to find a
host is
greatly
reduced
upon the
removal
of
antennae
or
maxillary
palp or
proboscis

7. Odorant-binding Protein (OBP)
 OBP genes encode small ,globular,hydrophilic proteins(14-
20 KDa and 11KDa respectively) that are excreted in
significant amount out of the cell into the peri-receptor
space.
 OBPs in the mosquito’s antennae and mouth parts function
in a way to concentrate odorant molecules and help carry
them to the insect’s actual olfactory receptors, leading to
odorant detection.
 NOTE: Mosquitoes have higher concentrations of these
scent-detecting substances in their sensory organs at
night.That’s why mosquitoes detect human scent better at
night

8. Sensory Neuron Membrane
proteins (SNMPs)
 SNMPs was first described as a pheromone binding
protein in the antenna.
 They are membrane receptors ,and are directly related
to CD36 family fatty acid transporter receptors which
have been shown to bind a number of ligands in
various biochemical pathways .
 Twelve to fourteen CD36 like genes have been
identified in the genome of Ae. aegypti and An.
gambiae.

9. Olfactory Properties of Mosquito
Antennae
 The mosquito antennae which is the principle adult
olfactory appendage,remains the least understood is so
far as the molecular basis of olfactory physiology.
 There is a growing appreciation of antennal expression
patters of several genes that encode signal
transduction components

10. Olfactory Properties of Mosquito
Labellum
 The proboscis is a long slender oragn housing the
feedind stylets of the adult mosquito, harbouring
numerous chemosensory sensilla at its bulbous tip
known as labellum.
 Three morphologically and functionally different
labellar sensilla have been identified.TYPE –I(T1) and
TYPE-II(T2) sensilla are found on the external surface
of the labellam while the TYPE-III(T3) is present on
the oral surface.All three types are sensitive to taste
stimuli and several odorant stimuli including
butylamin and several aliphatic carboxylic acids.

11.  Fig: The labellum of Anopheles
gambiae exhibits olfactory
responses.Schematic dorsal view
of mosquito head and
brain,anterior(A),posterior(P),do
rsal(D) and ventral axis,as
indicated.(A) neurons of the
labellum project to the
posteromedial region of the
Antennal Lobes(AL) in the
brain.B a subset of T2 sensilla on
the labellum respond to odorant
stimuli in single sensillum(SSR)
experiments.Coincidely the T2
sensilla express OR6 and
OR7.SOG: Sub Esophageal
Ganglion.OL:Optic Lobe

12. Olfactory Properties of The
Maxillary Palp
 The maxillary palp of mosquitoes consists of five
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segments.
The maxillary palp is considerably less complex
,harbouring a single type of chemosensory
sensillum,the Capitate peg.
these structures contain three ORNs that form
stereotyped triads within each capitates peg sensillum.
The capitates peg(Cp) A and B neurons renpond with
high sensitivity to Co2 and 1-octen-3-ol,respectively.
CpC neuron might be involved in the detection of the
mosquito pheromones.

13.  Fig: Fig: The labellum of Anopheles
gambiae exhibits olfactory
responses.Schematic dorsal view of
mosquito head and
brain,anterior(A),posterior(P),dorsal
(D) and ventral axis,as
indicated.(A)Palpal ORNs project to
a posteromedial region of the
antennal lobes(AL) in the brain.(B)
Capitate pegs(Cp) are located on the
ventral side of the 2nd,3rd and 4th
segment of the maxillary palp.Each
neuron in the Cp(CpA,CpB,CpC)
respond to different odorant stimuli
and expresses a different odorant
stimuli and expresses a different set
of odorant receptors(Or) or
gustatory
receptors(Grs).SOG:Suboesophageal
ganglion,OL:optic lobe.

14. Odorant receptors(ORs)
 Traditionally classified as G-protein coupled receptors(GPCRs),insect
ORs,Aa/AgORs belong to a novel membrane protein family.In addition to their
characterstic lack of primary sequence conservation, insect OR proteins are
recognised as functionally obligate heterodimers composed of conventional
and highly divergent OR and a highly conserved non-conventional co-receptor
called OR7 in mosquito.
CO2 Receptor
 A distinct set of AgOrs and AgGrs are expressed in the triad of ORNs in the
maxillary palp of An. gambiae(Jones et al 2007,Lu et al 2007).The CpA neuron
co-expresses three CO2 receptors,AgGr22,AgGr23, AgGr24.And the other two
ORNs express AgOr7 along with AgOr8 or AgOr28,respectively(Jones et al
2007,Lu et al 2007).Several other AgOrs could be detected from palpal
cDNAs,albeit only after 35 cycles of PCR amplification(Latron and Biessmann
2008).Three AaGrs, AaGr1, AaGr2 and AaGr3,are closed homologs of
AgGr22,AgGr23 and AgGr24 respectively and are presumably expressed in the
Ae. Aegypti equivalent of the CpA neuron.

15.  olfactory signal
transduction pathways.
 Here, a 7 transmembrane
odorant receptor protein (OR)
lying within the ORN dendrite
interacts directly with odorants
or alternatively (?) in the context
of odorant-binding protein
(OBP) complexes. In both cases,
subsequent interactions with
heterotrimeric G-protein
complexes (G/G′) activate
downstream effector enzymes
adenyl cyclase (AC) and
phospholipase C (PLC). This
leads to the synthesis of the
second messengers cyclic AMP
(cAMP), diacylglycerol (DAG)
and inositol 1,4,5 triphosphate
(IP3) that regulate several cation
(Na+, Ca++) channels that carry
the transduction current. In
addition, OBPS are hypothesized
(?) to interact with odorant
degrading enzymes (ODEs) to
clear unbound ligand from the
extracellular space. Signaling is
terminated (bulbed lines) by
decoupling of OR/G-protein
complexes by arrestins (A) and
G-protein coupled receptor
kinases (GRKs).

16. ROLE OF CO2 IN HOST-SEEKING
 Atmospheric levels of CO2 were estimated at 0.03-
0.04%), with an increase of 0.06% to 0.10% due to
plant respiration in a tropical forest.
 Schmidt & Nielsen (1975) estimate that 275 ml/min of
CO2 is given off by a human subject, resulting in a
concentration of between 0.01% and 1.0% in air
coming off the subject.
 mosquitoes may respond to a change in CO2
concentration rather than a certain threshold level.

17.  Experiment:
 An example of the
selected host-seeking
behavior of 60 female
mosquitoes monitored for
a short period (75 min) by
automated-device. CO2
was delivered for 2 sec at
15 min, 30 min, and 45
min (arrow). Note that
female mosquitoes show
"touch-down" behavior
(blue) in response to the
change of CO2
concentration (red).

18. BEHAVIOURAL RESPONSE
 Take off:When mosquitoes were exposed to an
airstream to which 0.2% carbon dioxide was added, the
rate of take-off was greatly increased for a period of
about two minutes before falling off again to a low
level.
 Sustained flight: In a tunnel,an airstream uniformly
permeated with carbon dioxide at a concentration of
0.2 or 0.05% was flowing, Mayer & James (1969) found
that Ae. aegypti failed to progress more than a short
distance up the tunnel. It is presumed that the
mosquitoes settled again soon after take-off.

19. ROLE OF LACTIC ACID IN HOSTSEEKING
 Acree et al. (1968) identified lactic acid as a mosquito
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attractant
L isomer of lactic acid was 5 times as attractive as the D
isomer.
Lactic acid is produced as a result of vertebrate muscle
metabolism and is only produced in the L(+) isomer .
Smith et al found that lactic acid evaporating from human
hands was within the range of 23-133µg/h.
an enzymatic decomposition of lactic acid eliminates the
attractive effect of human skin residues for A. aegypti.
This implies that all components which contribute to the
attractiveness of skin odour are only effective when lactic
acid is present.

20. FEMALE MOSQUITOES WILL BE ATTRACTED
TO A SOURCE OF
HEAT
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Mosquitoes landed three times as often
on a clothed robot when the robot's
"skin" temperature was 98°F (36.7°C)
than one with "skin" temperature of 5065°F (10.0-18.3°C).
Fig:Mosquito TRPA1 is a candidate
thermo-sensing protein for hostseeking behavior. (A-L) AsTRPA1 (+)
cells (green) are present along the
whole proboscis and in association with
sensory neurons (magenta: HRPpositive neuron, D and J). Note that
some sensilla contain AsTRPA1 (+)
signals (A, F, and H). All bars: 5 μm.
(M-O) TRPA1-activating agent (AITC)
disturbs the selected host-seeking
behavior. Female mosquitoes were
exposed to 1% (v/v) AITC at ZT16-19.
Note that volatile AITC(allyl
isothiocyanate ) reduces the selected
host-seeking behavior compared to
controls.

21. Experiment Combining heat and
odor
 Air was pumped in on one
side of the wind tunnel and
the mosquitoes were released
on the other, downwind,
side. Clean air was used as
control. The treatments
consisted of a heat source at
the air inlet, and a
combination of heat and foot
odor.
 The presence of a heat source
near the air inlet did not
affect the flying path, but
more mosquitoes landed
near the inlet When the air
contained foot odor.
 The results show that the
mosquitoes use both odor
and heat to find their host
and that either of these two
stimuli alone is not enough.

22. CONCLUSION
With this information,it should be possible to identify
and formulate chemicals that will act as attractants,
repellents and other agents targeted against the
olfactory basis of mosquito behaviours. Increased
awareness of the importance of these approaches will
be helpful in the fight against insect-borne diseases.

23. ACKNOWLEDGEMENT
 Sincere thanks to Dr. Debajyoty Chakraborty, the Head
of the Dept. of Zoology, Barasat Govt. College for
valuable suggestions and providing infrastructure
facilities.
 Thanks to Dr. Tuhin Saha, Dr. Madhumita Manna, Dr.
Debjani Sarkar, Dr. Sanjoy Poddar,Dr.Tanaya Dey, Dr.
Srikanta Guria, for providing encouragements and
various help during the entire period of study.
 Lastly, the co-operation received from the classmates is
also acknowledged.