Mosquitoes are among the most studied creatures on Earth, and their role in disease transmission and nuisance biting makes them worthy of such intense scrutiny. With over 3,500 species found across the globe, excluding Antarctica, mosquitoes exhibit fascinating and complex biology. While a small fraction of these species can transmit pathogens that cause human diseases, a substantial body of research has helped to understand mosquito-borne disease transmission and control methods. Beyond their role in disease, mosquitoes have incredible and unusual behavioral, anatomical, and physiological traits. Mosquitoes, like all animals, are driven by a fundamental set of needs. The patterns of behavior exhibited by mosquitoes are complex and driven by sensory systems that are adapted to the environments they inhabit and the ecological niches they exploit. Their life cycle, anatomy, physiology and behaviour make these creatures both an extraordinary object of study and crucial to human culture.
Mosquito Life Cycle and Habitat
Mosquitoes are holometabolous insects, undergoing complete metamorphosis from egg to larva, pupa, and finally, adult. The juvenile stages are aquatic, requiring females to lay eggs in or near water. The female’s choice of location for laying eggs is a critical factor for determining the offspring’s survival in the immature stages (eggs, larvae and pupae). These eggs are broadly classified into rapid-hatch eggs, laid directly on the water surface, and delayed-hatch eggs, laid adjacent to water or on moist soil. Delayed-hatch eggs can survive for extended periods, resisting desiccation and extreme temperatures.
Dictated by the availability of aquatic habitats, mosquitoes are found in a wide range of environments from the tropics to the Arctic circle. After hatching, the young mosquito larva harvests nutrients from the water. Larvae feed on decomposing organic matter, bacteria, and algae. After progressing through four larval stages, or instars, the mosquito is ready to pupate. The mosquito pupa is fully developed, it will rise to the surface of the water one last time. After resting briefly on the surface of the water, the adult mosquito must take a short shaky flight to find a refuge, typically in surrounding vegetation, where it rests to allow its newly pumped-up wings to dry and properly harden. The emergence patterns of different mosquito species vary.
Adult Mosquito Diet: Sugar vs. Blood
Once emerged as adults, both male and female mosquitoes feed extensively on sources of plant-based sugars, such as those found in nectar and fruit juices. Contrary to popular belief, mosquitoes don't feed solely on blood. In fact, only female mosquitoes feed on blood in order to reproduce. Mosquitoes are primarily vegetarians, and they only occasionally indulge in carnivorous cravings.
The Importance of Sugar
Newly emerged mosquitoes cannot survive for long without taking in a sugar meal and it is the consumpti… Mosquitoes enjoy feeding on plant nectar like juices from flowers and fruits, plant sap, honeydew and other fluids from plants. Sugar is a cornerstone of adult mosquito life. The adult mosquito's diet becomes broader rather than narrow. Like butterflies, bees, and many other insects, all male and female mosquitoes have a nutritional need for sugar, which can be supplied by the nectar of flowering plants. This sugary mosquito food gives them the energy they need to fly, reproduce and sustain themselves. In any case, flying between resources and in search of shelter requires a great deal of energy and adult mosquitoes need to feed in order to gather the energy that flight requires.
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The Female's Need for Blood
Only the adult female mosquito seeks a blood meal, which has the requisite nutrients to support egg development. Female mosquitoes take blood meals to get the protein and iron needed to produce eggs that will properly hatch. When the female does feed on blood it does not necessarily select humans as the source of this blood, and indeed humans are rarely a mosquito’s main blood source. Other mammals, birds or reptiles will also satisfy the palate of some mosquito species. It is when the mosquito selects a human for its blood meal that the insect earns its reputation as an annoyance and harbinger of disease.
To successfully reproduce, female mosquitoes require a specific protein found in the blood of mammals and humans to help generate their eggs. Female mosquitoes also feed on nectar, which is a large part of their diet. Female mosquitoes only feed on blood for the purpose of reproduction, while male mosquitoes don’t feed on blood at all!
Male Mosquito Diet
Male mosquitoes eat plant nectar, and unlike their female counterparts, they do not feed on blood. A male mosquito's diet consists of flower nectar, fruit juice, and other sweet plant secretions. In fact, male mosquitoes do not have the mouthparts necessary to pierce the skin and feed on blood like females do. Male mosquitoes do not require blood meals. Instead, they rely on sugar from plant nectar. They use their proboscis to retrieve the nectar from plants.
Host Preference
A female mosquito ready to lay eggs doesn't need to feed exclusively on human blood. Birds, like crows, jays, robins, and sparrows, must also fend off biting mosquitoes. The same goes for waterfowl, such as ducks, geese, and herons. Outside of the bird kingdom, small mammals such as raccoons have a place in the mosquito's diet, as do some snakes, lizards, frogs, and fish. Not all species of mosquitoes prefer to feed on humans, and some species prefer birds or reptiles. Still, conditions and competition may force them to feed on other hosts.
Mosquitoes are alerted to the presence of a nearby food source by a number of factors, including movement, smell, the source's carbon dioxide (CO2) emissions, and much more. The average human being exhales more CO2 than, say, a squirrel and makes a more significant blip on the hungry female mosquito's radar and an easier overall target. The human body also features odor‐producing chemicals that, while unpleasant or undetectable to us, make the female mosquito's mouth water.
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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.
The Exception: Toxorhynchites
It might also be noted that while the majority of mosquito species follow this general dietary pattern, there is a notable exception in the genus Toxorhynchites. This group includes the world’s largest mosquito, Toxorhynchites speciosus, whose wingspan may be four times larger than most other species. The idea of a giant mosquito may seem somewhat terrifying; however, the 90 or so species in this genus are particularly noteworthy because the adults do not take any blood meals at all and feed exclusively on plant sugars. This is on account of the predatory nature of the larvae, which kill live prey-including other mosquito larvae-in order to acquire sufficient reserves of protein for egg development when adult. So aggressive are the predatory (and even cannibalistic) appetites of Toxorhynchites amboinensis larvae that this species has been cultivated and distributed as an effective form of mosquito biocontrol, especially for the control of species of mosquitoes that typically breed on shipping containers and which are associated with the transmission of dengue and Zika virus (Collins and Blackwell, 2000).
Mosquito Sensory Systems and Host Seeking
The organs that the mosquito uses to inform the searches are all remarkable in their own way. Stimuli that are visual, chemical and aural all play important roles at some point in the life of the adult mosquito. The compound eyes of night-active mosquitoes are amongst the most sensitive to low light levels in the animal kingdom and the structure of the nocturnal mosquito eye is uniquely adapted to maximize this sensitivity. The mosquito antenna is another remarkable organ. It can detect odours that distinguish potential hosts, sugar meal sources and egg-laying (oviposition) sites.
Female mosquitoes hunt for hosts by smelling substances such as carbon dioxide (CO2) and 1-octen-3-ol (mushroom alcohol, found in exhaled breath) 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. 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 (as the presence of CO2 shows). Body odour, composed of volatile organic compounds emitted from the skin of humans, is the most important cue used by mosquitoes. 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. 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.
The Mechanics of Blood Feeding
Female mosquito mouthparts are highly adapted to piercing skin and sucking blood. Externally, the most obvious feeding structure of the mosquito is the proboscis, composed of the labium, U-shaped in section like a rain gutter, which sheaths a bundle (fascicle) of six piercing mouthparts or stylets. These are two mandibles, two maxillae, the hypopharynx, and the labrum. The labium bends back into a bow when the mosquito begins to bite, staying in contact with the skin and guiding the stylets downwards.
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For a mosquito to obtain a blood meal, it must circumvent its vertebrate host's physiological responses. Females of many blood-feeding species need a blood meal to begin the process of egg development. A sufficiently large blood meal triggers a hormonal cascade that leads to egg development.[63] Upon completion of feeding, the mosquito withdraws her proboscis, and as the gut fills up, the stomach lining secretes a peritrophic membrane that surrounds the blood. This keeps the blood separate from anything else in the stomach. Like many Hemiptera that survive on dilute liquid diets, many adult mosquitoes excrete surplus liquid even when feeding. This permits females to accumulate a full meal of nutrient solids. The blood meal is digested over a period of several days.[64] Once blood is in the stomach, the midgut synthesizes protease enzymes, primarily trypsin assisted by aminopeptidase, that hydrolyze the blood proteins into free amino acids.
Mosquitoes as Disease Vectors
Mosquito blood feeding habits allow disease pathogens to be transferred among people as well as animals. When a mosquito takes a blood meal, that mosquito injects its saliva into the host. If that mosquito is a carrier then the saliva likely contains pathogens obtained from an infected host the mosquito previously fed upon. If the same female mosquito feeds on another host, disease pathogens are introduced into a new, previously uninfected host. As a result, bites can spread West Nile virus and eastern equine encephalitis in addition to other diseases.
Mosquitoes in the Ecosystem
Mosquito larvae are among the commonest animals in ponds, and they form an important food source for freshwater predators. Among the many aquatic insects that catch mosquito larvae are dragonfly and damselfly nymphs, whirligig beetles, and water striders. Emerging adults are consumed at the pond surface by predatory flies including Empididae and Dolichopodidae, and by spiders. Several flowers including members of the Asteraceae, Rosaceae and Orchidaceae are pollinated by mosquitoes, which visit to obtain sugar-rich nectar. A few plant associations are specialized for mosquito pollination, such as the Platanthera orchids which are pollinated by Aedes species.[83] They are attracted to flowers by a range of semiochemicals such as alcohols, aldehydes, ketones, and terpenes. They may also use visual cues including UV reflectivity.[84] Mosquitoes have visited and pollinated flowers since the Cretaceous period.
Mosquito Control
To keep mosquitoes out of homes, ensure all screens, windows, and doors are in good repair. In yards, empty or dispose of anything that collects water, regardless of how small and unimportant the item may seem. Contact an Orkin Pro for help with identifying and reducing issues with mosquitoes feeding around homes and yards.