4.1. Biology

Mosquitos are small, two-winged, blood-sucking insects. This is the most important group of insects from a medical point of view. Mosquitos are found near water, where their larvae develop. The biological cycle has four stages: egg, larva, pupa, and adult, the first three of which are aquatic.

The eggs are laid individually either on moist land subject to flooding (in genus Aedes) or on the surface of the water (in Anopheles and Aedes); or they are laid in groups on the water surface (in Culex). Eggs laid on water hatch in two or three days, whereas eggs laid on land hatch after they are flooded. The submerged larvae position themselves parallel (Anopheles) or oblique (Culex and Aedes) to the surface of the water, and breathe air through a breathing tube (Culex and Aedes) or a pair of posterior spiracles (Anopheles). The larvae feed on organic matter suspended in the water, and pass through four progressively-larger stages before developing into pupae, which do not feed but release the adult insects after about two days.

The time required to complete all stages of development varies according to species, temperature, and the availability of nutrients. Adult mosquitos are able to mate as soon as they emerge; mating occurs in-flight. Both males and females feed on plant juices, but females also require periodic blood meals in order for their eggs to mature.

When seeking a host from which to draw blood, mosquitos may fly three kilometres or more from their larval development site, and wind-blown mosquitos may travel considerably greater distances. However, the first suitable animal encountered is most often targeted as a source of blood. Generally speaking, the closer a refugee camp is to a mosquito development site, the greater the likelihood of heavy infestation.

Animals give off carbon dioxide when they breathe, and carbon dioxide is the most important attractant of female mosquitos. When they detect it in the air, they fly upwind toward the source animal(s). Therefore, it is useful to know the direction of prevailing winds in an area, so that camps may be set up downwind of major larval development sites.

There are more than 3,000 species of mosquitos in the world but, in the tropical savannah zone of Africa, the three most important in disease transmission and nuisance are Anopheles gambiae, Anopheles funestus, and Culex quinquefasciatus, which is active only by night. The presence of black spots on the wings of An. gambiae and An. funestus easily distinguishes them from Cx. quinquefasciatus. Also, the body of a resting adult Anopheles forms a 45° angle with the supporting surface, whereas that of a Culex is parallel (fig. 1). There are also several species of Aedes mosquitos in forest zones, and they can be very aggressive biters, even during the day. The Aedes species, belonging to Stegomyia subgenus, are easily recognised by the presence of white bands on their black legs.

Figure 1 - Differential characteristics of mosquitos of the genera Anopheles and Culex

An. gambiae develops in the fresh, open water of small temporary pools (pl. 5), or larger bodies of water. Flooded brick holes, although the water they contain may be muddy, provide excellent larval development sites. Population density can increase rapidly during the rainy season. An. gambiae actually comprises a complex of species having different biological characteristics and varying capacities for transmitting malaria.

An. funestus colonises permanent and shaded collections of water, particularly ponds, paddy fields, and other irrigated plantations. This mosquito is particularly abundant in the dry season.

Cx. quinquefasciatus colonises polluted water in waste-water ditches and drains, septic pits, and cesspools. It is present all year round in areas having sufficient water for the development of its larvae.