Mosquitoes are small flies from the family Culcidae. They resemble flies from the family Tipulidae (crane flies) and Chironomidae (chironomid flies), but these do not feed on blood. There are more than 3,500 species of mosquitoes recognized around the world, with around 175 of them being found in the United States. Due to their wide range of species, mosquitoes also feed on a lot of various hosts, from vertebrates, including mammals, birds, reptiles, amphibians, to even some kinds of fish. Although the loss of blood is not so important for the host, what makes mosquitoes a nuisance is the fact that they cause a serious rash after feeding and are able to pass on various disease through their saliva, leading to infections such as malaria, yellow fever, West Nile virus, dengue fever and the Zika virus. Due to these actions, he mosquito is considered a vector of diseases and one of the deadliest animal in the world.
Description and distribution
Mosquitoes have long, slender bodies, made out of 3 segments: a head, a thorax, and an abdomen. The head presents a pair of eyes and a pair of long, segmented antennae, and it is used to feed and to receive sensory information. The compound eyes are separated from one another, one on each side of the head. The antennae detect odours, helping direct mosquitoes to different hosts and to different breeding sites. In this way, mosquitoes can “smell” human breath, by detecting the carbon dioxide we release when we exhale. They can also “smell” our sweat; this is made out more than 340 chemical odours, some of which smell like “dinner” to mosquitoes. In males, antennae are “bushier” and also contain auditory receptors, which detect characteristic whines of females. The head also presents what has to be the most defining characteristic of mosquitoes: the forward-projecting proboscis (the “stinger”), used to feed. On each side of the proboscis, there is one maxillary palp. In males, the maxillary palps are longer than the proboscis, while in females, they are shorter; this difference is especially obvious in blood-sucking species, where only the females feed on blood. Furthermore, in females, at the very tip of the proboscis, there are 2 taste receptors, which are believed to play a role in helping the mosquito find a “suitable” place to feed: a place with easily accessible blood vessels, the presence of which is thought to be detected through the warmth of the skin. However, no more about this process is currently known.
The thorax is specialised in locomotion, with 3 pairs of legs and 1 pair of wings being attached to it. Mosquitoes are good fliers, although they are unable to fly very fast. Individuals of the species Anopheles can fly continuously for 4 hours and travel for over 12km in only one night. Individuals of most species, however, usually remain within several hundred feet of where they hatched.
The abdomen is specialised in food digestion and egg development. The length of an adult is typically between 3-6mm. The smallest known species of mosquito measures around 2mm, while the largest around 19mm. Typical weight is of around 5mg. Adult body size will ultimately also vary according to the density of the larvae population and food supply within the water.
Mosquitoes have been present on Earth since the Jurassic period, and, today, are a cosmopolitan species. They can be found on every continent, with the exception of Antarctica and a couple of polar or subpolar islands, such as Iceland. In warm, humid environments (such as tropical regions), most species of mosquitoes are active during the course of the entire year, while in temperate and cold regions, adults frequently enter diapause, hibernating during the cold seasons. Otherwise, adults of some species can survive winter by finding shelter in microhabitats, such as buildings or hollow trees. Generally, eggs of mosquitoes from temperate zones are more tolerant of the cold than those of mosquitoes from tropical zones, with some even able to tolerate sub-zero temperatures.
Most mosquito species are crepuscular. This means that they are active only at sunset or sunrise, resting in a cool place during daytime. Some species, however, such as the Asian tiger mosquito (Aedes albopictus) fly and feed during the day.
Breeding and dietary information
As is the case with most flies, mosquitoes undergo 4 developmental stages: egg, larva, pupa, and adult. Female mosquitoes generally lay eggs in stagnant water; some species attach them to aquatic plants, while others only need them to be near a water source, and most often lay them at the very edge of the water. Some species may breed in lakes, others in marshes, while some will do even with temporary puddles; some prefer salt water, others fresh water, and others still are able to adapt to different levels of salinity. Each female chooses the best spot, according to species, habitat, environmental conditions, etc. There are also species that are adapted to breeding in phytotelmata: natural reservoirs. These lay eggs anywhere that water accumulates in nature (in hollow tree trunks, cupped leaves, and even artificial water containers), and it is important to note that these species often carry some of the most dangerous diseases. It is also important to note that these are the species that are most likely to come in contact with humans. Mosquitoes that breed and feed mainly in remote areas, such as wetlands and marshes, even if infected with a disease, are very unlikely to come across humans to infect in turn.
The first 3 stages of a mosquito’s life are aquatic; depending mainly on the species and the environmental conditions (most important of which is temperature), as the time period needed to reach adulthood will vary. Some species, in ideal conditions, can develop completely in as little as 5 days, but others, even in ideal conditions, can take up to 40 days. Mosquitoes that live in regions where some seasons are freezing or waterless spend part of the year in diapause, resuming development once the environmental conditions once again suit their needs. The eggs hatch to become larvae, which then change in pupae. The larva has a well-developed head, complete with mouthparts that are used to feed, as well as a large thorax (with no legs), and a segmented abdomen. It breathes through spiracles, which means that it frequently swims to the surface of the water, diving below only when it is disturbed. It swims through propulsion (using its mouthparts) or through jerky movements of its entire body, which has led to mosquito larvae often being called “wigglers” or “wrigglers”. Larvae have 4 stages of development (instars), in the last of which metamorphosing into pupae. The pupa is comma-shaped, with the head and the thorax merged into a cephalothorax and the abdomen curving around it underneath. Like the larva, the pupa frequently comes to the surface to breathe, but unlike the larva, it can actively swim, by flipping its abdomen, which has led to pupae being called “tumblers”. Once mature, the pupa rises to the surface of the water, the dorsal side of its cephalothorax splits, and so the adult mosquito emerges.
Adult mosquitoes are able to mate within just a couple of days after reaching adulthood. In most species, males will form swarms, and females will fly towards and into these swarms to mate. The females will then feed and rest for 2 -3 days, allowing the blood to be digested and the eggs to develop. The blood plays an important role in the process, seeing as it contains concentrated nutrients (such as lipids and proteins) that helps the eggs develop. The female can lay up to 300 eggs at a time, and once this is done, she resumes both mating and feeding. The cycle repeats itself until the female dies.
Total lifespan will depend on the species, sex, and environmental conditions, as well as on the ability of each individual mosquito to successfully obtain meals and defend itself from predators. Most bloodsucking mosquitoes can live as little as a week and as long as several months. Males typically live less, around 5 – 7 days, while females are more likely to live around 2 weeks.
When it comes to their feeding habits, it has been noted that larvae feed on algae, bacteria, and other microbes in the microlayer surface of the water, while pupae do not feed.
Males generally feed on reserves of sugar, such as nectar or plant juices, while females have 2 very different food sources: like males, they feed on sugar; and they also feed on blood. The first source of food gives them energy, while the second protein, which is especially important (as previously mentioned) in egg development. Thus, only females are acting as ectoparasites, as the blood is obtained from a host, by piercing the skin using tube-like mouthparts. Most species of mosquitoes are selective feeders, meaning that they specialise in hosts; some feed exclusively on monkeys, others on certain species of birds, etc. However, when competition is fierce, they often relax this selectivity.
Mosquitoes as pests
There is evidence to suggest that, when it comes to blood, humans are not mosquitoes’ first choice; most would rather feed from horses, cattle, or birds. However, when it does come to humans, mosquitoes prefer:
- those with type O blood;
- those with heavy breathing;
- those with a lot of body heat;
- those with a lot of skin bacteria.
Before, as well as during, feeding, mosquitoes inject saliva into the body of the host. This saliva serves as an anticoagulant, which helps prevent blood clots from clogging the mosquito’s proboscis. It is also the way in which mosquitoes transmit diseases.
Interestingly, because hosts may sometimes be difficult to find, when they have the opportunity, mosquitoes feed on as much blood as possible. A single mosquito’s abdomen is able to hold up to 3 times its weight in blood. However, digesting such a large amount of blood requires time and energy, which in turn requires sugar. This is the reason why mosquitoes have a digestive system that can store both food types (sugar and blood), for longer periods of time, giving access to either whenever they are needed.
Although the subtracted quantity of blood is hardly ever of great importance to the host, the saliva of the mosquito often causes irritation, leaving behind rashes and wheals (bumps, caused by histamines gathering to fight off the protein left from the bite). More dangerously, many species of mosquitoes carry many diseases, which, while not affecting them, are very easily transmitted as they feed. These range from serious infections such as malaria, west Nile virus, filariasis, yellow fever, dengue fever, etc., to arboviruses, such as Francesella tularensis, which causes tularemia. As opposed to popular belief, however, mosquitoes do not transmit HIV; the virus that causes it is actually digested in the mosquitoes’ stomachs, broken down without being passed on. There are 3 species of mosquitoes that are recognized to be primarily responsible when it comes to infectious diseases:
- the species Anopheles, the only known species that carries malaria; it also carries filariasis (elephantiasis) and encephalitis;
- the species Culex, which carries encephalitis, filariasis, and the West Nile Virus;
- and the species Aedes, which carries encephalitis, yellow fever, and dengue.
Overall, various species of mosquitoes are estimated to transmit various types of diseases to more than 700 people annually (mostly in Africa, South America, Central America, and much of Asia), with millions of resultant deaths. Not surprisingly, this has caused the Culcidae family to be rendered as the deadliest animal family in the world. Diseases can be contracted through biting already-infested humans, or through parasites, which enter the mosquitoes’ bodies, treating them as hosts themselves.
It is also important to note that, even among species that do transmit diseases:
- there is not a single disease (or set of diseases) that is transmitted by a single species;
- not all diseases are transmitted under the same circumstances.
Even if the species of mosquito you are normally exposed to are not dangerous as far as disease transmission is concerned, there is still no bigger nuisance than having a mosquito in your house or not being able to enjoy being outside, especially at night, without having to defend yourself from these small, but annoying, insects. Preventing them from taking over your yard and managing to get rid of a possible infestation is very important. You can find more details on these subjects in out other articles about mosquitoes, where we present ways in which you can ‘Prevent infestation with Mosquitoes’, but also ‘How to get rid of Mosquitoes’, with both natural and chemical solutions.
The only upside to having mosquitoes close to your home is that they represent a reliable food source for other animals, such as birds, bats, frogs, as well as other insects, such as dragonflies. Depending on what other creatures can be found on your property, you have to take into consideration that preventing or eliminating mosquitoes can also affect them as well.
