In this essay we will discuss about mosquitoes. After reading this essay you will learn about: 1. Introduction and Economic Importance of Mosquitoes 2. General Organisation of Mosquitoes 3. Life-History 4. Prevention against Mosquitoes.
Introduction and Economic Importance of Mosquitoes:
The mosquitoes are insects which belong to the order Diptera. These nocturnal insects are well-known as vectors of various disease producing parasites. Following genera of mosquitoes are well-known throughout the world.
A. Anopheles sp.—Carrier of malarial parasite which causes malaria.
B. Culex sp.—Causes malaria in birds and filariasis in man.
C. Stegomyia sp.—Causes yellow fever.
D. Aedes sp.—Responsible for dengue fever.
It may be mentioned here that only the female mosquitoes are of blood-sucking type, whereas males feed on plant saps and are innocent of carrying any disease.
General Organisation of Mosquitoes:
External Feature:
The body of these small size insects are divided into three parts—Head, thorax and abdomen. In between head and thorax, is a short neck.
Head contains:
(i) A pair of prominent bean-shaped compound eyes;
(ii) A triangular lobe clypeus, bearing a pair of elongated antennae (bushy in males) and
(iii) Number of appendages around the mouth.
Appendages around the mouth. In the female mosquito the mouth parts are planned for piercing and sucking, but in the male it is only for sucking.
The appendages around the mouth may be broadly grouped into two parts:
(1) Palp, and
(2) Proboscis.
1. Palp:
Paired processes, one on each side of the proboscis. In male, the palps are much longer than the female.
2. Proboscis:
Well-developed in female; consists of six elongated pieces united together at the base by a sheath.
It consists of following structure:
(a) Mandible—one pair, needle-like with smooth sharpened tip.
(b) 1st Maxilla—one pair, needle-like with serrated sharpened tip.
(c) 2nd Maxilla—one pair, fused together to form labium; the tip of the labium is bifurcated into two white lobes called labellae, which possesses numerous sensory hairs at the free end.
(d) Labium and Epipharynx:
These are impaired processes, united together by chitinous hypo-pharynx. The free end is very sharp and the ventral side is provided with a groove. The salivary ducts run along this groove and opens at the tip. The labium, epipharynx and hypo-pharynx together form a tube for sucking the fluid. In males the mandibles are absent and the hypo-pharynx is united with the labium.
Thorax:
It is divided into three segments of which one is very large and other two are very small. It carries three pairs of walking legs and one pair of wings. The legs are built up in the same plan of an insect. Only difference is that tarsus is long and coxa is short.
The wings are meant for flight and in the case of Anopheles these are spotted. The second pair of wings are reduced to two stumpy processes called halters which act as balances and sensory structure. Breathing aperture or stigmata is present on the lateral side.
Abdomen:
Long and narrow abdomen is divided into nine segments and is covered by imbricate scales. No appendages, excepting a pair of knob-like ovipositor in fences, are seen in the abdomen. Anal opening is on the eighth segment and the ninth segment carries the reproductive opening.
Internal Structures:
Of the different internal structures (as the process of nutrition is very peculiar in mosquito), only the alimentary system will be discussed.
Alimentary System:
It consists of alimentary canal and associated glands. The alimentary canal is built up in such a fashion that it helps to have a heavy meal.
It contains following structures:
1. Mouth—associated with several appendages which have already been described. It leads to pharynx.
2. Pharynx—acts as a pumping organ to suck the fluid.
3. Oesophagus—narrow straight tube with three reservoirs at two posterior end. Oesophagus opens into the stomach and the opening is guarded by an oesophageal valve to check the flow of blood to the stomach.
4. Stomach or midgut—a. straight tube continuous with the hindgut. At the posterior-most end thread-like Malphigian tubules open.
5. Intestine or hindgut—near the opening of Malphigian tubules originate the hindgut. It runs straight up to the anus.
In the thoracic region and under the alimentary canal, there are two sets of salivary glands. Each set consists of three lobes. The secretion of the glands, saliva, contains anti-coagulating enzymes which prevent the coagulation of blood in the mouth parts. At the time of feeding, the female mosquito sets the forked tip of the labium on the skin to support the entrance of the six piercing stylets.
The saw-like tip of the first maxillae cut, the skin and all the six stylets penetrate within it. The blood is sucked by the action of pharyngeal wall through a food-tube which is formed by labium-epipharynx and hypo-pharynx.
Immediately after piercing, the mosquito drains saliva which contains enzymes for the prevention of coagulation. Sporozoites of malarial parasites also enter into the blood stream at this stage. The meal of blood which is drawn is stored within the oesophageal diverticula.
Life-History of Mosquitoes: (Fig. 115):
Fertilization is internal. After mating the female mosquito comes near water to lay eggs. It prefers stagnant water which is of low depth. Eggs of anopheles remain scattered and are provided with lateral air floats. The female culex mosquito arrange her eggs by hind legs and glue them together to form a raft.
From the eggs develop short larvae which can dive into the water. The body of larva is divided into three parts—head, thorax, and abdomen. Head contains a pair of lateral compound eyes, a pair of antennae and mouth with a pair of maxillae, a pair of mandible and a bunch of hair-like feeding brushes.
The larvae eat voraciously different micro-organisms which are driven towards the mouth by the active movement of feeding brushes. Thorax is the broadest part and contains only lateral tufts of long hairs.
The narrow abdomen is provided with bundles of hairs on lateral side, long pointed siphon, former leaf-like tracheal gills and a bundle of elongated rudder bristles. The siphon, which contains the air tubes, is for aerial respiration and contains in its free end two spiracles with five terminal flaps.
The air tubes of siphon are in connection with the internal tracheal network. The tracheal gills are for aquatic respiration. A secredon from a special gland, called pre- spiracular gland, present around spiracle helps the larvae to alight on the surface of the water. The rudder bristles help in guiding movement under water.
The larva of anopheles mosquito floats parallel to the surface of the water and possesses a very short siphon, whereas the culex larva remains under water with its head downwards and possesses a long pointed siphon. Gradually, the larva changes into pupa. The pupa of mosquito is very mobile but do not eat anything. The anterior part of the body is broad and massive and is known as cephalothorax.
The posterior part is represented by a narrow abdomen having nine segments. A pair of short respiratory trumpets are placed on the dorsal side of the thorax and are guarded by numerous hairs which prevent the entrance of water into it. These trumpets act as respiratory siphon of the larvae. On the eighth abdominal segment there is a pair of large fins which work as organs of swimming.
Within the body of the pupa rapid transformation takes place. Most of the pupal structures are used up as nutritive material of the cells which are forming adult structures.
When the adult is fully formed, the pupa comes near the surface of water. The pupal skin breaks along the dorsal side and adult mosquito comes out. Immediately after exit, the mosquito rests for a while on the floating shell and after a few minutes drying up, takes off into the air.
Prevention against Mosquitoes:
The mosquitoes are menace to the human being. By transmitting deadly diseases like malaria, filaria, yellow fever, dengue, it not only kills many human and domesticated animal lives, but also incapacitate millions of working hands. To prevent the national damage suitable measures should be and must be adopted.
Following measures are particularly recommended:
1. To clean and remove the swamps and stagnant water, so that there is no breeding ground of mosquitoes.
2. To maintain cleanliness in and around the house.
3. To spray kerosene, detergents and insecticides in all ditches, crevices and small stagnant watery areas.
4. In ponds and pools where insecticides are poisonous, biological control should be introduced by the help of larvae-eating fishes.
5. To use mosquito-nets and curtains for preventing the entrance of adult mosquitoes.