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Mosquito-borne diseases such as malaria, Zika virus, and dengue virus are a menace to the human population. Although many mosquito species are not attracted to humans and do not feed on blood, human-biting female mosquitoes are strongly attracted to people and use chemosensory cues to identify a suitable host for a blood meal. Mosquitoes need blood components to reproduce, rendering them excellent vectors for blood-borne diseases. The three genera (Culex,Anopheles, andAedes) responsible for most of these diseases find hosts by using their peripheral sensory organs. These organs include the antennae, maxillary palps, and proboscis. All three contain diverse populations of highly sensitive neurons that express sensory receptors that can detect odorants, temperature, chemicals, and tastants. Although these organs are essential to the host-seeking behavior that results in biting, their small size and thick outer cuticle can hinder typical histochemical analyses. Here, we briefly review the role the peripheral sensory organs play in mosquito behavior. Then, we introduce how to investigate their gene expression profiles using immunohistochemical and RNA in situ approaches for both whole-mount and frozen-section preparations.

Immunohistochemistry analysis of mosquitoes is complicated by the outer cuticle that prevents reagents from penetrating peripheral tissues. This protocol incorporates a cryosectioning method that provides a higher resolution of the internal architecture of mosquito peripheral sensory tissues and enables the visualization of protein expression. This eliminates the need for enzymatic steps to digest the outer cuticle that encases these tissues. This protocol can also be adapted for other tissues, such as the brain and the legs, as chitin exoskeleton thickness does not affect antibody penetration once the sample is sectioned.

While most animals thermotax only to regulate their temperature, female mosquitoes are attracted to human body heat during pursuit of a blood meal. Here we elucidate the basic rules of Aedes aegypti thermotaxis and test the function of candidate thermoreceptors in this important behavior. We show that host-seeking mosquitoes are maximally attracted to thermal stimuli approximating host body temperatures, seeking relative warmth while avoiding both relative cool and stimuli exceeding host body temperature. We found that the cation channel TRPA1, in addition to playing a conserved role in thermoregulation and chemosensation, is required for this specialized host-selective thermotaxis in mosquitoes. During host-seeking, AaegTRPA1-/-mutants failed to avoid stimuli exceeding host temperature, and were unable to discriminate between host-temperature and high-temperature stimuli. TRPA1-dependent tuning of thermotaxis is likely critical for mosquitoes host-seeking in a complex thermal environment in which humans are warmer than ambient air, but cooler than surrounding sun-warmed surfaces. DOI: http://dx.doi.org/10.7554/eLife.11750.001, eLife digest Temperature can vary considerably in an environment. Living organisms have evolved sensory systems to detect and avoid excessive heat or cold: a behavior that is termed ‘thermotaxis’. In rare cases, animals use this ability to locate food sources in their environment. One example of such an adaptation is the female mosquito of the species Aedes aegypti. When a mosquito needs blood to produce her eggs, she becomes attracted to the body heat of warm-blooded hosts. But the range of temperatures that these mosquitoes prefer and the genes required for this behavior had not been been defined. Now, Corfas and Vosshall have found that female Aedes aegypti are highly sensitive to differences in temperature, and are capable of heat-seeking in a range of environmental temperatures. Furthermore, by seeking out things that are warmer than their surroundings, while avoiding those that are cooler or much hotter than their host’s body temperatures, these mosquitoes tune their thermotaxis toward targets that resemble a human to feed upon. Corfas and Vosshall also discovered that a protein called TRPA1 is required for this tuning of Aedes aegypti’s heat-seeking behavior. This protein is known to allow insects to detect chemical signals and regulate their own temperature, but it was not previously known that this protein was involved in mosquito thermotaxis. Mutant mosquitoes without the gene for TRPA1 failed to avoid high temperatures, which meant that they could no longer tell the difference between an overly hot target and a warm one that resembled their hosts. Following on from this work, the next challenge will be to characterize all the genes, sensory organs, and neural circuits that drive mosquito heat-seeking behavior. These findings may in the future inform the design of the next generation of repellents and traps for the control of mosquito-borne diseases, such as dengue and yellow fever. DOI: http://dx.doi.org/10.7554/eLife.11750.002