Chapter 6: Movement (Size of exoskeleton with growth)

Content standard:

Support, movement and growth in animals

Learning standard:

Relate the size of exoskeleton with growth

Lesson activity:
Hello everyone! How are you? Today we are going to learn about the growth of exoskeleton animal. Do you still remember what you have learnt in the last lesson? You have learnt that exoskeleton animals can grow in size. However, the way they grow are different from other organisms.

Diagram 1: Examples of exoskeleton animals

First let us discuss the meaning of growth. Growth can be described as irreversible increase in body size, mass and number of cells in an organism. Growth in multicellular organism can be measured using parameters like length, height, volume, dry mass and fresh mass over a certain period of time. Basically, the growth curve for most organisms is sigmoid curve or S-shape. The graph below shows example of human growth curve in sigmoid curve or S-shape.

Diagram 2: Growth curve of human

Now let us discuss the growth curve of an insect. It is not the same as human or other organisms growth curve. Why? Because their skeleton are outside their body and they need to remove their exoskeleton in order for them to grow in size. When the length of insects is plotted against the time, it shows very unusual, intermittent growth. The graph has a staircase shape.

Diagram 3: Growth curve of insect (exoskeleton animal)

The horizontal part indicate zero growth. It correspond to the time when the insects stop growing. The five vertical lines of curve are sudden growth spurts. This correspond to the time when the insects (nymphs) cast away their old external skeleton. This process is called moulting(menyalin kulit) or ecdysis.

Diagram 4: The moulting process of grasshopper

Ecdysis takes place each time it grows, right until it reaches the adult stage. During ecdysis, the old cuticle softens and a new exoskeleton forms below it. The old cuticle is then removed. The insects then swallows and retains air to inflate the new soft cuticle until it hardens. Before the new exoskeleton hardens, growth takes place actively to increase the size of the insect.This active growth during each ecdysis gives rise to the growth curve in the shape of a staircase. The diagram below shows the life cycle of an insects. e.g grasshopper.

Diagram 5: The lifecycle of grasshopper

Before we end the class today, let's watch this video.

Thank you for taking part in today's lesson. Make sure to submit your exercise today in WhatsApp group before 4pm. The exercise as below:

Exercise:
1. Draw the growth curve of an insect in an A4 paper.
2. Why does the growth curve of insects has a staircase shape?
3. Explain the metamorphosis (moulting) process of grasshopper based on the growth curve that you have drawn.

References:

• https://www.slideshare.net/nirmalajosephine1/biology-form-5-chapter-4-46-growth-in-multicellular-organisms
• https://www.slideshare.net/DhiaJiana/biology-form-5-chapter-4-47
• https://piximus.net/animals/this-is-how-grasshopper-moults
• http://animalessons.blogspot.com/2013/08/there-are-three-stages-in-grasshopper.html
• https://biologydictionary.net/exoskeleton/#foobox-2/0/Insect.jpg
• https://youtu.be/pMCg1oqWkT4