Biology

Habitats and species of Baix Llobregat

The life of mosquitoes is closely linked to the aquatic environment, the larvae and pupae develop in the water and the adult females must return to it to lay their eggs.

_{Polluted water, Culex pipiens. SCM files}
Almost any body of water is suitable for the development of one species of mosquito, as long as it is stagnant or flows very slowly. It does not need to be permanent; in some cases it is enough to stay there for a minimum of a week or ten days, so that a new generation of mosquitoes can appear.
There are species that colonize polluted waters and species of clean waters. Similarly, some tolerate low temperatures better than others, although in general, they are always favored by heat.

The larvae of Culex pipiens,one of the two most common species in the Baix Llobregat, can be found in water above approximately 15 ºC, although they occasionally withstand lower temperatures. In these latitudes, this results in a mosquito season that lasts from about April to October. Culex pipiens prefers polluted waters, although it is an opportunistic species that can be found practically everywhere. They are found in ditches, natural flooded areas and all kinds of objects including street drains.

_{Clean water. SCM files}
Larvae of 'Aedes albopictus, the tiger mosquito, on the other hand, develops in small spaces that we find mainly in urban areas: vases, buckets, drums, drains, ashtrays, plates, sewers, etc. Is this an invasive species, detected for the first time in the Iberian Peninsula in Sant Cugat del Vallès by our SCM in 2004. Scientific article here.

The next most abundant species, Aedes caspius, like all those belonging to the genus Aedes, it lays its eggs in land susceptible to flooding. The eggs remain inactive while waiting for the spring or autumn rains, which submerge them, making it possible to hatch. In the Delta, this species is found almost exclusively in the coastal marshes of Prat de Llobregat and Viladecans but also in some ditches, or abandoned fields in Gavà, Castelldefels or Molins de Rei.

_{Street storm drain. SCM files}
In mountain areas there are two very special habitats where mosquitoes breed. The puddles that remain in streams in places away from the current and the holes that are made in large trees and that are filled with rainwater. In these habitats we find species such as Anopheles claviger, Aedes vexans and Aedes geniculatus.     

Mosquitoes have numerous natural predators. The larvae are exposed to different organisms, especially fish (the invasive species Gambusia holbrooki), adult insects (Dytiscidae, Notonectidae) and also insect larvae (Odonata). The gambusia is an American fish that was introduced to the Llobregat Delta at the beginning of the century by the Antimalarial Commission, as part of a mosquito control program. It is a tremendously voracious fish, and really where there are gambusia you don't usually find mosquito larvae. The flying adults are prey for arachnids, dragonflies, bats and insectivorous birds.

Mosquitoes belong to the Diptera order, Culicidae family. They are insects with complete metamorphosis, that is to say; the eggs give rise to active aquatic larvae that transform into nymphs called pupae, from which adults emerge. More than 3,700 species of mosquitoes are currently known

https://mosquito-taxonomic-inventory, present in all latitudes. In Baix Llobregat there are 20 of them, distributed in 6 genres: Anopheles (3), Culex (4), Aedes (7), Culiseta (3), Coquillettidia (2), and Uranotaenia (1).

_{Biological cycle. Courtesy DIBA}
Anopheles atroparvus
Anopheles claviger
Anopheles plumbeus
Culex pipiens
Culex modestus
Culex hortensis
Culex theileri
Aedes albopictus
Aedes caspius
Aedes detritus
Aedes vexans
Aedes geniculatus
Aedes berlandi
_{Larvae un abandoned laundry. SCM files}Aedes pulcritarsis
Culiseta subochrea
Culiseta longiareolata
Culiseta annulata
Coquillettidia richiardii
Coquillettidia buxtoni
Uranotaenia unguiculata

However, only three, Aedes caspius and mainly Aedes albopictus and Culex pipiens, represent the vast majority of annoying mosquitoes in the area. Culiseta longiareolata is very common but it only bites birds.

Biological cycle

Eggs

_{Aedes albopictus eggs. SCM files}

Mosquito eggs are roughly cylindrical in shape and about a millimeter long. The females can lay them one by one (Aedes, Anopheles...) or in groups of 50 to 200, forming clumps called egg rafts for their resemblance to small ships (Culex, Culiseta). Its shape causes the eggs to float on the surface of the water, as do some isolated float-equipped eggs (Anopheles). Other eggs, such as those of Aedes, are laid in the soil and remain inactive until the ground is flooded. When the egg hatches the larva appears. Egg hatching is closely linked to the environment and, for example, is produced by floods or rain in the case of many species of Aedes.

Larvae

_{Culex pipiens larva. Courtesy A. Lindström}

The larva is always aquatic. Complete its metamorphosis after four growth instars growing from about 1 mm in length to about 12 mm (variable according to species). The length of the larval stage is greatly influenced by temperature. In the Llobregat delta the larval development of Culex pipiens or Aedes albopictus can last about 5 to 7 days in summer, and much longer in low temperatures.

The larvae move through very characteristic abdominal contractions and feed by filtering and scraping off finely particulate organic matter, unicellular algae and bacteria, among other organisms.

With the exception of the genus Coquillettidia, which presents a very particular adaptation (it lives attached to the roots of aquatic plants, breathing oxygen from plant tissues), the larvae of the other species capture atmospheric air. Most have a respiratory siphon located at the end of their abdomen that remains below the surface when they are in their usual upside-down position, allowing them to breathe air. Species of the genus Anopheles do not have a siphon and lie parallel to the surface of the water, breathing directly through the spiracle of the abdomen.

Pupae

_{Pupa. Courtesy A. Lindström}

The transformations that allow the mosquito to pass from the aquatic environment to the aerial environment, they begin at the end of larval development and continue, already in the nymphal stage, with the development of a completely new body.

This stage lasts only a short time, from 2 to 3 days. The pupa does not feed, uses the reserves accumulated during the larval stage and breathes through two "trumpets" located on the cephalothorax. The mosquito pupae, much more active than those of other insects, stay on the surface of the water, but when disturbed they dive with sudden contractions of swimming paddles located at the end of the abdomen.

At the time of emergence of the adult, the exoskeleton breaks longitudinally. The adult emerges from the pupal shell on the surface of the water, where it remains for a short period of time until its body unfolds and it is able to fly.

Here you can see a nice video of the life cycle of the tiger mosquito.

_{Culex pipiens male. Courtesy A. Lindström}

The usual food of adults, males and females, is the nectar of the flowers. Female mosquitoes are mainly famous for its biting behaviour to obtain blood, which means a supplementary supply of protein for egg maturation. For this reason, females are able to travel distances of kilometers, while searching for the blood they need.

Mosquitoes bite mainly vertebrates, but they tend to prefer a specific order. Some prefer birds (Culiseta longiareolata) or amphibians (Culex hortensis). Others like Culex pipiens, can produce a first egg raft without biting. It is called autogenous egg raft and is carried out at the expense of the energy reserves accumulated by the larva.

Most mosquitoes bite at specific times of the day, especially at sunrise and sunset, or during the night. At least some species, such as those belonging to the genus Aedes, bite at any time. Aedes albopictus usually bites during the day and outdoors.

 _{Culex pipiens egg raft. Courtesy A. Lindström}

Fertilization of the eggs takes place at the time of laying, but the pairing has been done before. There is generally a single copulation at the beginning of the adult's life; the sperm is stored in the spermathecae of the female where it is preserved throughout its life. Mosquitoes can, like other species of insects, form copulatory clouds made up solely of males, into which the females enter.

The number of eggs varies according to the species and the amount of blood sucked. If the female has been able to ingest sufficient blood without being disturbed, a single bite may be sufficient to produce a spawn, although if she has not obtained enough blood, she will continue to bite until she obtains. Once the target is reached it will not sting again until it has spawned.

The phenomenon of hibernation can occur in a particular stage (egg, larva, adult) or in several stages although it usually occurs in adults. Shortly before the beginning of the hibernation period, the females feed on all the nectar they are capable of in order to produce a special tissue, the fat body, which will serve as a food reserve throughout the hibernation. During this period, the females take refuge in protected places; the males die.

The life span of adult mosquitoes ranges from a few days to a couple of months, depending on the species and climatic conditions.