Mosquito
Mosquitoes, the Culicidae, are a family of small flies consisting of 3,600 species. The word mosquito is Spanish and Portuguese for little fly. Mosquitoes have a slender segmented body, one pair of wings, three pairs of long hair-like legs, and specialized, highly elongated, piercing-sucking mouthparts. All mosquitoes drink nectar from flowers; females of many species have adapted to also drink blood. The group diversified during the Cretaceous period. Evolutionary biologists view mosquitoes as micropredators, small animals that parasitise larger ones by drinking their blood without immediately killing them. Medical parasitologists view mosquitoes as vectors of disease, carrying protozoan parasites or bacterial or viral pathogens from one host to another.
The mosquito life cycle consists of four stages: egg, larva, pupa, and adult. Eggs are laid on the water surface; they hatch into motile larvae that feed on aquatic algae and organic material. These larvae are important food sources for many freshwater animals, such as dragonfly nymphs, many fish, and some birds. Adult females of many species have mouthparts adapted to pierce the skin of a host and feed on blood of a wide range of vertebrate hosts, and some invertebrates, primarily other arthropods. Some species only produce eggs after a blood meal.
The mosquito's saliva is transferred to the host during the bite, and can cause an itchy rash. In addition, blood-feeding species can ingest pathogens while biting, and transmit them to other hosts. Those species include vectors of parasitic diseases such as malaria and filariasis, and arboviral diseases such as yellow fever and dengue fever. By transmitting diseases, mosquitoes cause the deaths of over one million people each year.
Description and life cycle
Like all flies, mosquitoes go through four stages in their life cycles: egg, larva, pupa, and adult. The first three stages—egg, larva, and pupa—are largely aquatic, the eggs usually being laid in stagnant water. They hatch to become larvae, which feed, grow, and molt until they change into pupae. The adult mosquito emerges from the mature pupa as it floats at the water surface. Mosquitoes have adult lifespans ranging from as short as a week to around a month. Some species overwinter as adults in diapause.Adult
Mosquitoes have one pair of wings, with distinct scales on the surface. Their wings are long and narrow, while the legs are long and thin. The body, usually grey or black, is slender, and typically 3–6 mm long. When at rest, mosquitoes hold their first pair of legs outwards, whereas the somewhat similar Chironomid midges hold these legs forwards. Anopheles mosquitoes can fly for up to four hours continuously at, traveling up to in a night. Males beat their wings between 450 and 600 times per second, driven indirectly by muscles which vibrate the thorax. Mosquitoes are mainly small flies; the largest are in the genus Toxorhynchites, at up to in length and in wingspan. Those in the genus Aedes are much smaller, with a wingspan of.Mosquitoes can develop from egg to adult in hot weather in as few as five days, but it may take up to a month. At dawn or dusk, within days of pupating, males assemble in swarms, mating when females fly in. The female mates only once in her lifetime, attracted by the pheromones emitted by the male. In species that need blood for the eggs to develop, the female finds a host and drinks a full meal of blood. She then rests for two or three days to digest the meal and allow her eggs to develop. She is then ready to lay the eggs and repeat the cycle of feeding and laying. Females can live for up to three weeks in the wild, depending on temperature, humidity, their ability to obtain a blood meal, and avoiding being killed by their vertebrate hosts.
Eggs
The eggs of most mosquitoes are laid in stagnant water, which may be a pond, a marsh, a temporary puddle, a water-filled hole in a tree, or the water-trapping leaf axils of a bromeliad. Some lay near the water's edge while others attach their eggs to aquatic plants. A few, like Opifex fuscus, can breed in salt-marshes. Wyeomyia smithii breeds in the pitchers of pitcher plants, its larvae feeding on decaying insects that have drowned there.Oviposition, egg-laying, varies between species. Anopheles females fly over the water, touching down or dapping to place eggs on the surface one at a time; their eggs are roughly cigar-shaped and have floats down their sides. A female can lay 100–200 eggs in her lifetime. Aedes females drop their eggs singly, on damp mud or other surfaces near water; their eggs hatch only when they are flooded. Females in genera such as Culex, Culiseta, and Uranotaenia lay their eggs in floating rafts. Mansonia females in contrast lay their eggs in arrays, attached usually to the under-surfaces of waterlily pads.
Clutches of eggs of most mosquito species hatch simultaneously, but Aedes eggs in diapause hatch irregularly over an extended period.
Larva
The mosquito larva's head has prominent mouth brushes used for feeding, a large thorax with no legs, and a segmented abdomen. It breathes air through a siphon on its abdomen, so must come to the surface frequently. It spends most of its time feeding on algae, bacteria, and other microbes in the water's surface layer. It dives below the surface when disturbed. It swims either by propelling itself with its mouth brushes, or by jerkily wriggling its body. It develops through several stages, or instars, molting each time, after which it metamorphoses into a pupa. Aedes larvae, except when very young, can withstand drying; they go into diapause for several months if their pond dries out.Pupa
The head and thorax of the pupa are merged into a cephalothorax, with the abdomen curving around beneath it. The pupa or "tumbler" can swim actively by flipping its abdomen. Like the larva, the pupa of most species must come to the surface frequently to breathe, which they do through a pair of respiratory trumpets on their cephalothoraxes. They do not feed; they pass much of their time hanging from the surface of the water by their respiratory trumpets. If alarmed, they swim downwards by flipping their abdomens in much the same way as the larvae. If undisturbed, they soon float up again. The adult emerges from the pupa at the surface of the water and flies off.Feeding by adults
Diet
Both male and female mosquitoes feed on nectar, aphid honeydew, and plant juices, but in many species the females are also hematophagous ectoparasites. In some of those species, a blood meal is essential for egg production; in others, it just enables the female to lay more eggs. Both plant materials and blood are useful sources of energy in the form of sugars. Blood supplies more concentrated nutrients, such as lipids, but the main function of blood meals is to obtain proteins for egg production. Disease vector mosquitoes like Anopheles and Aedes are anautogenous, requiring blood to lay eggs. Some Culex species are partially autogenous, needing blood only for their second and subsequent clutches of eggs. The three genera of Malaya, Topomyia and Toxorhynchites, together comprising a small percentage of mosquitoes species, reproduce autogenously, never taking blood.Host animals
Blood-sucking mosquitoes favour particular host species, though they are less selective when food is short. Different mosquito species favour amphibians, reptiles including snakes, birds, and mammals. For example, Culiseta melanura sucks the blood of passerine birds, but as mosquito numbers rise they attack mammals including horses and humans, causing epidemics of Eastern equine encephalitis virus in North America. Loss of blood from many bites can add up to a large volume, occasionally causing the death of livestock as large as cattle and horses. Malaria-transmitting mosquitoes seek out caterpillars and feed on their haemolymph, impeding their development.Finding hosts
Most mosquito species are crepuscular, feeding at dawn or dusk, and resting in a cool place through the heat of the day. Some species, such as the Asian tiger mosquito, are known to fly and feed during daytime. Female mosquitoes hunt for hosts by smelling substances such as carbon dioxide and 1-octen-3-ol produced from the host, and through visual recognition. The semiochemical that most strongly attracts Culex quinquefasciatus is nonanal. Another attractant is sulcatone. A large part of the mosquito's sense of smell, or olfactory system, is devoted to sniffing out blood sources. Of 72 types of odor receptors on its antennae, at least 27 are tuned to detect chemicals found in perspiration. In Aedes, the search for a host takes place in two phases. First, the mosquito flies about until it detects a host's odorants; then it flies towards them, using the concentration of odorants as its guide. Mosquitoes prefer to feed on people with type O blood, an abundance of skin bacteria, and high body heat; they also favor pregnant women. Individuals' attractiveness to mosquitoes has a heritable, genetically controlled component.The multitude of characteristics in a host observed by the mosquito allows it to select a host to feed on. It activates odour and visual search behaviours that it otherwise would not use, when in presence of CO2. In terms of a mosquito's olfactory system, chemical analysis has revealed that people who are highly attractive to mosquitoes produce significantly more carboxylic acids. A human's unique body odour indicates that the target is actually a human host rather than some other living warm-blooded animal. Body odour, composed of volatile organic compounds emitted from the skin of humans, is the most important cue used by mosquitoes. Many of these volatile odor compounds are produced when skin-associated bacteria metabolize components of sweat and sebum, contributing to individual variation in human odour profiles. Variation in skin odour is caused by body weight, hormones, genetic factors, and metabolic or genetic disorders. Infections such as malaria can influence an individual's body odour. People infected by malaria produce relatively large amounts of Plasmodium-induced aldehydes in the skin, creating large cues for mosquitoes as it increases the attractiveness of an odour blend, imitating a "healthy" human odour. Infected individuals produce larger amounts of aldehydes heptanal, octanal, and nonanal. These compounds are detected by mosquito antennae. Thus, people infected with malaria are more prone to mosquito biting.
Contributing to a mosquito's ability to activate search behaviours, a mosquito's visual search system includes sensitivity to wavelengths from different colours. Mosquitoes are attracted to longer wavelengths, correlated to the colours of red and orange as seen by humans, and range through the spectrum of human skin tones. In addition, they have a strong attraction to dark, high-contrast objects, because of how longer wavelengths are perceived against a lighter-coloured background.
Different species of mosquitoes have evolved different methods of identifying target hosts. Study of a domestic form and an animal-biting form of the mosquito Aedes aegypti showed that the evolution of preference for human odour is linked to increases in the expression of the olfactory receptor AaegOr4. This recognises a compound present at high levels in human odour called sulcatone. However, the malaria mosquito Anopheles gambiae also has OR4 genes strongly activated by sulcatone, yet none of them are closely related to AaegOr4, suggesting that the two species have evolved to specialise in biting humans independently.