Introduction to mosquitoes

DR. VICTOR A.

BRUGMAN: This video will give an introduction to the biology of mosquitoes, what they are, where they are found, and why these small insects continue to cause such a huge problem to humans worldwide. Understanding the biology of one of humanity’s most important pests is the first step towards implementing effective control My name is Victor Brugman, and I research the behaviour of mosquitoes and other important biting insects. The aim of this section is to provide an introduction to the biology and disease transmission mechanisms of mosquitoes. This will involve defining the characteristics of mosquitoes to include the taxonomy, important genera, the mosquito lifecycle, their morphology, their feeding behaviour, nuisance biting and disease transmission, and finally, host seeking and selection behaviour.

Mosquitoes are small flying insects from the order diptera, comprising a single family, the culicidae. Adults are a few centimetres in length, and possess a single pair of wings which generate the characteristic whine that will be familiar to many. Mosquitoes are a hugely diverse group of insects. There have been more than 3,500 species described worldwide, and they are present on every continent except Antarctica. Mosquitoes undergo complete metamorphosis, which means they change their body form completely as they develop through four distinct life stages, egg, larva, pupa, and adult.

Female mosquitoes lay their eggs onto or at the edge of standing water, using odours produced by the composition of organic matter and even odours produced by other insects in the water to choose a suitable place to lay. These can be ponds or areas as small as a tiny cup of water, and indeed, both natural and manmade collections of water can provide habitat for mosquitoes. Human activity has facilitated the transport of some mosquito species worldwide. For example, the eggs of some Aedes species are highly resistant to dessication, or drying, and can survive the long journey across the world after they have been laid in used tyres stored outside.

After a period of time, the eggs can then hatch when they are next submerged in water, and these adults can then be emerging in a completely different country many miles away. So after a period of time, the first instar larvae will break out of their egg, using a so-called egg tooth located on the top of their head, and emerge into the water. The larvae use their brushlike mouth parts to feed on organic matter present in the water column, and they develop through four larval stages, each time moulting their exoskeleton to allow them to grow. Mosquito larvae breathe air via tubes called syphons, which are located on the rear end of the larvae.

The majority of species take in air from the water’s surface, but others, including members of the genera Coquilettidia and Mansonia, have modified syphons which they use to tap into plant stems underwater and take in air. These differences in biology have important implications for trapping. Now after the fourth larval stage, the mosquito will become a comma-shaped pupa. This stage, unlike the larvae, does not feed, but also requires air, which it obtains via its appendages, called respiratory trumpets. Pupae are highly sensitive to their environment and use the powerful paddle at the end of their body to propel themselves through the water column and avoid predators.

After a few days at the pupal stage, the mosquito will complete its development, emerging as an adult on the surface of the water. So when do mosquitoes become a problem? The answer lies with their feeding behaviour. Both males and females require sugar to survive, which they obtain from various sources, including plant nectar. However, the females of some mosquito species also require a blood meal to provide the necessary nutrients to produce their eggs. In contrast, males do not blood feed. Now, you can tell the difference between the sexes very clearly, even with the naked eye. Males have very feathery antenna compared to the females, as you can see in the leftmost image.

The proboscis of the female mosquito is uniquely adapted, allowing it to pierce the skin of a vertebrate host and take in a blood meal. Although it may seem like a single tube, the proboscis is in fact an intricate structure comprising six individual components. During blood feeding, a female mosquito can ingest a pathogen, such as the malaria parasite, or the dengue virus, and take it into its body. Now the pathogen will mature inside the mosquito, and after a period of time will migrate to the salivary glands, where it can be excreted the next time the mosquito feeds, and therefore infect the new host. The involvement of mosquitoes in disease transmission is a major reason for controlling them.

However, another major driver across the world is their role as nuisance biters of humans. Bites can be itchy, uncomfortable, and affect human well-being, and thus considerable cost and effort is expended in many parts of the world to control this nuisance biting. Some of the highest mosquito biting rates, which we can define as the number of bites received in a given time, can be found in the Arctic Circle during times of snowmelt, where biting rates upwards of 1,000 bites per hour have been reported. However, even in regions we wouldn’t immediately associate with mosquitoes, we may experience a lot of mosquito biting.

For example, in the United Kingdom, biting rates of over 200 per hour have been recorded in coastal regions of Kent. So a few slides previously, we learned that only the females of some mosquito species require a blood meal to complete egg development. There are other species, such as females of Culex pipiens form molestus, which became infamous for biting in Londoners sheltering in the Underground during the air raids of World War II, which can produce their first egg batch without a blood meal. This is important for them, as it allows them to survive in a challenging habitat where hosts are not always available. There are other mosquito species, too, that do not blood feed at all.

Members of the genus Toxorhynchites, for example, feed only on sugar sources as adults, and as larvae, in fact, feed on the larvae of other important mosquito species. These, therefore, have been trialled as a means of controlling other important pest mosquitoes. So how do mosquitoes find their hosts? They do this by the process of host location. The primary sensory organs of mosquitoes, the palps and antennae, are highly attuned to volatile odours. When we and other animals breathe, we produce carbon dioxide, which activates the host seeking process in mosquitoes. They will then follow the gradient plume of carbon dioxide and other volatile compounds to make their way towards the host.

As they get closer, they detect additional short range volatiles, and also use vision, heat, and movement in order to alight on the host and bite. However, we don’t all smell the same to a mosquito. Every one of us has a unique chemical scent that makes us more or less attractive to biting relative to others. Many of the factors that influence this are only just being uncovered. For example, recent twin studies have shown that how attractive you are to mosquito biting has a genetic component. Now, we can investigate which host mosquitoes feed on by selectively collecting resting blood fed specimens, extracting the DNA contained within their abdomens, and sequencing this to obtain the order of DNA bases.

We can then compare the sequence to an online database of known sequences to discover which host species the blood originated from. This selection of host that a mosquito will feed on is referred to as its host range, and this will affect what role a mosquito plays in the transmission of pathogens. Now, across the mosquito family Culicidae, mosquitoes will take blood meals from a huge variety of different hosts, including reptiles, birds, and mammals. This has important implications for disease transmission dynamics. For example, in areas where it is found, West Nile virus is maintained naturally in wild bird populations by mosquitoes feeding primarily on birds.

However, if a mosquito bites an infected bird and then subsequently bites a human or a horse, it can transmit the virus to these so-called dead end mammalian hosts. Mosquitoes that bite both birds and mammals in this way are described as bridge vectors. It is therefore really important to understand the host range of mosquitoes and the situations causing a switch in feeding behaviour in order to better understand disease transmission side cycles. The most important species worldwide are found in the genera Aedes, Anopheles, and Culex. Anopheles mosquitoes are vectors of the malaria parasite, while Aedes and Culex mosquitoes are associated with the transmission of viruses, such as dengue or West Nile virus.

Members of the latter two genera are furthermore frequently associated with causing a serious biting nuisance in localised areas. We can use some key features at each life stage of the mosquito, as illustrated in the diagram, to distinguish between members of these genera, although these are not always easy to see at first glance. So to conclude, mosquitoes are small flying insects which cause both biting nuisance and are important in transmitting serious human and veterinary pathogens. Not all mosquitoes species bite, and of those that do, only the females take a blood meal, which they do in order to support egg production.

Mosquitoes have a very sensitive olfactory system, and they use this to detect their hosts and the habitats in which to lay their eggs. Pathogen transmission dynamics are closely influenced by which host mosquito species feed on, and therefore, understanding this is a key part of identifying which species to direct resources towards for control efforts. Do you know how many mosquito genera and species are found in your country? Do you know of any invasive species that have recently been reported in your country? Do you know what sort of control programmes your local or national government has in place for mosquitoes, if any?

We’d love to hear from you, so please add your comments to the discussion boards below and share your experiences with mosquitoes.