Biology, SS 3, Week: 2
Topic: Development of New Organisms
Introduction
The development of an organism varies from one species to another. In sexual reproduction, the male animal produces the sperm while the female produces the egg or ovum. The fusion of the sperm and the egg in the Fallopian tube of the female during mating is called fertilization. The fusion of these sex cells, i.e., sperm and egg, results in the formation of what is called zygote. The zygote undergoes cell division to form a developing organism called embryo.
Stages in the Development of Toad
Frogs and toads, although they have several differences, share the same general life cycle with all amphibians. The process begins with a male and female toad laying and fertilizing eggs. It progresses through various stages. Along the way a young toad has a tail and uses external gills. Later in the process, it loses the tail, grows legs, and develops lungs instead of gills. At each stage the toad is referred to by a different name. Each stage takes place at a certain number of weeks into the cycle.
Spawn
The female toad returns to a river or stream. A male toad finds her and jumps on her back, a process called amplexus. He wraps his legs around her. As she lays eggs in a long chain, he fertilizes them. This egg mass is called spawn, which is different than the individual eggs. The spawn only refers to the group of eggs as a whole, but doesn’t refer to how each egg develops on its own.
Egg
The yoke within the egg splits into two sections. This splits again to create four sections and continues splitting in this fashion. The yoke begins to look like the raspberries inside a jello cup. After this the embryo begins to develop, it starts looking like a tadpole. The embryo grows longer. If you look closely, you will see it swim about in the egg, feeding upon the yoke to survive.
Tadpole
The egg begins to hatch between days 6 and 21, depending upon the type of toad and other factors, such as environment. It breathes through external gills, which can be seen from the outside. The tadpole does not swim for the first 7 to 10 days, but floats around in the plants, while it continues to grow. Four weeks into the process, the skin grows over the gills and they become internal gills. Between six to nine weeks after hatching, the tadpole grows legs, but keeps its tail.
Youth and Adulthood
Between weeks 9 and 12, the tail shortens. Front legs begin to grow and the lungs develop. At week 12, the toad looks like other toads, only much smaller. It does not leave the water until somewhere between 12 and 16 weeks after hatching. It will return again later to mate and fertilize more eggs. Until then, it lives away from water, eating insects and other bugs.
Role of Hormones in Development of Toad
In toad and other amphibians, metamorphosis is controlled by the hormone called thyroxine. It is produced by the thyroid glands in the region at the junction of the head and the trunk. For thyroxine to be made, there must be iodine in the water. The tadpole cannot change into adult form in the absence of this hormone. However, an increased level of thyroxine hastens metamorphosis.
Metamorphosis in Insects
Metamorphosis refers to the process through which insects develop, grow and change form. Metamorphosis actually infers “change.” The term is of Latin and Greek origins: Meta means “change” and morphe means “form.” Metamorphosis describes the series of changes through which an insect passes in its growth from the egg stage through the immature stages (e.g. nymph, larva or pupa) to the adult stage. Insects grow in stages. The cycle of stages is called metamorphosis. For many insects, the stages are so different from one another that you might not recognize them as the same animal.
Types of Metamorphosis
There are two forms of insect metamorphosis, these are:
- Incomplete Metamorphosis (Egg > nymph > adult)
Insects with incomplete metamorphosis have three life stages: egg, nymph and adult. Adults and nymphs of these insects usually feed on the same foods. These insects may have wings, which if present, develop externally. Young are known as nymphs and there is no resting stage (pupa). Examples of insects with incomplete metamorphosis are grasshopper, cockroach, dragonfly, locust, aphids, termites and praying mantis. - Complete Metamorphosis(Egg > Larva > pupa > adult)
Insects with complete metamorphosis have four (4) life stages: egg, larva, pupa and adult. Larvae are very different in form from the adult. The active immature stages are known generally as larvae. These insects also have a resting stage known as a pupa. The pupal stage is a transition stage, when the larva transforms into the adult. Wings, if present, develop internally within a pupa. The pupa molts to the adult form. Examples of insects with complete metamorphosis are housefly, mosquito, butterfly, bees, beetles and many other insects.
Differences Between Complete and Incomplete Metamorphosis
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Complete Metamorphosis |
Incomplete Metamorphosis |
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1. |
Four stages of development are involved |
Three stages of development are involved |
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2. |
There is presence of pupa (resting stage) |
There is absence of pupa (no resting stage) |
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3. |
There is no resemblance between pupa and adult |
There is a resemblance between nymph and adult |
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4. |
There is absence of nymph |
Nymph is present |
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5. |
Examples of complete metamorphosis are housefly, mosquito, etc. |
Examples of incomplete metamorphosis are cockroach, grasshopper, etc. |
Life History of the Housefly (An Example of Complete Metamorphosis)
The basic cycle of housefly begins with an egg, then develops through a larva phase, a pupa phase, and finally, into an adult. Fertilized egg to adult spans a mere seven to 10 days.
After a male housefly chases down and fertilizes a female counterpart, the female lays her eggs. Houseflies are solitary creatures. Like the rest of the insect world, males and females do not stick together after mating and, unlike nesting insects; females do not care for or protect eggs. Females simply leave the eggs where they will be safe from predators and have plenty to eat upon hatching.
The female housefly deposits her eggs in the crevices and corners of the same kinds of decaying organic matter adults feed on. Within a day, the first larvae begin to emerge from the eggs. Also known as maggots, these worm-like creatures are little more than fleshy, sectionless tubes with hooked mouth parts used for feeding.
The maggots grow rapidly. In less than two days they’ve doubled in size and therefore must molt. Molting is a process common to many invertebrates through which a growing insect sheds its former exoskeleton and grows a new one. A maggot will molt twice more, emerging larger and more developed each time.
Following its third molt, larvae will burrow deep into the substance they’ve been feeding on. Their skins will darken and harden as they enter the pupa stage. Inside this protective shell, the larva will fully develop the body segments and appendages of an adult housefly.
The only visible addition to the emerging housefly is a swollen bump on the fly’s head, used to break through the shell. Since the housefly doesn’t have teeth or jaws to chew its way out, it uses this fluid-filled pouch to break through the pupae shell. Once fully emerged, the bump deflates back into the fly’s head.
A new adult housefly has, at most, three months to reproduce before it dies. With so many predators, a housefly’s average lifespan is even shorter: 21 days. Each female can lay up to 900 eggs during her brief life.
Life History of Cockroach (An Example of Incomplete Metamorphosis)
The Cockroach Life Cycle and Behavior
As with many animals, cockroach reproduction relies on eggs from a female and sperm from a male. Usually, the female releases pheromones to attract a male, and in some species, males fight over available females. But exactly what happens after the male deposits his sperm into the female varies from species to species.
Most cockroaches are oviparous (their young grow in eggs outside of the mother’s body). In these species, the mother cockroach carries her eggs around in a sac called an ootheca, which is attached to her abdomen. The number of eggs in each ootheca varies from species to species. Many female cockroaches drop or hide their ootheca shortly before the eggs are ready to hatch. Others continue to carry the hatching eggs and care for their young after they are born. But regardless of how long the mother and her eggs stay together, the ootheca has to stay moist in order for the eggs to develop.
Other cockroaches are ovoviviparous (their young grow in an ootheca outside of the mother’s body, the cockroaches grow in an ootheca inside the mother’s body. In a few species, the eggs grow inside the mother’s uterus without being surrounded by an ootheca. The developing cockroaches inside feed on the eggs’ yolks, just as they would if the eggs were outside the body. One species is viviparous (its young develop in fluid in the mother’s uterus the), the way most mammals do. Ovoviviparous and viviparous species give birth to live young.
Ootheca and Nymph of the Oriental Cockroach
Whether mother cockroaches care for their young also varies from one species to another. Some mothers hide or bury their ootheca and never see their offspring. Others care for their offspring after birth, and scientists believe that some offspring have the ability to recognize their mothers. The number of young that one cockroach can bear also varies considerably. A German cockroach and her young can produce 300,000 more cockroaches in one year. An American cockroach and her young can produce a comparatively small 800 new cockroaches per year.
Newly hatched cockroaches, known as nymphs, are usually white. Shortly after birth, they turn brown, and their exoskeletons harden. They begin to resemble small, wingless adult cockroaches.
Nymphs molt several times as they become adults. The period between each molt is known as an instar. Each instar is progressively more like an adult cockroach. In some species, this process takes only a few weeks. In others, like the oriental cockroach, it takes between one and two years. The overall life span of cockroaches differs as well — some live only a few months while others live for more than two years.
Cockroaches generally prefer warm, humid, dark areas. In the wild, they are most common in tropical parts of the world. They are omnivores, and many species will eat virtually anything, including paper, clothing and dead bugs. A few live exclusively on wood, much like termites do.
