Life Cycles of Selected Animals

Embryo development and metamorphosis

Embryo development in mammals

In mammals, after fertilization, embryo development begins internally and proceeds through stages where the embryo gradually develops into a fetus. The fetus acquires features that are similar to those of an adult over time. After the gestation period, the fetus is expelled from the mother during birth. This process is largely controlled by hormones. The baby continues to grow into an adult without undergoing any drastic structural transformations, unlike in other animal groups such as insects, amphibians, and crustaceans, where development involves metamorphosis.

Metamorphosis

Metamorphosis refers to the process in which animals undergo a series of changes in structure and behavior as they mature from immature forms into adults. This process is characteristic of some species of insects, amphibians, crustaceans, and some fish.

Factors influencing metamorphosis

1. Hormones: Hormones like thyroxine play a crucial role in growth and development. For example, in amphibians, removing the thyroid gland prevents metamorphosis, even though growth continues.
2. Temperature: Higher temperatures often speed up metamorphosis, while lower temperatures can slow it down.
3. Nutrients: The quality of food (rich in proteins, fats, and minerals like iodine) affects the rate of metamorphosis. For example, amphibian larvae cannot metamorphose in water with insufficient iodine.

Types of metamorphosis in insects

Complete metamorphosis (Holometabolous):

• Insects like houseflies and butterflies undergo complete metamorphosis.
• The life cycle consists of four stages: egg, larva (caterpillar), pupa, and adult.
• Larvae (such as caterpillars) are distinct from adults in feeding habits and structures. They feed using mandibles to bite leaves, while adults have proboscises for sucking nectar.
• After multiple moults, the larva transforms into a pupa, secreting a protein case (cocoon). The pupa is immotile and non-feeding.
• The adult emerges from the cocoon, with wings fully developed, and begins its adult life.

Life cycle of a housefly:

• Eggs hatch into larvae (maggots) within 8–24 hours.
• Larvae feed and grow for several days, then stop feeding and form a cocoon (pupa).
• After several days, the pupa transforms into an adult housefly.

Incomplete metamorphosis (Hemimetabolous):

• Insects like cockroaches and grasshoppers undergo incomplete metamorphosis.
• Their life cycle includes three stages: egg, nymph, and adult.
• The nymph resembles the adult but lacks wings and functional reproductive organs. It grows in size through moults and eventually becomes an adult.
• Unlike the complete metamorphosis, there is no distinct larval or pupal stage.

Life cycle of a cockroach:

• Eggs hatch into nymphs, which undergo several stages (instars) before becoming adults.
• The nymph resembles the adult but lacks wings

Metamorphosis in amphibians

In amphibians like frogs and toads, metamorphosis occurs in five stages: egg, tadpole, tadpole with legs, young frog, and adult frog.

• Tadpoles begin life in water with tails and gills for swimming and breathing. They feed on algae.
• Metamorphosis begins with the development of hind legs, the appearance of lungs for breathing air, and changes to the digestive system to accommodate a carnivorous diet.
• Front legs emerge, and the tail is absorbed as the young frog matures into an adult, capable of living on land and feeding on insects.

Hormones in insect metamorphosis

1. Ecdysone (Moulting Hormone): Controls moulting, necessary for growth in insects. It triggers the shedding of the old exoskeleton.
2. Neotonin (Juvenile Hormone): Maintains the insect in its larval stage. High levels of this hormone prevent transformation into the pupa, while low levels allow the transition.

Importance of metamorphosis

1. Ecological niche separation: Insects that undergo metamorphosis often avoid competition between the larval and adult stages by occupying different ecological niches.
2. Growth and survival: Metamorphosis allows for better adaptation to different life stages (e.g., larvae feeding on different food sources than adults).

purposes.

Life cycle of a mouse

Mice, like all mammals, start their life cycle with fertilization, followed by the formation of a zygote. The mating process occurs during the oestrus period (lasting 4–6 days), after which a copulation plug forms, though it is not necessary for pregnancy. The gestation period lasts about 19–21 days, and the female gives birth to a litter of 3–14 pups. These pups are born blind and hairless and gradually develop fur, open their eyes, and become independent after around 21 days. Both female and male mice reach sexual maturity early, with females around 6 weeks and males around 8 weeks of age.