One of EID's strengths is that it knows its enemy, the mosquito, or rather mosquitoes. There are about 40 species on the French Mediterranean coast although only a small number of them bite Man. The most common belong to the genera Ochlerotatus and Culex. Male mosquitoes are harmless: only the females bite. After mating, the female has to feed on blood in order for the eggs to mature.
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Once her feed is over, she lays her eggs on the surface of water or on damp ground prone to flooding. |
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In the water, the eggs hatch and larvae, which are exclusively aquatic, emerge. Water is vital for hatching and larval development.
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In certain cases the eggs, like plant seeds, only hatch after having been dormant through a winter. The larval stage lasts less than 10 days for some species. Larvae feed on bacteria and plankton. |
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At the end of this period, the larva becomes a nymph. The nymph lives in the water for another 2 or 3 days, the time it takes to undergo major anatomical changes. |
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The nymph begins its metamorphosis by remaining stationary at the surface of the water. A tear opens up along its dorsal side... |
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...and the adult emerges slowly. Once free, it can open its wings and fly. |
| Beware: not everything that flies is a mosquito! Mosquitoes should not be confused with other similar insects that do not bite, particularly chironomes, which are the same size, or crane flies, which are much larger. |
Chironome |
Tipule |
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Eggs are generally spindle-shaped, measuring about 1 mm in length. They are whitish when laid and darken in the hours that follow. The female either lays single eggs on the surface of the water (Anopheles, Orthopodomyia) or lays clusters of 50-200 eggs (Coquillettidia, Culex, Culiseta, Uranotaenia). Eggs are also laid on damp substratum that is likely to be flooded subsequently (Ochlerotatus). Eggs laid on the water's surface can not sink because of floats (Anopheles), a ring (Orthopodomyia) or the way they are arranged in clusters. They hatch out as soon as the embryo has fully developed. Eggs from the genus Ochlerotatus, laid singly on a damp substratum, do not hatch until they have been submerged. Even then, a certain number of conditions (hatching factors) need to be met. These eggs are so-called quiescent. At hatching, the hatching button, a veritable can opener on the embryo's head, cuts the shell. |
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There are four stages to larval development, which is always aquatic. These stages are separated by moults which enable the larva to grow 2 to 12 mm. With the exception of the Coquillettidia which have an unusual biology, all larvae are mobile and breath at the water's surface, either directly (Anopheles), or through a siphon (Ochlerotatuss, Culex, Culiseta, Orthopodomyia, Uranotaenia) at the end of the abdomen. They move in jerks and generally feed by filtration, either at the surface or at the bottom of the larval site. The larvae of Coquillettidia live attached to plant roots. They have a special siphon that enables them to breath the air in the plant's tissues. They do not move around much and after each moult, they attach themselves close to the abandoned casing. The length of the larval stage is very variable, lasting from several days in summer to several months (sometimes up to a year for species that only produce one generation per year and hibernate during the larval stage, for example, Coquillettidia richiardii). Larvae can be found wherever water collects, from marshes to polluted ditches, from septic tanks to hollows in trees, but not in running water. These sites can be near the sea (Ochlerotatus mariae) or at high altitudes (Ochlerotatus pullatus, Ochlerotatus cataphilla, etc.), in areas with sparse vegetation (Ochlerotatus mariae) or in overgrown areas (Ochlerotatusrusticus, Ochlerotatus cantans), in fresh water (Anopheles), in polluted water (Culex pipiens) or even brackish water (Ochlerotatus mariae). In some conditions, the density of larvae is such that the larvae occupy the entire surface of the water (Culex pipiens, etc.). |
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The metamorphosis that allows the mosquito to move from an aquatic environment to the land starts at the end of larval development with the lysis of muscles and then during the nymph stage a totally new system develops. This stage only last 24-48 hours. The nymph does not feed, it draws on the reserves that were built up in the larval stage. It breathes through two trumpets that are on the cephalothorax and not at the end of the thorax as in the case of larvae. Nymphs of Coquillettidia are also fixed to roots by modified respiratory trumpets. Nymphs generally stay at the surface of the water but when disturbed, they dive by quickly furling and unfurling their abdomen which has two swimming paddles at the end. When the adult emerges, the cuticle splits longitudinally. The adult fills itself with air and extracts itself from its casing at the water's surface. |
Feeding
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During the first few days after emergence, the male and female adults rest in sheltered places. Their first feed is generally eaten at dusk and consists of nectar.
Eggs are fertilized at the point of lay although mating takes place prior to this. Generally, mating takes place only once at the start of adult life. The sperm is stored in the female's sperm bank where it is kept throughout her life. The male mosquito is attracted by the vibrations of the female's wings when she is flying. Mating can take place entirely in the air or finish on a support. This explains why it is difficult to breed some species in captivity because space is a determining factor. This drawback has been overcome by resorting to forced mating. Like many other insects, mosquitoes can form swarms which are usually made up exclusively of males.
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Mosquitoes are reputed for feeding on blood which is needed by the females of most species to provide additional protein for their eggs to mature. The females prefer biting vertebrates and nearly always prefer a particular order. Some prefer biting birds (Culiseta longiareolata, etc.) or batrachians (Culex hortensis, Culex impudicus). Others can lay their first eggs without feeding on blood (Coquillettidia richiardii, Culex pipiens, Uranotaenia unguiculata, etc.). These species are known as autogenous and use the energy reserves accumulated during the larval stage. We all know from experience that mosquitoes prefer biting at certain times of day, most often at dawn and dusk. Some rarer species are aggressive throughout the night (Culex modestus). |
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When the female has gorged herself on blood, she hides until the eggs have fully developed and then she looks for a place to lay. The number of eggs produced depends on the species and the amount of blood absorbed. The autogenous lays are always made up of a smaller number of eggs. A single feed of blood suffices for laying. If the female has fed on sufficient blood she will not bite again until after she has laid. For some species, egg maturation leaves marks in the ovaries (Anopheles, Coquillettidia), which means that the female's "physiological age " can be determined. This is an important concept particularly in the case of disease transmission. |
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Flying ability varies tremendously between species and also depends on weather conditions. Some species hardly travel at all when conditions are favourable (Culex pipiens in town areas) while others are capable of covering long distances especially when they are "carried" by hot damp winds (Ochlerotatus caspius, etc.). The distance covered depends on the energy reserve of glycogen which is synthesized from nectar and stored in muscles and fat. The maximum distance covered after a feed of nectar is around 30 km, though it is not necessarily covered in one go nor in the same direction. Dispersal flights occur at different periods of the mosquitoes' life (pursuing a host, species propagation, etc.) and involve mainly the females. |
Every mosquito has an endogenous rhythm of activity and rest. For example, it has been observed that laying takes place at a particular time, most often at dusk. This circadian rhythm also applies to the feeds of nectar and blood. The rhythm varies from one species to another although it is always around 24 hours and is maintained even if the activity does not actually take place because of unfavourable conditions such as strong winds, low temperatures, absence of a host for the blood feed, etc.
Wherever there is a winter, even a mild one, hibernation occurs. It can take place at a particular stage (egg stage in the case of Ochlerotatus, larval stage for Anopheles plumbeus and Coquillettidia richiardii, adult stage for Anopheles and Culex) or at several stages. In the adult stage, the females hide in sheltered places (cellars, sheep sheds, etc.). Even if they do feed on blood during this time, eggs are not produced until the season is right. Activity does not simply recommence when the temperature goes up because day length also plays an important part.
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Generally, it is thought that adult mosquitoes only live for a few days, a week or two at most. Some species do not live for more than 2-3 weeks, for example, Culex pipiens during the summer. On the other hand, other species live for several months, particularly those that hibernate during the adult stage. It should be noted that mosquitoes have numerous predators as well as being targeted by Man, factors which reduce their "life expectancy". A researcher showed that Anopheles atroparvus lives for 6 months on average in winter but rarely more than 6 weeks in summer. |
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