**3. Mosquito navigation**

A sophisticated olfactory system of mosquitoes enables them to communicate and responds to the diverse array of biological and environmental chemical stimuli throughout their life cycle. They use olfactory cues for locating a food source (nectar sugar), finding a mate partner, locating oviposition site, and most importantly selecting a vertebrate host for blood-feeding. Among these olfactory-guided behaviors, searching and locating the desired plant for nectar-feeding involves both visual and chemical cues emanating from different plant species [13]. Volatiles such as monoand bicyclic monoterpenes are major floral odors for mosquito attraction, and lightercolored plant flowers have an additional benefit for successful sugar feeding [14]. But, detection of blood-feeding host requires the integration of olfactory, visual, thermal, and humidity cues [15, 16]. The pattern of host-seeking behavior and selection of a certain host are strictly species-specific. However, the navigation trajectory of all the blood-feeding mosquitoes may have some common events (**Figure 3**).


#### **Figure 3.**

*Mosquito navigation trajectory according to odor plume. The random, non-oriented navigation becomes oriented when mosquitoes sense a gradient of different host odors such as CO2, lactic acid, 1-octen-3-ol, etc. olfaction along with vision, thermosensation, and hygrosensation facilitates the navigation process and blood meal uptake.*

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*Neuro-Olfactory Regulation and Salivary Actions: A Coordinated Event for Successful…*

c.When mosquitoes reach in the close vicinity of their potential host through olfactory-guided random to specific navigation, the cumulative role of visual thermosensors enhances. The compound eyes of mosquitoes enable them to visualize a particular host at a distance of 5–10 m by discriminating the light

d.After host localization, a mosquito must first land over the host for biting. Temperature and humidity also play a crucial role in short-range host orientation and landing. Then, mosquitoes trace a suitable site for probing through mechanosensory system. The olfactory appendages labellum and stylets along with the peripheral appendages (legs) of mosquitoes finally determine the

It is not difficult to admire that the multimodal sensory system of mosquitoes is the critical regulator of different behavioral processes and thus has a potential impact on disease transmission. In addition, the wide diversity of host preference in mosquitoes is governed by the different genetic makeup of individual species which has strong epidemiological consequences. Therefore, decoding species-specific molecular factors of the mosquitoes' olfactory system may unravel the mechanism of their behavioral plasticity. Two primary components of the chemosensory system are the peripheral system where the chemical information is detected and the central processing unit where the initial signal of odor is processed. The appendages present on the head of the mosquitoes act as the principal detection system, which includes paired antennae, paired maxillary palp, and a labium [8, 17]. These peripheral appendages are equipped with fine hair-like structures called the sensilla, which are distributed nonrandomly across these antennae, maxillary palp, and labium. The type and number of sensilla present on the olfactory organ are highly speciesspecific [8, 17]. Odorants are thought to penetrate through the numerous pores present on the wall of the sensilla and then traverse through the aqueous sensillar lymph towards the array of molecular receptors present on the dendrites of olfactory receptor neurons (ORNs) [18]. Binding of the diverse odorants with their cognate receptors either activates or inhibits the receptors by changing the ORN action potential. More than two decades of research on insect olfaction uncover several molecular factors that are responsible for odor detection and downstream signal transduction processes. These include odorant-binding proteins (OBPs), odorantdegrading enzymes (ODEs), odorant receptors (ORs), sensory neuron membrane proteins (SNMPs), G proteins, arrestins, and other signaling molecules [8].

**4. Mosquito sensory system and olfactory signal transduction**

**4.1 Odorant-binding proteins and odorant-degrading enzymes**

Odorant molecules are hydrophobic in nature which require cargo to traverse through the sensillar lymph to reach the receptor molecules, which are present on the dendritic membrane [19–21]. This role is carried out by the odorant-binding proteins (OBPs), which act as a passive carrier of the chemical odorant molecules. OBPs are water-soluble globular proteins containing six α-helical domains with conserved cysteine residues [22]. The number of genes encoding different OBPs varied across different mosquito species and is also dependent on the number of odorant receptors [22]. The availability of this wide and diverse spectrum of OBPs in the insect's tissues facilitates their rapid adaptation in distinct environment. The OBP family broadly includes the pheromone-binding proteins (PBPs) which

*DOI: http://dx.doi.org/10.5772/intechopen.90768*

intensity and color of the respective host.

probing site for successful blood meal uptake.

*Neuro-Olfactory Regulation and Salivary Actions: A Coordinated Event for Successful… DOI: http://dx.doi.org/10.5772/intechopen.90768*

