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Life Cycles and Dispersal : Question 3
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Insects, like all arthropods, grow in stages. This is because they have an exoskeleton surrounding their body. The exoskeleton is hard and it cannot expand. Every so often the insect will lose its old exoskeleton and grow a new one (this is called moulting). The new exoskeleton is soft to begin with so the insect's body can expand for a while. While the insect is moulting it cannot feed itself so it will not grow in mass.

Usually the insect inflates its soft new exoskeleton with air or with water (if it is an aquatic insect). This makes room for new growth inside the exoskeleton. When the exoskeleton hardens, the insect starts to feed again. The space inside the exoskeleton will be gradually filled up with body tissues, such as muscles. This will continue until the is no more room inside the exoskeleton so the insect will have to moult once again. Usually an insect moults 5 times during its life.

Now study the two graphs below which show the growth of a stick insect.

 (A) A graph showing the change in mass of a stick insect as it grows

 (B) A graph showing the change in length of a stick insect as it grows

(a) What is the stick insect's length at 100 days, 110 days and 130 days?

(b) Why does the graph of the stick insect's length increase in steps?

(c) What does each vertical part of the graph of the stick insect's length represent? (for example: at 18 days, 38 days and 62 days)

(d) What is the mass of the stick insect at 100 days, 105 days and 120 days?

(e) When the length if the stick insect increases what happens to its mass? (for example: between 105 days and 120 days)

(f) When the mass of the stick insect increases what happens to its length? (for example between 105 days and 135 days)

(g) Use the information given at the beginning of this question to explain the differences between the graph of the length and the graph of the mass of the stick insect.