Mosquitoes occur all over the world. They are most plentiful in the tropics, though in the brief Arctic summer, enormous swarms occur in Lapland, Finland, northern Siberia and North America. There are over 1,500 species known, twenty – eight of which are native to Great Britain. The female Anopheles mosquito remains the most common species due to its notorious habit of feasting on human blood and transmitting the organism called Plasmodium that causes malaria
The female lays up to one hundred eggs singly in water. They are boat-shaped, less than one millimeter long and equipped with two lateral floats. In a few days, the larva emerge. They are very active creatures, commonly known as “wrigglers”. The head is large, the three thoracic segments are fused to form a broad middle region, and the abdomen has nine segments. On the head are short antennae, a pair of plumed brushes at the sides of the mouth, and mandibles and maxillae which comb food particles out of the mouth, and mandibles and maxillae which comb food particles out of the brushes. These brushes sweep through the water at the rate of 150 strokes per minute, small particles are enlarged in them: these constitute the food. The penultimate segment bears two spiracles. When it comes to the surface, the larva lies supported by the surface film with the spiracle just breaking the surface: they secrete an oily substance which prevents the entry of water. The body is supported horizontally by tufts of palmate setae which cling to the surface film. The head is rotated through 180o so that its ventral surface is upward and the larva proceeds to sweep the underside of the film for food. After a period varying from several days to several months, the pupa is formed. Under good conditions of temperature and food, the larva is full grown in less than a week.
The pupa of the mosquito is very active also. It has a large head and a curved body and is sometimes described as comma – shaped. It does not feed but has to come to the surface to breathe: the spiracles are now borne not feed but has to come to the surface to breathe: the spiracles are now born on the dorsal surface of the head. This stage, during which the metamorphosis takes place, usually lasts less than a week. The pupal skin splits along the dorsal side and the imago emerges in a few minutes.
The sexes of the mosquito can be distinguished by the condition of the antennae. In the male, they are tufted while in the female they are slender and beset with fine setules. The body is very slender and covered with setae and fine scales. On the head are the large compound eyes, but no ocelli. The mouth parts are adapted for piercing and sucking, though it is only the female that sucks blood. The fore – wings are narrow, sparsely veined and fringed with setae. The halters are well marked and very typical. In adaptation to settling gently, it has very long and extremely delicate legs. The adults often hibernate in dark sheltered places.
Mosquitoes are notorious for conveying pathogenic organisms but it must be remembered that relatively few species do this. The most dangerous mosquitoes are Anopheles maculipennis, which conveys malaria parasites, Culex fatigans, conveying filarial worms, and Stegomyia fasciata (Aedes aegypti) which transmits the unknown organism causing yellow fever. Malaria was common in South – eastern England until the end of the last century: the disease was known as dengue.