Anopheles larvae below water surface
Characteristics & Habitat
Depending on the species, the adult Anopheles mosquito may measure from 4 to 10mm (0.2 to 0.4 in.) which is similar to its subfamily, Culex. In the female, palps as long as the proboscis are characteristic of anopheline mosquitoes. Anopheles mosquitoes tend to sit with their heads low and their rear ends raised high, especially when feeding. The larvae tend to float horizontal at the surface of the water when not in motion. They mainly breed in polluted stagnant water and clogged drains.
Males feed on nectar and other sources of sugar. Females will also feed on sugar sources for energy, but usually require a blood meal for the development of eggs. Like other mosquito species, the female locates organic substances produced by the host, in this case, human produces carbon dioxide, humidity and its optical ability to recognitise humans.
Like all mosquitoes, Anophelines go through four stages in their life cycles: egg, larva, pupa, and imago (the imago is the last stage an insect attains maturity during its metamorphosis, its process of growth and development). The first three stages are aquatic and last 5–14 days, depending on the species and the ambient temperature. The adult stage is when the female Anopheles mosquito acts as malaria vector. The adult females can live up to a month (or more in captivity), but most probably do not live more than two weeks in nature.
Adult females lay 50–200 eggs per oviposition. The eggs are quite small (about 0.5 × 0.2 mm). Eggs are laid singly and directly on water. They are unique in that they have floats on either side. Eggs are not resistant to drying and hatch within 2–3 days, although hatching may take up to 2–3 weeks in colder climates.
The mosquito larva has a well-developed head with mouth brushes used for feeding, a large thorax and a nine-segment abdomen. It has no legs. In contrast to other mosquitoes, the Anopheles larva lacks a respiratory siphon, so it positions itself so that its body is parallel to the surface of the water. In contrast, feeding larva of a nonanopheline mosquito species attaches itself to the water surface with its posterior siphon, with their body pointing downwards.
Larvae breathe through spiracles located on the eighth abdominal segment, so must come to the surface frequently. The larvae spend most of their time feeding on algae, bacteria, and other microorganisms in the surface microlayer. They dive below the surface only when disturbed. Larvae swim either by jerky movements of the entire body or through propulsion with the mouth brushes.
Larvae develop through four stages, or instars, after which they metamorphose into pupae. At the end of each instar, the larvae molt, shedding their exoskeletons, or skin, to allow for further growth. First-stage larvae are about 1 mm in length; fourth-stage larvae are normally 5–8 mm in length.
The process from egg-laying to emergence of the adult is temperature dependent, with a minimum time of seven days.
The larvae occur in a wide range of habitats, but most species prefer clean, unpolluted water. Larvae of Anopheles mosquitoes have been found in freshwater or saltwater marshes, mangrove swamps, rice fields, grassy ditches, the edges of streams and rivers, and small, temporary rain pools. Many species prefer habitats with vegetation. Others prefer habitats with none. Some breed in open, sun-lit pools, while others are found only in shaded breeding sites in forests. A few species breed in tree holes or the leaf axils of some plants.
The pupa is comma-shaped when viewed from the side. The head and thorax are merged into a cephalothorax with the abdomen curving around underneath. As with the larvae, pupae must come to the surface frequently to breathe, which they do through a pair of respiratory trumpets on their cephalothoraces. After a few days as a pupa, the dorsal surface of the cephalothorax splits and the adult mosquito emerges. The pupal stage lasts around 2–3 days in temperate areas.
The duration from egg to adult varies considerably among species, and is strongly influenced by ambient temperature. Mosquitoes can develop from egg to adult in as little as five days, but it can take 10–14 days in tropical conditions.
Like all mosquitoes, adult Anopheles species have slender bodies with three sections: head, thorax and abdomen.
The head is specialized for acquiring sensory information and for feeding. It contains the eyes and a pair of long, many-segmented antennae. The antennae are important for detecting host odors, as well as odors of breeding sites where females lay eggs. The head also has an elongated, forward-projecting proboscis used for feeding, and two maxillary palps. These palps also carry the receptors for carbon dioxide, a major attractant for the location of the mosquito’s host.
The thorax is specialized for locomotion. Three pairs of legs and a pair of wings are attached to the thorax.
The abdomen is specialized for food digestion and egg development. This segmented body part expands considerably when a female takes a blood meal. The blood is digested over time, serving as a source of protein for the production of eggs, which gradually fill the abdomen.
Anopheles mosquitoes can be distinguished from other mosquitoes by the palps, which are as long as the proboscis, and by the presence of discrete blocks of black and white scales on the wings. Adults can also be identified by their typical resting position: males and females rest with their abdomens sticking up in the air rather than parallel to the surface on which they are resting.
Adult mosquitoes usually mate within a few days after emerging from the pupal stage. In most species, the males form large swarms, usually around dusk, and the females fly into the swarms to mate.
After obtaining a full blood meal, the female will rest for a few days while the blood is digested and eggs are developed. This process depends on the temperature, but usually takes 2–3 days in tropical conditions. Once the eggs are fully developed, the female lays them and resumes host-seeking.
The cycle repeats itself until the female dies. While females can live longer than a month in captivity, most do not live longer than one to two weeks in nature. Their lifespans depend on temperature, humidity, and their ability to successfully obtain a blood meal while avoiding host defenses.
Disease & Risks
One important behavioral factor is the degree to which an Anopheles species prefers to feed on humans (anthropophily) or animals such as cattle, pets like dogs, cats or birds. Anthropophilic Anopheles are more likely to transmit the Malaria parasites from one person to another.
Anopheles gambiae is one of the best known, because of its predominant role in the transmission of the most dangerous Malaria parasite species (to humans) – Plasmodium falciparum.
Some species of Anopheles also can serve as the vectors for canine heartworm disease, Dirofilaria immitis, the filariasis-causing species Wuchereria bancrofti and Brugia malayi, and viruses such as one that causes O’nyong’nyong fever.
Anopheles might also transmit a virus or other agent that could cause human brain tumor.
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