Class 9 Science Note
Life Cycle of some Insects
The life cycle of the mosquito completes in four stages.
A mosquito lays egg usually in lakes, ponds, fields, puddles and in stagnant water. An anopheles mosquito lays 40 to 400 eggs at a time. Those eggs remain floated in water singly without being stuck to one another. On the other hand, a culex mosquito lays 300 eggs at a same time. Those eggs remain floated on water remaining attached together forming a shape of a boat.
Under favorable condition, eggs hatch out larva within 2 to 3 days. They are seen transparent and live on algae. At this stage, they are voracious. A larva has broad thorax with three segments joined together. The abdomen is also segmented. The head has a pair of compound eyes. The bristles are found on thorax and abdomen too. On the last segment of the abdomen, there are four gills and a respiratory siphon. Larva remains suspended near the surface of the water. An anopheles larva lies parallel to water surface whereas a culex larva lies making an angle with water surface.
The larva develops into pupa within a week if it gets suitable temperature and sufficient food. This is in fact, a resting stage in many insects but a mosquito pupa is also very active. It swims a lot and breathes through siphon. However, it does not eat any food. The duration of pupal stage is 2 to 7 stage. The head and thorax of a pupa are fused together called cephalothorax. The abdomen is curved.
A fully developed mosquito emerges out of the pupal case and flies away. The female one begins to lay eggs after it mates with the male one at maturity.
Disadvantages of mosquito
When a female anopheles mosquito bites a person infected with malaria, it suck up blood along with plasmodium. When the mosquito bites a healthy person, the plasmodium enters into the blood along with saliva. Thus, malaria is transmitted to healthy persons from infected persons.
Preventive measures against malaria are as follows:
a. Mosquito net should be used while sleeping.
b. The pits, damp, ditch with dirty water should be filled up with soil or insecticides should be used.
c. Anti mosquito coils, mats, incense should be used or anti mosquito cream should be applied.
Silkworms are the butterfly like insects that produce silk. They feed on the mulberry plants
A silkworm completes its life cycle in about 45 days. It has four stages in its life cycle such as egg, larva, pupa and adult.
A female silkworm produces 300 tiny eggs on a mulberry leafs only once and dies. Eggs remain dormant for many days in cool place. But they hatch out larva within a week if temperature is increased. It takes about 12 days to hatch larva in a suitable temperature.
The larva grows on eating the mulberry leaves. They resemble the caterpillar of the butterfly. They feed on the leaves for 25 to 32 days. During every moulting, they do not eat and move for 20 to 24 hours. A fully grown larva starts to produce silk fibers and encloses itself within the fibers forming a cocoon.
The pupal stage is an inactive stage. The pupae don’t eat. They remain at rest at a place; however changes occur inside them. The duration of stage is 12 to 14 days.
A fully grown pupa has three pairs of legs and a pair of wings. It flies away after drying out wings after sometime. An adult silkworm survives for 5 to 6 days.