Life Cycle

Mosquitoes undergo complete metamorphosis, passing through four distinct stages during their life cycle: egg, larva, pupa, adult/imago. 

A complete cycle from egg to egg (whole developmental cycle) can be as fast as about 14 days or as long as several months in diapausing species. Larval diapausing as well as hibernation or aestivation in females has been reported in some mosquito species. 

Egg

The eggs of mosquitoes are brown or blackish and are laid singly or in rafts (1) on the water surface of diverse types of water collections; (2) on the undersides of floating vegetation, to which they are 'glued'; and (3) on wet mud, leaf litter, other damp substrates near the edge of water. In the latter case they usually hatch when flooded. 

In detail, anopheline eggs are black, laid on the water surface and possess lateral floats. They cannot withstand desiccation and hatch within 2-3 days, although hatching can take up to 2-3 weeks in colder climates. 

Regarding culicine eggs, eggs of Aedes and Ochlerotatus species are black and usually deposited on damp substrate just beyond the water-line. They can withstand desiccation, and dry eggs may remain viable for months or even a few years. They hatch when becoming flooded. Eggs of Culex and Coquillettidia are brownish and laid upright and together, thus forming egg rafts that float on the water surface. Eggs of Mansonia are laid in a sticky mass that is glued to the undersides of floating vegetation. 

In nature, usually 4 to 5 ovipositions per female occur, with 30-500 eggs each. The site of oviposition is species-specifically chosen according to water chemistry and a circadian rhythm. 

In the tropics, eggs hatch within 1-2 days, in cooler climates this process might take 1-2 weeks.  

Larva

Mosquitoes of the family Culicidae possess aquatic larval stages which pass through four larval instars. Mosquito larvae, or wrigglers, occur in a variety of aquatic situations, depending on the species.  

With the exception of the species Mansonia and Coquillettidia, larvae have to come to the water surface to breathe, usually using a breathing tube (siphon) at the posterior end of the body.  

The larvae of anophelines lack a siphon (breathing tube) and lay parallel to the water surface where they breathe and feed. They breathe through a pair of spiracular plates at the posterior end. They only descend, when they are disturbed. Their larval habitats range from small water aggregations (hoofprints, puddles) to rice-fields and marshy areas, including salt-water marshes and mangrove swamps. Very few species breed in water-filled tree holes and some in leaf axils e.g. of bromeliad plants. Generally clean water without animal or vegetable matter is preferred. 

Culicine larvae have a siphon (short or long), usually used for breathing at the water surface. The larvae of this subfamily hang down from the water surface. Culex larvae have several pairs of hair tufts on their breathing tube, which is relatively long and slender, whereas Aedes larvae have a short and stout breathing tube with only a single pair of hair tufts. Larvae of the species Mansonia and Coquillettidia insert their modified siphon into aquatic plants to obtain oxygen via plant tissues. Pupae also obtain oxygen from plants by inserting their specialised respiratory trumpets into them. Larvae and pupae of these two genera remain submerged. Some larvae feed near the water surface while others feed on the debris at the bottom of the larval habitat. The larval habitats of culicines show a wide variety. Especially Aedes and Ochlerotatus breed in natural and man-made container-type habitats (tree holes, cut bamboo stems, discarded cans and tyres, etc.). Culex larvae are mainly found in ground collections of water (pools, ponds, ditches, borrow pits). Some breed in rice-fields, while others tolerate high levels of organic pollution, using latrines, septic tanks and soak-away pits. Mansonia and Coquillettidia larvae and pupae are only found in waters with aquatic plants to attach themselves. 

The larvae of most species feed on algae and organic debris, but also feed on a variety of aquatic microorganisms.  

In tropical climates the larval period can be as short as 7-10 days. In cooler climates it may take weeks to months, and several species even overwinter in this stage. 

Pupa

The pupa is also called tumbler. It is aquatic, quite active, and breathes at the surface through a pair of small trumpet-like structures on the thorax. It does not feed. 

With the exception of Mansonia and Coquillettidia species, the pupae remain at the water surface, where they breathe. When disturbed, they rapidly swim up and down, as seen in larvae. 

Pupal development takes 2-3 days in tropical and a week or more in temperate areas. 

Adult

Adults possess a long proboscis projecting forwards from the head, have scales on the wing veins and a fringe of scales along the posterior margin of the wing. Their wing venation is characteristic with the second, fourth and fifth longitudinal veins being branched (Goma, 1966). 

Males and females can be differentiated by the form of the antennae. In males they are very plumose, while in females they only have a few short hairs.  

The maxillary palps of anopheline adults are long in both sexes (in contrast to the short maxillary palps in females of the culicines), and clubbed in the male. Adult Anopheles wings are usually spotted, due to groups of differently coloured scales on the wings. In a resting position adults of this genus hold the body and proboscis in a straight line and at an angle to the surface on which they rest. Some species almost seem to "stand on their head".  

The wings of culicine adults are not spotted. Culicine adults hold their body in a resting position more or less parallel to the surface with the proboscis bent down 

Adult mosquitoes are usually active during the twilight hours or at night, or in dense shade: many spend the day in hollow trees, under culverts, or in similar resting places. Some overwinter in such places. 

Only the females are blood-sucking, males and occasionally also females feed on naturally occurring sugary secretions including nectar. 

Few hours to one day after emergence adults are ready for mating. Most females require a blood meal for egg development. Blood digestion is temperature-dependent and takes 2-3 days in tropical areas, respectively 1-2 weeks in temperate climates. After digestion the female is ready to lay eggs, which means it is gravid. The gonotrophic cycle is repeated several times during a mosquito's life time. Some species may show alterations of this scheme (e.g. multiple blood meals before becoming gravid in Aedes aegypti). 

The life expectancy of mosquitoes is depending on climatic conditions. In tropical climates females live on average 1-2 weeks, in temperate zones they might survive 1-2 months or even remain in hibernation for up to 9 months. Males usually have a much shorter life. 

References

Adult

Goma LKH: The mosquito. 1966, Hutchinson Tropical Monographs, Hutchinson & Co. (Publishers) LTD, London 

Further Reading

Mehlhorn H: Mosquitoes. In: Mehlhorn H (ed.): Encyclopaedic reference of parasitology. Biology, structure, function. 2nd edn., 2001, Springer Verlag, Berlin, pp. 378-84 

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