Read this article to learn about the Structure and Life Cycle of Mosquito !
Genus: Anopheles or Culex
Mosquito IS a common insect found almost everywhere. In some species of mosquito, the females feed on humans, and are therefore vectors for a number of infectious diseases affecting millions of people every year.
The head is small and spherical in shape. It bears two large compound eyes and a pair of long, many-segmented antennae.
The thorax has three segments prothorax, mesothorax and metathorax (fig. 8.5, 8.6). Each thoracic segment bears a pair of legs. Mesothorax bears a pair of wings and prothorax a pair of spiracles near the legs.
The abdomen is long, slender and made up of 10 segments. Second to eighth abdominal segment are normal and bear a pair of spiracles, 8th segment bears the terminal anus and 9th bears the terminal gonopore. In females the 10th segment bears a pair of anal cerci, sandwiching a small post-genital plate. In males 9th segment bears a pair of clawed claspers and 10th modified into a copulatory organ aedigus.
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.
The mouthparts of male mosquitoes are ‘sucking type’ to suck the nectar of flowers while those of females are of ‘piercing and sucking type’ to pierce the skin of warm-blooded vertebrate hosts and suck their blood for feeding.
Life cycle of mosquitoes:
Mosquitoes copulate while flying during the night. It is believed that the pitch of sound produced during flight is higher in females, and this helps the male mosquitoes to locate the female mosquitoes and copulate.
After copulation the female Anopheles lays about 40 to 100 and female culex about 150 to 300 eggs after midnight in standing water of some pond, ditch, pool, puddle, lake, well, water-storage tanks etc., or even in water containers in our houses. A blood-meal by the female is necessary before oviposition.
While laying its eggs one-by-one, the female culex holds these upright upon water surface with the help of its hind legs and plasters these with each other. Hence, its eggs occur in boat-shaped floating clusters called “rafts”. Female anopheles lays its eggs singly.
Eggs of culex are somewhat elongated and cigar shaped with their narrower end directed upwards in the floating rafts. The lower, broader end bears a micropyle cap. In the beginning, the eggs are white, but gradually these acquire a grey colour.
The eggs of Anopheles are smaller, spindle-shaped and black. On each side of its middle, thicker part, the egg bears an umbrella-like membranous structure filled with air and called “air float”. These floats give buoyancy to the egg.
Within one to three days, the embryonic development is completed in an egg and a larva, called wriggler, hatches out in water from it. The larva of Culex hatches out by breaking open the micropyle cap. In the beginning, it is about 1 mm. long and transparent. It actively swims in water by wriggling, feeds upon aquatic micro-organisms at bottom, and grows by undergoing four moults. The larva of Anopheles, however, feeds upon the water surface.
The body of larva is distinguished into head, thorax and abdomen. The head is relatively large and somewhat flattened. On each lateral side, it bears a large compound eye and a small simple eye or ocellus. Just in front of each compound eye is a shot antenna.
The tip of the head is marked by larval mouth. The mouth is ventrally bounded by a lower lip or labium and laterally by a mandible and a bristle-bearing maxilla on each side. Dorsally, a pair of plate-like lobes, bearing hard setae, project infront from upper part of the mouth. The larval mouthparts are of chewing type. Food particles, coming in contact with mouthparts, are caught and chewed before swallowing.
Soon after the fourth moult, the larva becomes inactive, sinks down to the bottom and metamorphosis into a comma-shaped stage called pupa. Unlike the pupa of housefly, the pupa of mosquitoes is without a tough covering and it is as active as the larva. The pupa of Culex is grayish, while that of Anopheles is greenish grey. Its body is differentiated into two regions—a cephalothorax in the front region and abdomen in the back region.
The abdomen is narrow, 9-segmented and curved towards the ventral side. A pair of small, trumpet-shaped ‘respiratory horns’ helps in respiration. It has terminal spiracles and remain connected with tracheal system of body. Most of the time, the pupa remains at water surface with its respiratory horns protruding out in air for breathing.
In the pupa of Anopheles, the respiratory horns are relatively shorter, but the spiracles are broader (fig. 8.7, 8.8).
The 8th abdominal segment of pupa bears a pair of large, backwardly directed, leaf-like fins or paddles which help it in darting. Each paddle terminally bears a single, slender spine in the pupa of culex and two in case of anopheles. Around the spiracle at the tip of each respiratory horn, there is a crown of fine bristles which prevent entrance of water into the horn. Besides these structures, all abdominal segments bear tufts of long bristles.
Metamorphosis of pupa:
Pupa has no mouth or anus. Hence, it is non-feeding. It depends only upon stored food. That is why, its life is very short (2 to 7 days). During this period, active histolysis and histogenesis occur inside its body as described in case of housefly. These processes of metamorphosis in the pupa can be observed from outside through the semitransparent pupal skin.
Metamorphosis in the pupa results into the formation of young mosquito. Eventually, the pupal spin splits in mid-dorsal line of cephalothoraxes, between the respiratory horns, and the young mosquito, called imago, hatches out from it. The pupa at this time essentially keeps floating at water surface. After hatching, the imago keeps sitting upon the dead pupal skin for a while to dry its wings and then flies away.
From egg to imago, the life cycle of mosquito is completed in about a month. The imago becomes sexually mature after about a week of hatching. The life span of male mosquito is hardly of three weeks. It generally dies soon after copulation. The female mosquitoes remain alive for one to several months.
Mosquitoes are a vector agent that carries disease-causing viruses and parasites from person to person without catching the disease themselves. The principal mosquito borne diseases are the viral diseases yellow fever, dengue fever and Chikungunya, transmitted mostly by the Aedes aegypti, and malaria carried by the genus Anopheles. There are many methods used for mosquito control.
Depending on the situation, source reduction, bio-control, larviciding (control of larvae), or adulticiding (control of adults) may be used to manage mosquito populations. These techniques are accomplished using habitat modification, such as removing stagnant water and other breeding areas, spraying pesticide like DDT, natural predators. The dragonfly nymph eats mosquitoes at all stages of development and is quite effective in controlling its populations.