Mosquitoes

There are more than 4,000 types of mosquitoes in the world. Some mosquitoes are carriers of diseases to human beings and animals for instance, yellow fever mosquitoes (Aedes Aegypti) and Aedes Albopictus are carriers of dengue haemorrhagic fever. Culex mosquitoes are carriers of encephalitis. Anopheles, carry malaria. Mansonia mosquitoes carry filariasis. All of the diseases mentioned are diseases that affect human beings. Mosquitoes also have a large impact on animals because they carry a number of animal diseases. Culex mosquitoes, for instance carry heart parasite disease in dogs and malaria in birds. Some mosquitoes bite cows causing cows to lose weight and produce less amount of milk. Part from being harmful to human beings and warm-blooded animals, mosquitoes are also harmful to cold-blooded animals.

Medical Significance may be divided into two types;

1. Causing direct effect- skin is inflammed because of an allergy to mosquito saliva. One will feel itchy. Scratching the inflamed skin may lead to wound that may be infected by bacteria.

2. Mosquitoes are carriers of the following diseases;

Carrier Disease
Anopheles Malaria
Culex Encephalitis
Aedes Dengue Fever, Yellow fever, Chikungunya
Mansonia Filariasis

 

Biological and Ecological Features

Life cycle and important characteristics

 

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Mosquitoes grow into Complete Metamorhposis or Holometabola which means the growth in which there are stages of distinct transformation. There are 4 such stages in mosquitoes’ life cycle;

Between the growth in each stage, molting needs to occur. It is controlled by 3 important hormones namely Brain Hormone, Ecdysone and Juvenile Hormone.

Egg Phase

Mosquito eggs differ in size and characteristics. One may be able to tell the type of mosquitoes from the way eggs are laid. Mosquitoes like to lay eggs on the water surface or damp areas such as the edges of containers above water level. There are 4 types of mosquito egg laying.

  1. Single egg laying on the water surface- Anopheles
  2. Raft laying on the water surface- Culex
  3. Single egg laying along the edges above the water level- Aedes
  4. Laying egg next to leaves of water plants- Mansonia or black mosquitoes

 

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The egg phase takes 2-3 days. Then eggs are hatched and mosquito larvas emerge. For some types of mosquito, eggs are able to stay in dry conditions for several months or a year. Once there is some water around, larvas will emerge. Different types of mosquitoes go to different places to lay eggs. For example; aedes like to lay eggs in a container with water or a swamp; culex like to lay eggs in dirty water or sewage water from the drainage pipes. However, if they cannot find a suitable place, mosquitoes may lay eggs on water that has different conditions than what they are used to. Several scientists report that the factor that allows female mosquitoes to realise where to lay eggs is a certain chemical in water. The chemical may be diglycerides that is produced by larvas living in that water or fatty acid from bacteria or phenolic compounds from water plants.

Larva Phase

Different types of mosquito larvas live in different types of water for instance; containers with water; ponds; swamps; streams; tree hollows; or water bound leaf spathes. Most mosquito larvas come up to the surface to breathe with breathing pipes called siphons except anopheles larvas which do not possess breathing pipes. Instead, they lie themselves down paralell to the water surface, then their palmate hairs will help them to float and they will breathe through spiracles. Mansonia larvas use short and pointy breathing pipes to pierce through water plants and get oxygen through the roots of water plants. Food for mosquito larvas include small living organisms in water such as bacteria, yeasts and algaes.

Larvas go through 4 moltings. The last molting turns them into pupae. The larva period is approximately 7-20 days depending also on the types of larva, food, temperature and density of larva.

 

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Pupa Phase

Pupae are of different shape from larvas. The head section is connected to the thorax. A pupa has a body similar to a comma (,). In this phase, pupae do not eat. They move quickly. There is a pair of breathing pipes on a pupa’s head called Trumpets. Pupae of culex have breathing pipes sticking out on top of the cephalothorax. Such pipes are long and narrow. Pupa of anopheles have breathing pipes that are wide and short and in the shape of cones coming out from the middle of cephalothorax. Pupae of aedes have breathing pipes that are shorter than culex. This phase takes only 1-3 days.

 

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Adult Phase (mosquito) The body of a mosquito is divided into 3 parts;

Head

  • Round and is connected to the thorax.
  • One pair of compound eyes
  • One pair of 15-joint antenna. The antennae can be used to indicate gender of mosquitoes. Each joint is covered with hair. Female mosquitoes have hairs around antenna that are short and sparse called Pilose Antenna while male mosquitoes have bushy and long hairs. Their antenna is called Plumose Antenna. Mosquito antennae are organs that are used to receive sound waves for instance female mosquitoes flapping wings, air humidity, scents and chemicals.
  • One pair of maxillary palpi which is divided into 5 sections that are adjacent to proboscis. In female anopheles, palpi are straight and are as long as proboscis whereas the male have an enlarged end of palpi like a club.
  • There is one proboscis. It is narrow and long like a needle. This is for piercing and sucking food.

Gender differences of mosquitoes

 

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Differences of maxillary palpi in different types of mosquitoes.

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Thorax

  • There is one pair of wings attached to the top of the thorax. They are covered with rough hair and scales that have different colours and patterns. We can use the differences in patterns to differentiate the types of mosquitoes.
  • There is one pair of Halteres 1 in the last section of the thorax. They look like small buttons next to the wings. When mosquitoes fly, halteres will vibrate quickly which is beneficial to the balance of mosquitoes. There are scales on mosquitoes’ legs which can also be used to differentiate between different types of mosquitoes. Wings are narrow and long.

Abdomen

  • Abdomen of a mosquito is round and long. It consists of ten sections. However, only 8 sections are clearly visible. Sections 9-10 are modified into sexual organ. For male mosquitoes, one may be able to use this to differentiate between ttheir different types.

A picture showing different parts of a mosquito

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Daily Life

Food

Adult mosquitoes of both sexes eat honeydew from flower pollens to survive. However, most female mosquitoes also need protein from human or animal blood to help with the growth of eggs and the generation of energy. Only female mosquitoes bite humans and animals. Different types of mosquitoes have different preferences of blood. Those that like to suck animal blood are called zoophilic while those that like to suck human blood are called anthropophilic. Blood will facilitate the growth of eggs. Only a few types of mosquito eggs can grow fully using only the accumulated food and not blood. These are called autogeny. Mosquitoes of this type include Aedes togio and Culex molestus. Different types of mosquitoes have different times that they fly out to look for food. Aedes like to look for food during the day, culex go during the night and hen mosquitoes like to find food early in the night and early in the morning. However, should a mosquito needs blood, it may look for food at any time.

Flying

Different types of mosquitoes have different types of flying characteristics. Aedes aegypti do not go far. They are able to fly for approximately 30-300 metres. Aedes albopictus can fly for approximately 400-600 metres. Anopheles can fly for approximately 0.5 – 2.5 kilometres and culex can fly from 200 metres to a few kilometres. Female mosquitoes can fly further than the male ones.

Breeding

Male mosquitoes casts off their pupae before female mosquitos and they stay near the egg laying areas. One or two days after female mosquitos cast off their pupae, male and female mosquitoes will mate and breed. Afterwards, female mosquitoes will look for blood sources. However, some mosquitoes need blood before breeding for instance anopheles culicifacies. Moreover, anopheles have a habit of flying around in groups to mate. This is called swarming. It occurs when the sun is setting. The light that fades quickly has an effect of stimulating this activity. Aedes mate without the need of swarming. The male mosquitoes will respond to the sound of wing-flapping of female mosquitoes. Male mosquitoes are able to find female mosquitoes within the distance of 25 centimetres.

Life span of mosquitoes

Usually, male mosquitoes have shorter life span than female mosquitoes. Male mosquitoes have the life span of approximately 1 week except when their food is in abundance and the humidity is appropriate in which they may live for as long as a month. Female mosquitoes’ life span is approximately 1-5 months. However, life span of mosquitoes depends on several factors, for instance, in the summer, mosquitoes have a lot of activities causing their life span to fall to an average of approximately 2 weeks. In winter, mosquitoes have fewer activities thus allowing them to live longer. In some areas, mosquitoes can hibernate throughout the winter.

Types of mosquitoes that are significant medically.

1. Genus Aedes

Aedes aegypti

  • Dengue fever carrier in Thailand and yellow fever carrier in South American
  • Originated from Africa
  • Tend to stay in the house or around the house
  • Breeding grounds include containers with water for instance earthen jars, concrete containers in bathrooms, cabinet feet saucers, old tires, cans, vases, rain gutters with water, coconut shells and leaf spathes.
  • They lay eggs individually along the edges of containers above the water. Eggs of aedes may stay in a dry place for a year waiting for the right humidity and temperature (28-30 degrees Celsius). Once the water level reaches the eggs, larvas will come out. The period of incubation in the eggs is approximately 2.5-3.5 days.
  • Aedes larvas take about 7-10 days to grow.
  • Once larvas become adults, aedes are able to breed at the age of 24 hours.
  • A female mosquito breeds once but is able to lay eggs many times.
  • A male mosquito is able to breed more than ten times in an hour.
  • Aedes Aegypti like to suck human blood. They go out to find food during the day. However, they also bite during the night.
  • Resting places of Aedes Aegypti after blood-sucking include dark places that wind cannot reach, toilets/bathrooms, hanging objects in a house such as clothes, nets and curtains.
  • Aedes Aegypti do not like water with bad scents.
  • The difference between Aedes Aegypti and Aedes Albopictus is that the back of the thorax has 2 white scales while there is only one for an Aedes Albopictus.

Aedes albopictus

  • Originated in Asia
  • Their habits are similar to Aedes Aegypti however, they are usually found in the countryside for instance fruit plantations, rubber plantations and various parks.
  • The water sources that are used to breed are natural water sources such as tree hollows and bamboo tubes. However, Aedes Albopictus are able to breed in human-made containers/places such as earthen jars and plant pots.
  • Aedes Albopictus fly further than Aedes Aegypti.
  • They are carriers of dengue fever.

2. Culex quinquefasciatus

  • Found mostly in African and Asian countries.
  • In Myanmar, India and Indonesia, this type of mosquito is the main carrier of filariasis (elephantiasis). They are also the carrier of encephalitis.
  • They lay eggs in a raft on waste water and sewage water. Their breeding grounds are close to houses. One raft has approximately 200-250 eggs. Eggs hatch within 30 hours at the temperature 24-30 degrees Celsius.
  • Larvas of Culex have slender breathing pipes floating with an angle against the water surface. They move in an (s) shape. They float under water by fixing their breathing pipes with the water surface.
  • An adult is grey or dark brown throughout its body. Its wings are rather transparent. Its abdomen may be in the form of white sections but its legs are not striped or its legs may have white sections but its thorax and abdomen are not striped. The end of female abdomen is of an obtuse angle.
  • They suck human blood and are nocturnal.

3. Genus anopheles

  • They are carriers of Malaria which is caused by a protozoa called Plasmodium
  • They lay individual eggs on the water surface. They lay eggs at night.
  • Each of its eggs is of ship-shaped. On both sides in the middle of an egg has air film that acts like a float causing the eggs to float on water. This is its unique characteristic.
  • Incubation period for an egg to become a larva is approximately 2-3 days.
  • Larvas of anopheles lay themselves parallel to the water surface. They have palmate hairs on almost each of their sections to help them float. This is their unique characteristic.
  • There is no siphon. They have to breathe through spiracles.
  • When they become adults, anopheles are able to breed immediately after coming out of their pupas. One breeding allows them to lay 5-6 batches of eggs but they need to get some blood first. Then they will wait until eggs are ready then they will look for a suitable place to lay eggs.
  • Adult anopheles have narrow and slender legs without hard hairs. Their bodies are rather black. Their wings have light-coloured and dark-coloured scales. To cling onto something, an anopheles’ mouth and thorax will be in a straight line making 40-90 degrees angle against the surface.

4. Genus mansonia

  • เThey are carriers of filariasis which is caused by Brugia Malayi. They are mostly found in the South of Thailand.
  • Breeding grounds of mansonia include ponds or swamps with water-bound plants such as water lettuce and water hyacinth.
  • Their period of growth is rather long. It takes approximately 23-30 days for them to grow from eggs to adults.
  • Mansonia lay eggs in batches under leaves of water plants. Their eggs are dark in colour and come in clusters shaped like flower petals.
  • Each cluster has approximately 75-200 eggs.
  • The special characteristic of mansonia larvas is the siphon which looks like a short cone with pointy end. The siphon has wavy edges like saw blades. The siphon attaches itself to the stem or root of a water plant.
  • Larvas breathe by getting oxygen from water plants. They move in S-shape.
  • Adult mansonia, when they have bred and consumed blood, will rest on grass-top waiting for the eggs to be ready. Then they will lay eggs in breeding grounds in ponds or water sources with water plants such as water lettuce and water hyacinth.
  • They like human and animal blood.
  • They usually go to look for food early in the night and before sunrise. One may only find mansonia flying and looking for food during the day in the areas with high humidity and enough shade.
  • They are different from other types of mosquitoes as their wing scales are rather colourful and have interesting patterns. Most of them are brown with round scales that are larger than other types of mosquito.

 

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Aedes aegypti
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Aedes albopictus
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Culex quinquefasciatus

 

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Genus anopheles
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Genus mansonia

 

Methods to control mosquitoes

1. Biological Control- the use of organisms living in nature to control mosquitoes effectively, the number of such organisms must be high enough to control the population of carrier mosquitoes. Biological control may involve the use of fish to eat larvas, parasites, fungi, bacteria and protozoa.

2. Chemical Control- the approach to control mosquitoes using harmful substances for instance, the use of synthetic pyrethroid, naturally extracted substances, organo-chlorine, organophosphate and carbamate substances. Chemical control through the use of harmful substances needs to be planned carefully based on the knowledge of biological habits of carrier mosquitoes and epidemiology of diseases. In terms of public health, there are not many harmful substances that can be used safely especially those that are used to spray. Some have residue effects while some are used for a long period of time in the agricultural sector causing carrier mosquitoes to be able to resist such substances. Hence the use of chemical control through the use of harmful substances needs to be used together with other measures.

3. Mechanical Control- the use of mosquito nets, wear proper clothing, the use of wire screen and the use of lids.

4. Genetic Control- change chromosomes of carrier mosquitoes to prevent them from carrying diseases around, sterilise mosquitoes using radioactive materials or harmful substances.

5. The use of Insect Growth Regulators (IGRs) for instance, juvenile hormone-like substances, substances that prevent the build-up of thorax membranes/walls.

6. The use of mosquito trap to survey the population of mosquitoes. A machine is able to lure mosquitoes in through the method of photo-catalysis and the emission of carbondioxide (CO2). Female mosquitoes and blood-sucking