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Mosquito Biology

By H.A.Standfast
International Vector Consultants
 
Mosquitoes are classified in the family Culicidae. Professor Kettle lists 544 species of mosquito from the Australasian Region, which includes New Guinea and New Zealand. In 1984 Dr. Marks listed 214 described and 50 undescribed species from Australia. Fortunately, only a small number of these attack humans and even fewer transmit disease.
 
LIFE CYCLE
Fertilised eggs are laid by female mosquitoes in most instances in the body of water in which the larval will develop. The females carefully select the site according to temperature, salinity and oxygen level so that Ochlerotatus vigilax, the salt marsh mosquito, selects saline sites and does not normally oviposit in fresh water.
 
The eggs of Anophelines are laid singly on the pool surface and are readily recognised by their canoe like shape and the pair of floats on either side.
 
The eggs of Culicine mosquitoes are laid side by side and stick together to form rafts which float on the surface. All species of Aedes lay their eggs singly while some species which have desiccation resistant eggs place them in mud or on vegetation bordering the breeding site.
 
The embryo in the egg completes development in one to two days dependent on temperature. In those eggs laid in water the embryo emerges as a first instar larvae and commences larval development. However in those species which have drought resistant eggs the embryo remains in the egg until next it is flooded, when the egg hatches.
 
After a period of growth the first in star larva moults, shedding its skin to form a larger second instar larva. This process is repeated twice more to produce a much larger fourth instar Larva. At the end of the development phase of the fourth instar larva it moults to form a pupa, which is the comma shaped stage sometimes referred to as tumblers. In this stage the larval tissue is completely reformed to produce an adult insect which emerges from the pupa.
 
Male mosquitoes normally emerge 24 hours prior to the females. The insects on emergence are very vulnerable until their cuticle hardens. Usually within 24-48 hours of emergence the female mates and goes in search of a blood meal. The development time for eggs is from 48-96 hours dependent to temperature, then the cycle starts again with the female selecting a suitable ovipositor site, depositing her eggs and going in search of a blood meal to mature the next batch of eggs. The number of eggs laid can vary from Less than fifty to more than two hundred- depending on the species and nutritional state of the insect. The period of aquatic development varies with temperature. The higher the temperature the shorter the development time.
 
LARVAL BEHAVIOUR
Breathing
Larvae come to the surface to breathe. The organ used for breathing is the siphon, which is a tube at the anterior end of the larva. containing a pair of tubes, which join to the trachea which carry oxygen to the insect. Two genera, the Mansonia and the Coquillettidia have specially modified siphons which they use to penetrate the roots and stems of water plants drawing oxygen from the plants, removing the requirement to come to the surface to breathe.
 
Feeding
Larvae use their mouth brushes to set up a current of water, which carries food particles to the mouth. Several groups are predators on other mosquito larvae. They have specially modified heavily chitinized mouthparts, which allow them to seize other mosquito larvae and to eat them. Ochlerotatus alternans is one such species frequently found in association with Ochlerotatus vigilax. When collecting larvae for study, care must be taken to see that predacious larvae are removed from the collection or when you come to examine the insects in the laboratory you may find one large predacious larva and the remains of the rest of your collection.
 
Distribution
Larvae are not evenly distributed in the breeding site, tending to concentrate in areas where they will gain most shelter from predators. Shallow pool fringes, particularly if grassy, provide protection from predators. Some species have specially modified hairs on the siphon which allow then to anchor themselves to strands of blue green algae. Often if disturbed even if only by a shadow passing over a pool the larvae will move to the bottom and remain there for lengthy periods.
 
Pupae
The pupal stage, which can be as short as 12 hours in some New Guinea Anophelines usually, takes 24-48 hours in summer in south Queensland. The stage is one when the larval tissue is reorganised to form an adult insect. The pupa does not feed and breathes at the surface through a pair of respiratory horns. It swims with a rapid tumbling action propelled by flexing its extensive tail, which contains the abdomen of the developing adult.
 
ADULT BEHAVIOUR
Activity times 
Mosquito species can be categorised as:
  • Diurnal - active during the day
  • Crepuscular - peaks of activity at dawn and dusk.. 
  • Nocturnal - active during the night
 
Activity is controlled by temperature, humidity; and light. Low temperatures below 200C inhibiting activity of tropical insects but the same species may still be quite active at 150C or lower in the south. Generally low humidities below 70% Rh inhibit activity.
 
Reproduction 
Females need only mate once but often mate more frequently. On mating the sperm in the form of a sperm plug are transferred into the vagina of the female. then migrate to the Spermathecae where they are stored until required. When the mosquito is ovipositing the sperm enters the egg as it passes down the genital tract. One mating provides enough sperm to last the insect's lifetime. Dependent on temperature, the first batch of eggs may take 24-48 hours longer to mature than subsequent batches and may require more than one blood meal. Females lay a batch of 50-200 eggs every 48-96 hours dependent on species and temperatures.
 
In some species males form a swarm and the females fly into the swarm where they couple with a male. In some species mating takes place on the host vertebrate shortly after the female takes a blood meal.
 
Feeding 
Both male and female mosquitoes rely on plant nectar and plant juices to supply the carbohydrates needed to provide the energy required for flight and other activities. Female mosquitoes require a blood meal to provide the protein needed to mature their eggs.
Some species are very selective and will only feed on a particular vertebrate species; others will feed on almost any warm-blooded animal, while others specialise on lizards and frogs. A number of species can produce a limited number of eggs without taking a blood meal, relying on the food reserves from the larval stage to provide the protein required for egg development.
 
Host Finding
There are several stages in the feeding behaviour of mosquitoes - activation, orientation, landing and probing. The time of activation is set by the insects biological clock but will be modified by climatic influences eg. if it is too windy, too cold or if the humidity is too low. The activity is often triggered by CO2, with the insect finding the host by flying up the odour plume generated by the host (CO2 + body odours). A number of authors list the range of attraction to be approximately 20 metres with a visual range of 7 metres or less. The final stages of landing and feeding require the visual cues of colour and shape as well as the odour plumes. None of the traps devised to attract the animal using CO2, light, heat and various chemicals is as attractive as the intact animal.
 
Flight Range and Dispersal
Some species eg. Ochlerotatus vigilax - are knows to regularly travel long distances from the breeding site while others are never found more than a few hundred meters from the breeding site. Many estimates of pest range are based on unsatisfactory data and should be treated with caution.
 
Useful discussions of mosquito biology can be found in thye following publications:
 
  • Mosquitoes and Mosquito-borne Disease in Southeastern Australia by Richard C. Russell, obtainable from Department of Medical Entomology, Westmead Hospital. Westmead, NSW 2145
  • Medical and Veterinary Entomology by D.S.Kettle, obtainable from the University of Queensland Bookshop.
 
If you can locate them in your library, the long out-of-print classics Mosquito Behaviour by R.C. Muirhead-Thompson 1951 and The Natural History of Mosquitoes by Marston Bates (The Macmillan Company, New York) 1949 make interesting reading and contain a wealth of detailed observations on mosquito biology.
 
Other reference books include:
  • Mosquitoes of Victoria by N.V. Dobrotworsky (Melbourne University Press) 1965.
  • Mosquitoes of the South Pacific by John N Belkin (University of California Press) 1962.
 
 

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