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Life cycles A: General stages -
Butterflies and frogs
Life cycle stages
The life cycle of all organisms usually involves some
variation of each of these stages:
- Mature adults produce gametes (sperm and eggs in animals; pollen
and ovules in flowering plants).
- In animals, adults attract a mate of the same species but opposite
sex using sounds (frog calls), scents (dogs on heat), visual appearance
(breeding plumage in birds), and courtship rituals (lyrebird dance,
sea gull displays). This ensures the fittest individuals breed at
the best time to aid survival of the species.
- Mating occurs to bring gametes together (pollination in flowers
is helped by insects, birds, and wind).
- Fertilisation of gametes occurs to form the zygote, the first cell
of the new individual. This may occur externally as in fish, or internally
in land animals as the sperm must swim to the eggs.
- The zygote starts dividing to form an embryo which continues to
develop (in shelled eggs, or internally inside the mother).
- Adults build a nest (or the female's womb develops) to protect developing
embryos.
- The young hatch from eggs, or are born.
- Parents protect the young and help development of survival skills
through learning and instinct.
- The young grow and develop to reproductive maturity.
Any long-term disruption of any stage can result in extinction.
Some life cycles studied in Primary Schools: Butterflies and frogs.
Butterflies
This life cycle involves the adult male detecting a female by smell,
brief courtship display, and mating (sperm enters female and fertilises
eggs). The female lays eggs on a plant. The embryo develops into a caterpillar
using food stored in the egg. The caterpillar (larva) hatches, eats
the eggshell and then the plant. As it grows, it moults and grows new
skin several times to accommodate increased size. It increases in birth
weight by approximately 3000 times. Finally, it spins a cocoon in which
the pupa (chrysalis) changes drastically (metamorphosis) into the emerging
adult butterfly. The role of the short-lived adult is to fly to a new
area, mate and lay more eggs.
Frogs
Adult frogs locate and attract mates by "singing" a call
recognised only by that species. This prevents the loss of time, energy
and gametes in mating with other species resulting in no offspring.
The male grips onto the female's back and deposits sperm as he squeezes
out her jelly-covered eggs into the water. The jelly surrounding the
eggs protects against germs, sticks them together, and absorbs warmth
from the Sun, but it is not a food source. Over 14 days, the developing
tadpole uses the egg's food and grows rapidly, then it escapes. It uses
its sucker mouth to attach first to the jelly sac and later to weeds
which it eats. In many common species, hind leg buds appear at 5 weeks,
and by 8 weeks they are fully developed. At 3 months a complete body
change occurs (metamorphosis). It casts its skin, gills disappear, forelimbs
grow, the tail is absorbed as a food supply, and the froglet is formed.
It now eats insects, breathes with lungs, and grows to sexual maturity.
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