Mosquito

• 1. Introduction

  ›

  Mosquitoes kill more than one million people a year simply by transmitting diseases through a bite. The family behind that toll, the Culicidae, numbers around 3,600 species of small fly. Their name comes from Spanish and Portuguese, built from mosca, meaning fly, and the diminutive suffix -ito, so mosquito literally means little fly. A mosquito's body is slender and segmented, carried on one pair of wings and three pairs of long, hair-like legs. It also carries a specialized, elongated proboscis built for piercing and sucking. Every mosquito drinks nectar from flowers, but females of many species have adapted to also drink blood. The group first diversified during the Cretaceous period. Biologists who study evolution describe mosquitoes as micropredators, small animals that feed on the blood of larger ones without killing them outright. Medical parasitologists see the same insect differently, as a vector carrying protozoan parasites and bacterial or viral pathogens from one host to the next. How does an insect this small manage to be this dangerous, and how did it come to feed on blood in the first place?

• 2. Life Cycle

  ›

  Like all flies, a mosquito passes through four stages: egg, larva, pupa, and adult. The first three are largely aquatic, beginning with eggs laid in stagnant water. Those eggs hatch into larvae that feed, grow, and molt repeatedly until they become pupae, and the adult finally emerges from the mature pupa as it floats at the water's surface. An adult mosquito can live for as little as a week or as long as around a month, and some species survive winter as adults by entering diapause. The adult body is usually grey or black and slender, typically only 3 to 6 millimeters long, carried on a single pair of scaled wings and long, thin legs. Most mosquitoes are small, but the genus Toxorhynchites includes the largest, reaching up to 18 millimeters in length with a 24-millimeter wingspan, while Aedes mosquitoes are far smaller, with wingspans of only 2.8 to 4.4 millimeters. In hot weather, a mosquito can develop from egg to adult in as few as five days, though the process can also take a month. Within days of pupating, males gather in swarms at dawn or dusk, mating as females fly in; a female mates only once in her life, drawn in by the male's pheromones. In species that need blood for their eggs to develop, the female then finds a host, drinks a full blood meal, and rests for two or three days to digest it. She then lays her eggs and repeats the cycle. In the wild, females can live for up to three weeks, depending on temperature, humidity, their success at finding blood, and their luck at avoiding the hosts they feed on. Most mosquito eggs are laid in stagnant water, whether that means a pond, a marsh, a temporary puddle, a water-filled hole in a tree, or the water-trapping leaf axils of a bromeliad. Opifex fuscus manages to breed in salt-marshes, and Wyeomyia smithii breeds inside the pitchers of pitcher plants, where its larvae feed on insects that have drowned there. Egg-laying itself varies widely between species. Anopheles females fly over water, dapping down to place roughly cigar-shaped eggs, complete with floats along their sides, one at a time; over her lifetime, a female can lay 100 to 200 eggs. Aedes females instead drop their eggs singly onto damp mud near water, and those eggs hatch only once they are flooded. Culex, Culiseta, and Uranotaenia females lay their eggs in floating rafts, while Mansonia females lay theirs in arrays attached to the undersides of waterlily pads. Once hatched, a larva's head carries prominent mouth brushes for feeding, above a large, legless thorax and a segmented abdomen. It breathes through a siphon on its abdomen, forcing it to surface often, and spends most of its time feeding on algae, bacteria, and other microbes in the water's surface layer, diving down whenever it is disturbed. It develops through several molts before metamorphosing into a pupa. Aedes larvae, once past their earliest age, can withstand drying out entirely, entering diapause for months if their pond disappears. The pupa fuses its head and thorax into a single cephalothorax, with its abdomen curving beneath. Known as a tumbler, it swims by flipping that abdomen and, like the larva, must surface often to breathe, doing so through a pair of respiratory trumpets on the cephalothorax. It does not feed, spending most of its time hanging from the water's surface by those trumpets, diving down if alarmed and floating back up again if left undisturbed.

• 3. The Hunt For Blood

  ›

  Both male and female mosquitoes drink nectar, aphid honeydew, and plant juices, but in many species only the females also become hematophagous, feeding on blood. For some of those species, a blood meal is essential before eggs can develop at all; for others, it simply allows the female to lay more eggs. Blood supplies concentrated nutrients such as lipids, but its main job is protein for egg production. Anopheles and Aedes mosquitoes are anautogenous, meaning they cannot lay eggs without blood, while some Culex species are only partially autogenous, needing blood solely for their second clutch of eggs onward. A small number of species, in the genera Malaya, Topomyia, and Toxorhynchites, reproduce autogenously and never take blood at all. Among species that do feed on blood, hosts are not chosen at random. Culiseta melanura normally sucks the blood of passerine birds, but as mosquito numbers rise it turns to mammals, including horses and humans, and has driven epidemics of Eastern equine encephalitis virus in North America. Most species hunt at dawn or dusk and rest through the heat of the day, though the Asian tiger mosquito is known to fly and feed during daylight. Females track hosts by smell, detecting carbon dioxide and 1-octen-3-ol, a mushroom-scented compound in exhaled breath, and by sight. Culex quinquefasciatus is drawn most strongly to nonanal; sulcatone is another key attractant. Of the 72 types of odor receptor on a mosquito's antennae, at least 27 are tuned specifically to chemicals found in human sweat. In Aedes, the hunt unfolds in two phases: the mosquito flies about until it detects a host's odorants, then flies toward the source, following the rising concentration. Mosquitoes prefer people with type O blood, abundant skin bacteria, and high body heat, and they favor pregnant women; a person's attractiveness to mosquitoes is even a heritable trait. Malaria itself alters a person's scent: infected people produce more of the aldehydes heptanal, octanal, and nonanal, mimicking a healthy human odor and making themselves more attractive to the mosquitoes that could spread the infection further. A study comparing a domestic, human-biting form of Aedes aegypti with a form that bites animals found that the preference for human odor tracks with higher expression of an olfactory receptor called AaegOr4, tuned to sulcatone. The malaria mosquito Anopheles gambiae also carries OR4 genes activated by sulcatone, but they are not closely related to AaegOr4, suggesting the two species evolved a taste for human blood independently of each other.

• 4. Bite Mechanics And Blood Digestion

  ›

  A female mosquito's mouthparts are built specifically for piercing skin and drawing blood; males, which drink only sugary fluids, carry far less specialized mouthparts. The visible feeding structure is the proboscis, formed by the labium, a U-shaped sheath like a rain gutter that encloses a bundle of six piercing stylets: two mandibles, two maxillae, the hypopharynx, and the labrum. As the mosquito begins to bite, the labium bends back into a bow, staying pressed against the skin while it guides the other stylets downward. The labrum and maxillae, sharpened to extremely fine tips, saw back and forth into the skin using only a thousandth of the force a needle would need, so the insertion goes unfelt. Mosquito saliva carries enzymes that assist sugar feeding and antimicrobial agents that control bacterial growth in that sugar meal. Getting a blood meal, though, means overcoming the host's own defenses. Saliva proteins block the host's hemostasis system, reducing vascular constriction, blood clotting, and platelet aggregation so the blood keeps flowing. Other saliva proteins dampen the host's immune response: they lower angiogenesis and immunity, trigger inflammation, suppress the release of tumor necrosis factor from mast cells, suppress interleukin-2 and interferon-gamma production, suppress populations of T cells, reduce interferon-alpha and interferon-beta expression, raise natural killer T cell counts in the blood, and cut cytokine production. Once a blood meal is large enough, it triggers a hormonal cascade that starts egg development. The mosquito withdraws her proboscis and, as her gut fills, the stomach lining secretes a peritrophic membrane that walls the blood off from everything else inside the stomach. Like other insects living on dilute liquid diets, adult mosquitoes excrete surplus liquid even while still feeding, which lets a female pack in a full meal of nutrient solids. Digestion of that meal takes several days. The midgut produces protease enzymes, mainly trypsin assisted by aminopeptidase, which break the blood's proteins down into free amino acids. Those amino acids are then built into vitellogenin, the protein that becomes egg yolk.

• 5. Distribution And A Changing Climate

  ›

  Mosquitoes occur on every land region except Antarctica and a handful of islands, making their distribution close to cosmopolitan. Polar and subpolar climates cause them problems, though: the weather there can freeze, then warm suddenly in mid-winter, coaxing mosquitoes out of diapause inside their pupae before freezing again and killing them before the cycle can finish. Eggs from temperate-zone mosquitoes tolerate cold far better than eggs from species native to warmer regions, with many able to survive subzero temperatures, while some adults ride out winter sheltering inside buildings or hollow trees. In warm, humid tropical regions, some species stay active all year, while those in temperate and cold regions hibernate or enter diapause instead. Arctic and subarctic mosquitoes may be active for only a few weeks each year, once melt-water pools form on the permafrost, but during that window they emerge in huge numbers; a single swarm can draw up to 300 milliliters of blood a day from each animal in a caribou herd. Transmitting disease depends on favorable humidity, temperature, and precipitation, and El Niño already shifts the location and number of outbreaks across East Africa, Latin America, Southeast Asia, and India, the regions where mosquito-borne disease has long been most common. An emergence of such disease in Europe was recorded early in the 21st century, and climate models built on historic outbreak data now forecast further spread. One projection holds that by 2030, the climate of southern Great Britain will suit Anopheles mosquitoes carrying Plasmodium vivax malaria for two months of the year, and that by 2080 the same will hold true for southern Scotland. Dengue fever is moving north for the same reason. Its vector, the Asian tiger mosquito Aedes albopictus, had by 2023 established itself across southern Europe and as far north as much of northern France, Belgium, Holland, and even Kent and West London. In 2025, specimens of Culiseta annulata turned up in Kjósarhreppur, Iceland, a country that had until then been free of mosquitoes entirely.

• 6. Predators, Pollinators And Deep Time

  ›

  Mosquito larvae rank among the most common animals in ponds, making them an important food source for freshwater predators such as dragonfly and damselfly nymphs, whirligig beetles, and water striders. Vertebrate predators include fish like catfish and the mosquitofish, amphibians such as the spadefoot toad and the giant tree frog, freshwater turtles like the red-eared slider, and birds such as ducks. At the pond surface, emerging adults are eaten by predatory flies, including the Empididae and Dolichopodidae, and by spiders, while flying adults are caught by dragonflies and damselflies, by birds such as swifts and swallows, and by bats. Mosquitoes themselves are parasitised, in turn, by hydrachnid mites, ciliates such as Glaucoma, microsporidians such as Thelania, and fungi including species of Saprolegniaceae and Entomophthoraceae. Several flowering plant families, including the Asteraceae, Rosaceae, and Orchidaceae, are pollinated by mosquitoes visiting for sugar-rich nectar; the Platanthera orchids are specialized for pollination by Aedes species alone. Semiochemicals such as alcohols, aldehydes, ketones, and terpenes draw mosquitoes to these flowers, alongside visual cues including UV reflectivity, and the relationship has persisted since the Cretaceous period, when mosquitoes first visited and pollinated flowers. It is possible that feeding on plant sap is what originally exapted mosquitoes toward feeding on blood. Evolutionary biologists classify blood-feeding mosquitoes as micropredators rather than true parasites, borrowing Edward O. Wilson's description of such creatures as "predators that eat prey in units of less than one." Micropredation is one of six major evolutionarily stable strategies within parasitism, distinguished by leaving the host alive and able to reproduce, unlike parasitic castrators or parasitoids, and by drawing on multiple hosts rather than just one. A 2023 study identified Libanoculex intermedius, preserved in Lebanese amber from the Barremian age of the Early Cretaceous, around 125 million years ago, as the oldest known mosquito, though other researchers dispute the identification and consider it a chaoborid fly instead. Molecular clock estimates place the mosquito lineage's true origin even earlier, in the Jurassic, with its major diversification only picking up later, during the Cretaceous.

• 7. Naming Mosquitoes

  ›

  More than 3,700 mosquito species have been described across 112 genera, split into two subfamilies that carry very different diseases: Anophelinae transmits protozoal illnesses such as malaria, while Culicinae spreads viral diseases such as yellow fever and dengue fever. The family name Culicidae itself comes from the Latin culex, genitive culicis, meaning midge or gnat, and was first proposed by the German entomologist Johann Wilhelm Meigen in a seven-volume classification published between 1818 and 1838. Mosquito taxonomy took its next major leap in 1901, when the English entomologist Frederick Vincent Theobald published a five-volume monograph on the Culicidae. Theobald worked from specimens sent to the British Museum, Natural History, from around the world, gathered on the 1898 instruction of the Secretary of State for the Colonies, Joseph Chamberlain, who had written: "in view of the possible connection of Malaria with mosquitoes, it is desirable to obtain exact knowledge of the different species of mosquitoes and allied insects in the various tropical colonies. I will therefore ask you... to have collections made of the winged insects in the Colony which bite men or animals." Within Culicinae, researchers count 11 tribes, 108 genera, and 3,046 species, dwarfing the 3 genera and roughly 430 species that make up Anophelinae. A 2009 phylogenetic analysis by Kyanne Reidenbach and colleagues, drawing on nuclear DNA and the morphology of 26 species, confirmed that Anophelinae sits near the base of the mosquito family tree, though the deeper branches of that tree remain poorly resolved.

• 8. Disease, Defense And Mosquito Lore

  ›

  Mosquitoes act as vectors for bacteria, viruses, and protozoan parasites alike, and nearly 700 million people acquire a mosquito-borne illness every year. Yellow fever, chikungunya, and dengue fever, the most common viral diseases they spread, are carried mostly by Aedes aegypti. Malaria, caused by Plasmodium parasites, travels exclusively through female Anopheles mosquitoes, while lymphatic filariasis, the leading cause of elephantiasis, is spread by a much wider range of species. Control efforts range from eliminating breeding sites and installing window screens and mosquito nets to biological control using fungi, nematodes, fish, copepods, dragonfly nymphs and adults, and certain lizards and geckos as predators or parasites. Some programs also release sterile males or explore genetic approaches such as gene drives. Repellents applied to skin offer only short-term protection. DEET repels many mosquitoes, and the CDC also recommends picaridin, eucalyptus oil, and IR3535, while pyrethrum, drawn from Chrysanthemum cinerariifolium and Chrysanthemum coccineum, works as a plant-based option. A bite itself can trigger anything from a mild skin reaction to a genuine allergy, an overreaction to proteins in mosquito saliva, and species from Aedes aegypti to Anopheles stephensi can set it off; because salivary proteins cross-react between species, almost any mosquito can theoretically cause the reaction. Treatments include oral antihistamines such as diphenhydramine, topical corticosteroids such as hydrocortisone, and aqueous ammonia at a 3.6 percent concentration. Mosquitoes have crept into human storytelling for just as long as they have fed on human blood. Ancient Greek fables such as "The Elephant and the Mosquito" and "The Bull and the Mosquito," which both trace back to Mesopotamia, share a single moral: the large beast never even notices the small one. Among the Ostiak people of Siberia, a myth tells of a man-eating giant named Punegusse, killed by a hero but unwilling to stay dead, whose ashes become mosquitoes once the hero finally burns the corpse. The writer Lafcadio Hearn recorded that in Japan, mosquitoes were seen as reincarnations of the dead, condemned by past misdeeds to the condition of Jiki-ketsu-gaki, or blood-drinking pretas. In 1912, Winsor McCay released How a Mosquito Operates, one of animation's earliest films, depicting a giant mosquito tormenting a sleeping man in a style still praised as ahead of its time. Twelve Royal Navy ships have carried the name HMS Mosquito, and the de Havilland Mosquito, a high-speed aircraft built between 1940 and 1950, served in many wartime roles. Each July, the Russian city of Berezniki holds a mosquito festival running from the 17th to the 20th, crowned by a competition in which women stand for 20 minutes in shorts and undershirts, and whoever draws the most bites wins.