Mosquitoes

DESCRIPTION

Order: Diptera (‘two-winged’)

Characteristics:
Adults: Globular head, a large part of the surface of which is taken up by the compound eyes; a pair of antennae, which are about three times as long as the head, are somewhat hairy in the female and quite bushy in the male; this difference provides a ready means of distinguishing between the sexes with the naked eye. Also on the head a pair of palps, one on each side of the proboscis; in the female they are smooth, in the male they have tufts of bristles. Mouthparts form a long thin projecting proboscis which in the female are designed for piercing; there is a single pair of membranous wings which bear tiny scales along the veins as well as a fringe of scales along the hind margin. The wing venation is also typical with 6 longitudinal veins of which the second, fourth and fifth are forked; tarsi are 5-segmented.

Larvae:
Entirely aquatic. They have a well developed head followed by a swollen unsegmented thorax; abdomen is segmented and there is a pair of spiracles on the last but one segment through which they breathe at the water's surface. Tufts of bristles arise from many of the body segments.

Pupae:
Comma-shaped with a curved tail which ends in a pair of paddles; head is equipped with a pair of respiratory trumpets through which the pupa breathes. The fused head and thorax show traces of the wings and long legs of the adult which are developing inside.

Family: Culicidae (mosquitoes)
Metamorphosis is complete with egg, larval, pupal and adult stages. There are about 3000 species of mosquitoes in the world.

Sub-Family: Anophelinae
Genus: Anopheles spp.


Characteristics:
Wings speckled with dark and pale-coloured scales; there are no scales on the abdomen; female palps almost as long as the proboscis, the male palps are usually clubbed. When resting the abdomen is held at an angle from the surface on which the mosquito is standing, forming a straight line with the proboscis (e.g. Anopheles gambiae).

Sub-Family: Culicinae
Characteristics:
Female palps are short and the abdomen is covered in scales. The resting stance is parallel to the surface. Genus: Culex spp.
Characteristics:
Vary in size but all have a blunt tipped abdomen and usually lack distinctive markings on the body or wings (e.g. Culex quinquefas-ciatus).

Genus: Aedes spp.
Characteristics:
Medium to large; most have pointed abdomen and well separated eyes; usually distinctly marked by patches of dark and light scales; often general appearance of black with silver markings (e.g. Aedes aegypti).

Genus: Mansonia spp.
Characteristics:
Medium sized and heavily covered in scales which are of mixed colours giving a ‘salt and pepper’ appearance. Larvae are rarely seen since they remain submerged and pierce plant tissues with their breathing siphons to obtain oxygen (e.g. Mansonia uniformis).

Note: There are several other Genera of mosquitoes some of which are implicated in disease transmission but others are of no risk to humans. Space does not allow consideration of all the other Genera here.

DISTRIBUTION
Mosquitoes are ubiquitous insects, they can be found in nearly every type of climatic region of the world from the arctic regions to the tropics, surviving severe winters or dry seasons depending on the region. Depending on species they can be found breeding in all types of water; from heavily polluted, to clean; from small collections of water in tin cans, to pools or streams; such is their adaptability. Their distribution is increased and aided by transport systems such as boats and aircraft and non-indigenous species have been introduced to new territories in this way, even infected mosquitoes have been transported to temperate climates, thereby transmitting tropical diseases.

SIGNIFICANCE
Primarily a seasonal biting nuisance in cooler climates but in the tropics they can also be major vectors of disease. They can transmit many diseases to both humans and animals, e.g. malaria, filariasis, yellow fever, encephalitis, and dengue fever. Only a few species of mosquitoes are implicated as vectors of human diseases. It is those which utilise humans as their primary host and are also willing to enter houses in their pursuit of a blood meal which are the most important. Some species do not feed on humans at all but utilise other mammals, birds or even reptiles as their hosts.

Statistics show that 270 million people are infected with malaria alone and of those up to two million will die annually. It is this that makes the mosquito the most dangerous creature in the world.

Examples of human disease transmission by mosquitoes