The sensory arsenal mosquitoes use to find us

Abstract

Female mosquitoes that home in on people for blood meals are exquisitely effective at finding us. This is because they are endowed with an uncanny ability to sense virtually every cue people provide. These include exhaled CO₂, the image of the host, and volatile body odors, which can be detected at distances of multiple meters. When they traverse to under one meter, they sense thermal infrared. Within a few centimeters of a human, they detect convection heat and humidity emanating from skin. Upon landing, mosquitoes taste non-volatile chemicals, and sense conduction heat before electing to engorge on blood or fly away. This review focuses on the cellular and receptor mechanisms underlying the sensory detection mechanisms that mosquitoes use to home in on us.

Finding hosts through multimodal integration

Mosquitoes are our most dangerous micropredators. No other animals are as effective at tracking us down, sucking our blood, and in so doing, transmitting viruses and parasites that sicken, or even kill us. It is impossible to get exact statistics, but according to a World Health Organization report in September 2024 (https://www.who.int/news-room/fact-sheets/detail/vector-borne-diseases), each year ~350 million people suffer from mosquito-borne diseases, and around 700,000 die. To make matters worse, with climate change and world-wide travel, the geographical range of anthropophilic (see Glossary) mosquitoes is expanding. There are >3,500 known species of mosquitoes; however, only a handful are capable of spreading disease-causing agents when they attack humans. The most pernicious mosquito micropredators include multiple Anopheles (An.) species, such as An. gambiae, An. coluzzii and An. stephensi, which spread the plasmodium parasite that causes malaria, and two Aedes (Ae.) species, Ae. aegypti and Ae. albopictus, which transmit the flaviviruses that afflict humans with dengue, yellow fever, Zika and chikungunya. In addition, multiple species of Culex infect people with the viruses that lead to West Nile fever, Japanese encephalitis and other diseases. The bias of different anthropophilic species for humans over other mammals, or even birds or reptiles varies, with Ae. aegypti showing particularly high preference for people [1].

The suffering inflicted by mosquitoes is made possible by their ability to zero in on humans with uncanny effectiveness. This is due in part to their persistence. The female mosquitoes are determined to find blood meals to nourish their eggs. Their tenaciousness would be less of a concern if they were not such virtuosos at sensing humans. This skill is aided by their acumen in integrating multiple types of sensory cues. Multi-sensory integration is essential [2–4] since no single host-derived cue is sufficient for anthropophilic mosquitoes to differentiate their favorite targets, humans, from other animals.

The arsenal of cues detected by female mosquitoes depends on how far away we are from them. Remarkably, at distances of 10 or even 15 meters they can detect our exhaled CO₂, the volatile emanations from our skin, and the fuzzy images of our bodies [5] (Figure 1). As these micropredators surge towards us, at least one intermediate range cue comes into play. Our surface body temperature is ~34 °C, and this heat is converted into electromagnetic radiation, thermal infrared, which can be detected effectively by mosquitoes at distances of up to ~0.7–0.8 meters [6] (Figure 1). Then, as they move into close range, within ~10 centimeters, they begin to perceive convection heat and humidity from our bodies [7,8] (Figure 1), as well as continuing to sense odors from our skin and the image of the host. Finally, when the mosquitoes land, they sense body heat through conduction, and sample non-volatile chemical cues on skin (Figure 1). However, the precise attractiveness of different volatile and non-volatile chemicals is not identical for all anthropophilic mosquitoes [9]. It is only after landing that female mosquitoes make the critical decision as to whether to probe for a vein and withdraw blood, or fly away.

Figure 1. Sensory cues used by female mosquitoes to detect human hosts.

Indicated are the cues that mosquitoes use to sense a human at long range (~1 – 15 meters), mid-range (<0.8 meters), short range (< 10 centimeters), and on contact after landing (figure prepared by J. Carrillo-Ortiz)

Depending on environmental conditions, different human-derived cues are more or less salient. If the human is moving quickly, or if there is a very strong wind, CO₂ and human odors may be less effective directional cues. Visual cues cannot help the mosquitoes if the host is in a completely dark room. Thermal infrared is of little use if the environmental temperature matches, or closely approximates the surface temperature of our bodies. In their quest for a blood meal, female mosquitoes must also evade defensive moves from the host to avoid being killed [10–14]. Thus, humans and other mammalian hosts are both prey and predators for female mosquitoes.

A major goal for many of the scientists focusing on mosquito/host interactions has been to identify the receptor cells and molecules that enable mosquitoes to find us. Until recently, this was a daunting endeavor due to challenges in targeting mutations in genes encoding candidate receptors, and to the equally immense hurdles complicating the creation of the repertoire of transgenic tools that have contributed to making Drosophila melanogaster such a powerful model organism. Fortunately, with the advent of CRISPR/Cas9, the goal of revealing the nature of the sensory receptors in mosquito disease vectors is beginning to be realized. This review focuses on some of the exciting and transformative progress in this area over the last few years.

Homing in from afar

Sensing CO₂

One of the key cues that alert anthropophilic mosquitoes to the possible presence of humans is CO₂. We exhale CO₂ at a concentration of ~4%, which is ~100-fold higher than atmospheric levels, and these plumes cause mosquitoes to surge in the direction of the source. Once they detect elevated pulses of CO₂, their responsiveness to other potential human-derived stimuli, such as skin odors, and visual cues is enhanced [2,3,15]. Because exhalation of CO₂ is intermittent, and the plumes are affected by wind, the mosquitoes detect short puffs of elevated CO₂ rather than a continuous stream.

Mosquitoes are endowed with three distinct organs that serve in the detection of olfactory stimuli. The antenna senses organic molecules emanating from the host, and the maxillary palp detects CO₂ [16–18] (Figures 2A and 2B). The labella at the end of the proboscis (Figures 2A and 2C), in addition to functioning in taste, also responds to a small set of volatile chemicals from human skin [19,20]. The detection of olfactory cues in these three mosquito organs occurs through sensory bristles (sensilla) that house two to five olfactory receptor neurons (ORNs), with two or three ORNs being the most typical. The most abundant olfactory sensilla on the antenna, and the most important type for sensing human odors are the long trichoid sensilla, which detect volatiles such as lactic acid and ammonia (Figure 2D) [9,21]. In addition to ORNs, there are several types of support cells including those that secrete odorant binding proteins into the lymph bathing the ORN dendrites. Olfactory sensilla are endowed with multiple pores, thereby allowing entry of volatile chemicals (Figure 2D). The main sensilla on the maxillary palp that sense human-derived odors are clubbed-shaped sensilla (dubbed capitate peg sensilla), which house three neurons, including the CO₂-sensitive neuron, called cpA (Figure 2E) [17,22]. The finding that only a single ORN functions in CO₂ detection is remarkable given the great importance of CO₂ in host-seeking. It is also notable that the cpA neuron can be activated or inhibited by a variety of organic volatiles, and that persistent activation of this neuron by an organic compound can disrupt the ability of mosquitoes to employ CO₂ as part of their sensory toolkit for finding hosts [17,22–24]. Thus, organic compounds that either inhibit or cause long-term activation of cpA could provide an approach for limiting the capacity of mosquitoes from homing on their hosts.

Figure 2. Female mosquito with key sensory organs highlighted.

(A) Illustration of a female mosquito. Indicated are an antenna, compound eye, labellum (marked by the red box), maxillary palp, proboscis, and the tarsus on a foreleg. (B) Cartoon of an antenna. The 1^st and 13^th flagellomeres are labeled. (C) Drawing of the end of the proboscis with two labella. The labrum is the stylet that contains neurons and withdraws blood. (D) Cartoon of a bristle-like olfactory trichoid sensillum present in the distal flagellomeres of the antenna. The trichoid sensilla have multiple pores. (E) Cartoon of a capitate peg sensillum, which are olfactory sensilla on the maxillary palp. These sensilla have multiple pores and include the cpA ORN that senses CO₂. (D-F prepared by J. Carrillo-Ortiz) (F) Cartoon of a bristle-like gustatory sensillum. Gustatory sensilla have one distal pore. (G-I) Three types of hygrosensory sensilla present in the antenna (prepared by D. M. DeGeorge). (G) Tip coeloconic sensillum (peg-in-pit sensillum) present in the 13^th flagellomere. (H) Sensillum ampullaceum (peg-in-tube sensillum) present in the 1^st and 2^nd flagellomeres. (I) Side coeloconic sensillum (peg-in-pit sensillum) found in the 10^th and 12^th flagellomeres. GRN, gustatory receptor neuron; MSN, mechanosensory neuron; ORN, olfactory receptor neuron.

A critical question concerns the identity of the CO₂ receptor. In Drosophila, two members of the gustatory receptor (GR) family, GR21a and GR63a, are expressed in ORNs, and are required for CO₂ detection [17,25,26]. Thus, despite their names (gustatory receptors), these proteins are actually olfactory receptors. GR21a, GR63a and other members of the GR subfamily consist of seven transmembrane domains. However, they are not G-protein coupled receptors (GPCRs), which also consist of seven membrane spanning segments. Rather, opposite to GPCRs, the N- and C-termini of insect GRs are inside and outside cells, respectively [27], and based on recent high resolution structures, they are tetrameric ionotropic cation channels [28–30]. Due to these features, they belong to a family of proteins referred to as 7-transmembrane domain ion channels (7TMICs) [31].

A requirement for GRs for CO₂ detection is conserved in mosquitoes. In mosquitoes, three Grs are expressed in the cpA ORN. These include the Gr21a homologs (Aedes Gr1, Culex Gr1, and Anopheles Gr22), the Gr63a homologs (Aedes Gr3, Culex Gr3 and Anopheles Gr24), and a third Gr (Aedes Gr2 and Anopheles Gr23) [17,25,32]. Mutation of Gr3 disrupts CO₂ sensation, demonstrating that it is essential [2]. Aedes GR2 and GR3 [33], as well as the Anopheles GR22 and GR24 [17] are sufficient to generate a CO₂-receptor since co-introduction of these GR pairs in Drosophila ORNs that are devoid of olfactory receptors, confers CO₂ sensitivity. Further addition of the third GR (either Aedes Gr1, Culex Gr1, and Anopheles Gr23) enhances the CO₂ sensitivity of the other two [17,32,33]. Thus, any two of the three GRs in the cpA ORN appears to be minimally sufficient to confer CO₂ sensitivity, while all three may be required for a maximum response. Consistent with this conclusion, mutation of Anopheles Gr23 or Gr24 eliminates CO₂ detection, while knockout of Gr22 only reduces sensitivity [34]. Nevertheless, the attraction of Ae. aegypti to humans under semi-field conditions is only slightly impaired in mutants lacking Gr3 [2]. These data should not be interpreted to mean CO₂ detection is not very important. Rather, they support the theme that mosquitoes have a rich arsenal of sensory mechanisms to find humans, and that abrogation of one type of stimulus (e.g. CO₂) is insufficient to prevent mosquitoes from successfully hunting us down.

Detection of highly volatile odors from skin

Human skin releases a daunting array of volatile chemicals, some of which can be detected by mosquitoes at distances of 5 – 10 meters or more. Most of the odorants, such as lactic acid, ammonia, and short- and middle chain carboxylic acids are produced by the microbial flora on our skin by metabolizing chemicals that we produce in our sweat [9,35,36]. While these human-derived odors are very attractive, female mosquitoes pay far greater attention to them in the presence of CO₂. The relative attraction to lactic acid, ammonia, various carboxylic acids and other organic odorants differs between An. gambiae, Ae. aegypti and Culex quinquefasciatus [9].

It is long known that mosquitoes find some individuals more appealing targets than others [37–39]. Interestingly, in Ae. aegypti, female mosquitoes will learn to avoid the scent of some humans, if it is associated with an aversive stimulus, such as swatting [40]. Nevertheless, in the absence of pairing an aversive stimulus with particular human scents, differential attraction to some humans over others is due largely to variations in the organic bouquets emanating from the skin of different people [41–43]. The most attractive humans emit higher levels of certain carboxylic acids, ketones and other organic volatiles [41–43], and these differences between individuals could be related to distinctions in microbial flora [9,35,36]. Moreover, infection of mice with the flaviviruses that are spread by Ae. aegypti can cause alterations in the mouse microbiome, thereby modifying the volatiles released by the host, which in turn makes them about two-fold more attractive to mosquitoes than uninfected mice [44]. If the same occurs in humans, this could potentially lead to increased transmission of these disease-causing viruses.

Male and female antennae are sexually dimorphic. While both include 13 segments called flagellomeres (Figure 2B), in Anopheles males, the olfactory sensilla are limited to the two most distal flagellomeres, while olfactory sensilla are distributed throughout flagellomeres 2 to 13 in females [21]. Females are endowed with a greater number of olfactory sensilla, and this sexual dimorphism is reflected by the critical importance of olfaction in helping females detect and navigate to their hosts.

In addition to the GRs that function in CO₂ detection, members of at least two other large families of receptors function in olfaction in mosquitoes, one of which is the so-called ‘Olfactory Receptor (OR) family [45]. ORs range in numbers from 72 in Anopheles gambiae to 117 in Aedes aegypti, and 180 in Culex quinquefasciatus (Table 1) [9,46]. Similar to GRs, the ORs are 7TMICs since they are tetrameric ionotropic receptors consisting of seven transmembrane segments, and have an inverse topology relative to GPCRs [47–51]. In mosquitoes, ORs are comprised of one subunit that binds the odorant (tuning subunit), and three co-receptor subunits (ORCO) [52]. Based on work in Drosophila [53], mosquito ORCO is most likely required for trafficking of the channel to the plasma membrane. Thus, loss of orco is tantamount to eliminating all OR-dependent olfactory responses.

Table 1.

Number of members of selected families of sensory receptors

Receptor	Abbreviation	Aedes aegypti	Anopheles gambiae	Culex quinquefasciatus
Gustatory Receptors	GR	107	90	126
Olfactory Receptors	OR	117	72	180
Ionotropic Receptors	IR	135	110	69
Pickpockets/ENaCs	Ppk	32	26	48
Transient Receptor Potential channels	TRP	10	9	10
Opsins	Op	10	11	13

Mutation of the orco gene in Ae. aegypti or An. coluzzii (a very close relative of An. gambiae) virtually eliminates the electrophysiological responses of olfactory sensilla to a variety of odorants tested, and blunts the behavioral response to the aroma of odorants present in nectar, as well as the attraction of females to human odor alone [54,55]. However, there is no deficit exhibited by the Ae. aegypti orco mutant to the much stronger attraction elicited by human odor in the presence of elevated CO₂ [54]. These findings indicate that there exist OR-independent mechanisms for sensing human odors.

The other major family of olfactory receptors are referred to as ‘Ionotropic Receptors’ (IRs) [56], and as their name indicates, they serve as receptors and ion channels. Although IRs and ORs were originally thought to be expressed exclusively in distinct ORNs, surprisingly, many ORNs in the antenna and maxillary palp express both classes of olfactory receptors [57]. Insect IRs are distantly related to mammalian ionotropic glutamate receptors, and are likely heterotetramers [58]. The mosquito IR families include 135 in Ae. aegypti, 110 in An. gambiae, and 69 in Culex quinquefasciatus (Table 1), although only a subset is expressed in olfactory organs [9,46]. This reflects roles for mosquito IRs in multiple senses in addition to olfaction, as described in subsequent sections in this review focusing on humidity and thermosensation. Based on work in Drosophila, IRs in mosquitoes are likely to also function in taste [59–63]. Insect IRs consist of multiple subunits, and include one or two of the three co-receptors (IR8a, IR25a and IR76b), and one or more tuning subunits [64–66]. In An. coluzzii males, expression of IRs is limited to the distal two flagellomeres. However, in females the IR co-receptors and more than half of the 20 olfactory IRs examined are distributed throughout the majority of the antenna, and most prominently in flagellomeres 5–13 [67]. Smaller subsets of IRs are expressed primarily in the distal segments or the proximal segments [67]. The functions of some of the tuning receptors have been unraveled through in vitro expression and gene knockout experiments. For example, co-expression of Anopheles IR41a or IR46c in Xenopus oocytes along with IR25a and IR76b confers responses to distinct amines, while co-expression of IR75k with IR8a elicited responses to carboxylic acids [66]. Knockout of Ir8a in Ae. aegypti eliminates the attraction to lactic acid in the presence of CO₂, and impairs the ability to detect human odors–a phenotype that is not dependent on CO₂ [68]. Furthermore, Aedes Ir25a and Ir76b mutants exhibit a reduced attraction to humans [41]. Since the orco mutant retains a robust response to human odors in the presence of CO₂, the combination of these findings indicates a greater importance for IRs relative to ORs for detecting human odors.

Seeing the fuzzy image of the host

It has been known for many years that vision is an important part of the mosquitoes’ sensory arsenal, enabling them to find their hosts [69]. Using vision, mosquitoes can detect potential objects at distances of up to ~15 meters [5]. However, the exact distance over which mosquitoes can detect a human host through visual cues is not resolved. Mosquitoes have compound eyes, which are comprised of hundreds of repeat units, referred to as ommatidia. Individual ommatidium consist of eight photoreceptor cells, each of which includes a microvillar structure, called the rhabdomere, where light reception and phototransduction takes place. In mosquitoes, the rhabdomeres are fused, thereby enhancing light sensitivity, but causing them to have poor visual acuity [70]. Consequently, their ability to use vision alone to differentiate between humans and other animals, or even inanimate objects is limited. Anthropophilic mosquitoes, such as the diurnal Ae. aegypti, and even nocturnal mosquitoes, such as An. coluzzii, use vision to help them navigate to a feature that might be a potential host. This behavior is increased after they detect elevated CO₂ [3,71]. Several studies have examined the effects of CO₂ on the orientation of tethered Ae. aegypti to visual features. According to one report, their orientations in relation to visual features is not affected by elevated CO₂ [72]. However, in other studies CO₂ either enhanced their contrast sensitivity [73], or increased the mosquitoes object tracking in response to vertical bars [74]. In addition, using a genetically-encoded calcium sensor, CO₂ affected responses in the optic lobes of Ae. aegypti [74]. Nevertheless, it has been suggested that the increased propensity of flying mosquitoes to reach a visual feature in the presence of CO₂ might be due to the higher activity levels induced by CO₂, rather than increased attraction to the visual cue [72].

Mosquitoes have biases for some colors over others. Ae. aegypti find black, red, orange and cyan especially attractive, and this discrimination depends on exposure to CO₂ [3,69,75–78]. In fact, in the presence of CO₂ Ae. aegypti are most attracted to colors consistent with the tones of human skin, such as orange and red [76].

The phototransduction cascade in the compound eyes of mosquitoes is not well defined, although it seems plausible that it employs a similar set of signaling proteins as in Drosophila. In fruit flies, light sensation initiates with photoactivation of multiple rhodopsins, which couple to a trimeric G protein (Gq) [79]. The Gq engages a phospholipase C, and the cascade culminates with the activation of the TRP and TRPL channels. Ae. aegypti encodes 10 opsin genes, five of which are expressed in the compound eyes [80–84]. Op1 is the major rhodopsin in Aedes and is expressed in the R1-R6 cells and many R8 cells [83]. Elimination of Op1 in combination with loss of Op2, which is expressed in a subset of R7 cells [80,85], virtually eliminates vision-guided target attraction, upon stimulation with CO₂ [86], including the detection of long wavelengths that approximate skin tones [76]. However, the op1,op2 double mutants are not blind since they exhibit an optomotor response (walking in the same direction as rotating black and white strips), although this behavior is reduced relative to wild-type [86]. Ae. aegypti also encode TRP and TRPL channels, and mutation of trp diminishes the amplitude of the response to bright light, demonstrating that TRP contributes to the phototransduction cascade in these mosquitoes [12].

Thermal infrared (IR)—a cue for sensing us at mid-range

Our surface body temperature is ~34 °C, and the female mosquito’s ability to sense this host cue occurs through all three modes of heat transfer: conduction, convection and infrared radiation (IR) [6,7,87]. They detect conduction heat only after landing, while sensation of convection heat requires that they are in very close range of <10 cm [7]. The thermal IR emanating from our bodies is detected by Ae. aegypti at distances up to ~0.7 – 0.8 meters, thereby comprising a mid-range navigational cue. As with CO₂, human odors, and visual cues, host seeking is aided by thermal IR only in combination with other cues [6], consistent with the theme that no single host-derived stimulus allows the mosquitoes to home in on humans effectively. Earlier studies conclude that Ae. aegypti are not capable of using thermal IR for host seeking [88,89], most likely because they assayed thermal IR in isolation.

The neuron that responds to thermal IR is housed in a specialized peg-in-pit sensillum (tip coeloconic sensillum) in the terminal (13^th) flagellomere of each antenna [6] (Figure 2G). These sensilla are housed at the bottom of a cuticular housing with an apical aperture, which allows for reception of thermal IR in the direction of the source. The thermal IR, which peaks at ~10 μm, is too low energy to directly activate a protein or a chromophore, such as a rhodopsin. Rather, the mechanism appears to involve local heating of the cuticle, which then warms the endolymph surrounding the neuron in the peg-in-pit sensilla, causing thermal activation of TRPA1 [6]. At low intensities of thermal IR, two opsins (Op1 and Op2) are also required for sensing thermal IR [6]. Altogether, the data fit the model that at lower levels of thermal IR that are insufficient to directly activate TRPA1, two opsins function as the thermal sensors, which are required for indirect activation of TRPA1 through an amplification cascade [6]. While thermal IR can be used as part of their sensory arsenal, this cue does not come into play if the environmental temperature is very similar to body temperature, due to loss of thermal contrast. Nevertheless, thermal IR has the advantage that it is not compromised by wind currents.

Almost there

Sensing convection heat

A female mosquito needs to navigate very close to its potential host before convection heat from skin emerges as a salient host cue. This is because air temperature equilibrates with the environment within <10 centimeters from skin [90,91]. Convection heat is sensed by the antenna near the distal tip [92–95], and is used in combination with other cues, such as CO₂, and human odors [2,95,96].

The cellular mechanism for sensing body heat at short distances involves warm-activated neurons, which promote attraction, and also cool-activated neurons, which cause avoidance [95]. The Warming Cell is stimulated by increases in temperature and inhibited by cooling, while the Cooling Cell is stimulated by cooling and inhibited by warming [93,95,97]. The baseline activity of Cooling Cells increases with a decrease in temperature, rather than by a specific absolute temperature [95]. Since activation of Cooling Cells causes repulsion, while activation of Warming cells promotes attraction, the convection heat from a warm-bodied animal causes behavioral attraction through dual mechanisms—inhibition of Cooling Cells, and stimulation of Warming Cells.

In An. gambiae, Ir21a and Ir93a are expressed in Cooling Cells at the distal end of the antenna, and loss of either of these receptors eliminates the increased spiking by a decrease in temperature [8,95]. Mutation of these receptors also greatly reduces but does not eliminate heat seeking in the presence of CO₂ [8,95]. In Drosophila, in addition to IR21a and IR93a, another IR (IR25a) also functions in Cooling Cells, raising the question as to whether mosquito IR25a also has a role in Cooling Cells in mosquito disease vectors. Currently, the receptors required in the Warming Cell in mosquitoes are not known.

Humidity detection

Once female mosquitoes are within a <10 cm range of the host, they are capable of sensing the increase in vapor pressure of water due to humidity emanating from skin. This ability depends two types of neurons: Moist Cells, which are activated by moist air, and Dry Cells, which are activated by dry air [8,98,99]. These two opposing types of humidity sensing neurons are housed in three types of sensilla distributed on the antenna [99] [8,100]. Moist Cells and Dry Cells are both found in flagellomeres 1 and 2 in peg-in-tube sensilla (sensilla ampullacea), which are located internally below a tube beneath the surface of the cuticle (Figure 2H). In flagellomeres 10 and 12, the two cell types are present together in peg-in-pit sensilla (side coeloconic sensilla) (Figure 2I). The Moist Cell, but not the Dry Cell, is also contained in a peg-in-pit sensillum (tip coeloconic sensillum) in the most distal flagellomere (13^th) that houses a Warming Cell (Figure 2G).

Recent work established that IRs function in either the Moist Cell, the Dry Cell, or both, similar to the contribution of IRs for humidity sensation in Drosophila [101–103]. In Ae. aegypti, IR40a is required for the response of the Dry Cell to dry air, while IR68a functions in the Moist Cell for the sensing moist air [99]. In addition, IR93a, which is widely expressed in the antenna and has a role in Cooling Cells, is required in both the Moist Cell and Dry Cell [8]. Thus, IR93a might act as a co-receptor for other IR subunits, such as IR40a in Dry Cells, IR68a in Moist Cells, and IR21a in Cooling Cells. Due to the wide expression of IR25a, and its requirement in Cooling Cells, this latter receptor may also be a co-receptor in Cooling Cells. Whether IR25a functions and acts as a co-receptor in Dry Cells or Moist Cells is not known. Interestingly, while mutation of Ir93a impairs blood feeding [8], loss of either Ir40a or Ir68a does not [99], suggesting that increased activity of the Moist Cell or decreased activity of the Dry Cell is sufficient to assess changes in humidity in proximity to a host, and promote blood feeding. Moreover, mosquitoes that are doubly mutant for both Ir40a and Ir68a are impaired for blood feeding to a similar extent as the Ir93a mutant, further supporting the conclusion that the Dry Cells and Moist Cells have functionally redundant roles in mediating this behavior [99].

Making the final decision

Once a female mosquito completes its navigation to a host, it still has not decided whether to search for a vein and take a blood meal, or fly away. A mosquito typically probes for a vein for ~30 – 60 seconds while it assesses the temperature and chemicals on the host surface [104]. Females take 3 – 4 minutes to withdraw blood [104], which is then directed to the midgut. This is distinct from nectar, which is sent to a food storage compartment, the crop. The ~3 – 5 μL that is consumed is very significant as this doubles or even triples a female’s body weight during engorgement [105,106]. Stimulants that promote blood feeding include adenosine phosphates [106–108], which appear to bind to a receptor that remains to be identified [109]. Biting is also promoted by a combination of NaCl and certain amino acids, but not by either alone [110].

Mosquitoes evaluate the surface temperature of the host by sensing conduction heat through their forelegs [87]. A recent study showed that if a mosquito’s ability to sense host odors is impaired, by mutating orco, then their sensitivity to conduction heat is increased [87]. This sensory compensation is dependent on Ir140, which is upregulated in the forelegs of the orco mutant [87]. It is not known if IR140 is a subunit of a thermosensory channel. Moreover, the mechanism through which expression of Ir140 is upregulated in forelegs, as a consequence of a deficit in ORNs, is a fascinating question, which remains to be resolved.

Following landing, female mosquitoes also use taste to sample a wide set of chemicals on skin, ranging from ammonium to amino acids, fatty acids, NaCl and others [110]. The main taste organs that sample the chemicals on a host are the tarsal segments of the forelegs and midlegs, which are the first organs to contact a host, and the mouthparts at the end of the proboscis. The tarsi and the bilaterally symmetrical labella at the distal end of the proboscis (Figures 2A and C) are decorated with taste sensilla. Each taste sensillum has a single distal pore (Figure 2F), allowing the entry of chemicals, which are then detected by two to five gustatory receptor neurons (GRNs) [104]. In Ae. albopictus, each labellum has 15 taste bristles, including five large, six intermediate, and four small sensilla [110]. Some of the GRNs are activated by sugars, NaCl and ammonium, while others are inhibited by chemicals such as bitter compounds and amino acids [110]. Centered between the two labella is a sheath (labium) housing a set of six needle-like stylets in females, one of which, the labrum (Figure 2C), harbors neurons that sense different nutrients in blood, but are insensitive to nectar [106]. This discovery helps explain the mechanism through which female mosquitoes can distinguish blood from nectar [106]. The labrum is also used to withdraw blood after it and other stylets pierce a vein.

Concluding remarks and future perspectives

It is now well established that female mosquitoes employ multisensory integration for attacking their hosts. However, many of the receptors and sensory receptor cells that promote this pernicious behavior remain to be identified (see Outstanding Questions). Currently, we do not know the taste receptors that sense nutrients in blood, including ATP, or the gustatory receptors that detect the attractive chemicals on skin, such as ammonium, and amino acids (see Outstanding Questions). Blood-sucking mosquitoes encode many members of large receptor families that are related to known taste receptors in Drosophila [79], and are therefore prime candidate taste receptors in mosquitoes. These include dozens of members of the gustatory receptor (GR) family, IRs, as well as smaller numbers of pickpockets (Ppks), TRP channels and opsins (Table 1) [46,104,110–113].

Outstanding questions.

Are there additional host-derived cues that female mosquitoes use to navigate to hosts that remain to be identified?

What are the taste receptors that mosquitoes employ to sense attractive compounds on skin, and which motivate them to continue to probe for a vein?

What are the mechanisms that endow a mosquito with the ability to find a vein and effectively insert its stylets?

What are the taste receptors that are employed to detect nutrients in blood and encourage a mosquito to continue to engorge?

What are the signals that mosquitoes use to establish that they are fully engorged and terminate blood feeding?

Does impairment in senses other than olfaction cause sensory compensation?

There are many additional questions that remain to be resolved (see Outstanding Questions). To help them navigate to a host, mosquitoes may be endowed with the ability to sense additional mammalian-derived cues that remain to be identified. This is plausible because mosquitoes are efficient at finding us, even when some of the known host cues are not available due the environmental conditions. Female mosquitoes that are compromised in olfaction exhibit increased sensitivity to conductive heat from sources near the surface temperature of humans [87]. Given these findings, sensory compensation might occur in mosquitoes when sensory modalities other than olfaction are compromised. It would be of particular interest to dissect the mechanisms underlying sensory compensation. After landing on a host, it not clear what mechanisms enable a mosquito to effectively find a vein as it probes our skin. Mechanosensation, thermosensation, or both might contribute to this behavior. Also of importance is the identification of the mechanisms that mosquitoes use to determine that they have consumed a sufficient quantity of blood, and terminate feeding. Finally, it will be important to leverage the insights gleaned from answering these and other questions to establish improved strategies to interfere with successful host-seeking and blood feeding, and reduce the incidence of mosquito-borne disease.

Glossary

Anthropophilic

Attracted to humans.

CpA

CO₂ responsive neuron that is associated with the capitate peg sensillum in each maxillary palp.

Flagellomere

Individual segments in the antenna.

Labellum

One of two bilaterally symmetrical organs at the end of the proboscis. It is the largest taste organ, although it can also detect a few volatile chemicals.

Labium

Sheath between the two labella that houses the stylets.

Labrum

One of six stylets that females use to penetrate skin that has sensory neurons to detect chemicals in blood and has as tube used to withdraw blood.

Maxillary palp

Senses a subset of olfactory cues, such as CO₂.

Ommatidium

One of the hundreds of repeat units of the compound eye.

Rhabdomere

Microvillar portion of photoreceptor cells where photoreception and phototransduction takes place.

Sensillum

In insects, a peripheral sensory organ comprised of one or a small group of sensory neurons housed in a cuticular structure. Some sensilla are bristle-like extending up from the body surface, while others a contained in pits and other structures that do not extend out above the body surface.

Thermal infrared (IR)

Wavelengths longer than visible and up to 1 μm. The thermal IR radiating from a ~34 °C (approximate surface body temperature) has a peak of ~9.4 μm, and extends from 3 – 30 μm. Thermal IR is absorbed by molecules and cause heating.