Many female mosquitoes actively seek potential hosts in the semi-darkness of the evening or the darkness of night, and most mosquitoes depend upon olfactory, rather than visual, information to locate potential hosts. This is true, also, for diurnal females. While visual cues are quite important for long-range detection of potential hosts, virtually all female mosquitoes depend upon olfactory cues for locating hosts in short-ranges. This is just one example of the significance of olfactory information in the biology of insects, and we might consider this in a little more detail.

Carbon dioxide (CO₂) is a key component of host odors. CO₂ stimulates two aspects of host seeking behavior. CO₂ activates mosquitoes, that is, it induces take off and helps maintain flight behavior. For a second role, CO₂ also helps female mosquitoes orient themselves with respect to a potential host. While most olfactory receptors are located on the antennae of insects, the mosquito CO₂ receptors are located on the labial palps.

The olfactory responses to CO₂ are integrated with other olfactory information to produce the host seeking behaviors of female mosquitoes. Recognizing that information from the palps is integrated with information from the antennae, we note the integration of information from the peripheral system is integrated in the central nervous system. We also note CO₂ is a necessary component of host odors. In the absence of CO₂, other odor components can not stimulate host orientation behaviors.

Lactic acid (LA) is probably the primary host odor to which female mosquitoes orient themselves during flight. Lactic acid is a ubiquitous product of energy metabolism. While high levels of LA are produced under anaerobic conditions, low levels are also produced under aerobic conditions. LA evaporates from the skin of humans and other mammals. Mosquitoes have LA receptors on the sensilla basiconica, or grooved pegs, on their antennae. The LA receptors are specific to lactic acid, with very little activity in response to other small, organic acids. In humans, the LA flux rate is about 1.5 x 10^-12 moles/second, which is well within the range of sensitivity of the mosquito LA receptor. There are actually two LA receptors in the antennae of female mosquitoes. One of the receptors is stimulatory and the other is inhibitory, that is, there is increased frequency of nerve impulses in one and a reduced frequency in the other. The biological significance of the inhibitory lactic acid receptor is not well understood.

The physiology of the LA receptor is of some interest because the expression of the LA receptor sensitivity coincides with host-seeking behavior. Newly emerged mosquitoes, and recently blood-fed mosquitoes do not seek potential hosts for blood meals. In both situations, the lack of host seeking behavior is related to activity of the LA receptors. Newly emerged females do not respond to LA, however, as the females develop, there is a progressive increase in LA sensitivity. By the time females are of host-seeking age, usually 24 or more hours after emergence, the LA receptors are sensitive to very low concentrations of LA. It was once though the sensitivity of the LA receptors was tied to juvenile hormone levels, because LA sensitivity is correlated to pre-vitellogenic ovarian development. But the correlation is not physiologically linked: females whose corpora allata were removed at adult emergence undergo a normal progression of LA sensitivity.

Information about LA and CO is integrated with other information, as well. Quite sensitive thermoreceptors are present in the sensilla coeloconica, and efforts to generate an artificial host odor reveal that temperature is one component of host odor used to orient with respect to potential hosts.

We considered two aspects of sex pheromones in insects. First, we saw that sex pheromone biosynthesis can be understood as slight variations on fatty acid metabolism in many insect species. Second, we considered the physiological regulation of sex pheromone biosynthesis, seeing again that insects have evolved a variety of mechanisms to achieve a single end, in this case a timely release of sex pheromones. Sensing and responding to sex pheromones lies on other side of biosynthesizing and releasing airborne sex pheromones. Let us look at the olfactory reception of sex pheromones in male moths.

The males of many moth species are activated to sustained upwind flight by airborne sex pheromones released by females of the same species. Males can detect sex pheromones released from females over an impressive distance, from a few meters to a few thousand meters. The physiological mechanism of sex pheromone reception is not much different from the mechanisms of detecting host odors, although we have quite a bit more information from male moths.

The physiology of detecting a sex pheromone probably begins with releasing the hormone into the air. Pheromone molecules are deposited from the air onto the surface of antennal hairs. The molecules sort of flow through pores in the hairs and make their way into a gel that surrounds the dendrites of antennal nerves. The gel is mostly protein, specifically proteins comprising the pheromone binding proteins of male moths. Glen Prestwich and some of his colleagues have tried to understand the molecular biology of these pheromone binding proteins. Finally, pheromone molecule, coupled to its pheromone binding protein, interacts with cell surface receptor sites located on the outer surface of the pheromone receptor neuron. Again, a neuron is just a nerve cell, and in antennae these nerve cells are arranged with the dendrite projecting within the shaft of the hair, cell bodies at the base of the antennae, and axons projecting from the cell body to the brain.

The receptors for pheromones are conceptually similar to all receptor sites. They are transmembrane proteins with amazing specificity for particular pheromone molecules. Pheromone receptors interact in a physiologically relevant way with one pheromone, and only one pheromone. To be just a little more precise, the receptors interact with one and only one component of a pheromone. The pheromone-receptor interaction generates a depolarization of the neuron membrane, and if the depolarization is strong enough an action potential flows from the cell body at the base of the antenna to the brain. By strong enough depolarization, we are referring to multiple interactions between several pheromone molecules and their receptors.

The neurophysiology and behavioral physiology of pheromone reception is a very important field of inquiry, and many people are attracted to the work. Leaders in the field include John Hildebrand who works on males of the sphinx moth, Manduca sexta. John moved from Columbia University to the University of Arizona in the 1980's, where he successfully lured Reg Chapman from UC Berkeley. Tom Baker was a student of Wendell Roelofs at Cornell and is now head of the Department of Entomology at Iowa State, is also a leader in understanding the integration of pheromone signals. Tom moved from UC Riverside to Iowa. Thanks to the work of Glen Prestwich, we have information on the interaction of pheromone components with their specific receptors. Many people have published work on the electrophysiological patterns of action potentials that follow from interactions with pheromones. The contemporary questions revolve around how blends of pheromones are detected and integrated.

The main idea comes from a beautiful combination of morphological and electrophysiological research. Male, but not female, moths have a structure at the base of their antennae known as the macroglomerular complex (MGC). The MGC receives information exclusively from sex pheromone receptor neurons. Thousands of sex pheromone receptor neurons are present on the antennae of male moths. Action potentials from these neurons are directed, by way of the antennal nerve, to the MGC. The breakthrough in understanding the detection and integration of pheromone blends came with appreciating the structure of the MGC. The MGC is subdivided into about 7 subcompartments, each of which receives olfactory information from exactly one component of a pheromone blend.

Let us consider a two-component pheromone system, taken from a recent article by Tom Baker and one of his post-docs. While pheromone molecules can and will be deposited on all antennal sinsillae, molecules of each each component will bind to specific binding proteins and interact with specific receptors tuned to a specific molecule. The pheromone-receptor interactions generate depolarizations, which can lead to action potentials. The action potentials run down to the appropriate subcompartment of the MGC. Two possibilities can follow from here. In one scenario, the axons form synaptic connections with interneurons which serve as integrating interneurons. These can integrate signals about relative intensities of each of the two components, and project an integrated signal deeper into the central nervous system, eventually to the protocerebrum. Alternatively, interneurons specific to each receptor neuron can project deeper into the central nervous system, where integration of the information can take place.

This basic scheme works for multi-component pheromones. The frontier of this research field lies in understanding how multi-component pheromones are integrated into a single piece of information that will direct the behaviors of male moths.

Again, this is a contemporary and very active area of insect science. We can complete our discussion of this area by asking how to determine a direct link between pheromone receptor neurons and specific subcompartments of the MGC. The key lies in modifying some neuron staining procedures from the early days of neural anatomy. One procedure is called "back-filling". A friend of mine at UC Berkeley used this to work out the neural anatomy of selected nerves in a cricket. The goal is to place a nerve fiber into a solution of cobalt dye, then let the dye move through the axons, and follow the path. The work is done in 3-dimensional preparations, and it is much more difficult than it sounds. The technique has been modified to that a sensillum is cut, then placed in an electrode filled with a cobalt solution. The sensillum was stimulated with a pheromone component, and the cobalt solution moved down the sensillum and into the MGC. In many of these preparations, we see specific staining in separate subcompartments of the MGC.