All living organisms must take in oxygen in order to burn the food they
have eaten and release energy. This process is called respiration. When food is burnt with oxygen in the bodies
of all creatures, a waste gas called carbon dioxide is produced. Carbon
dioxide is toxic and must be removed from the body quickly and fresh oxygen
brought in. Humans and other mammals have lungs which exchange oxygen
and carbon dioxide. Insects however, do not have lungs, instead they depend on a system of fine tubes that bring oxygen directly to the living
tissue and take carbon dioxide away. These microscopic tubes are called tracheoles and open to the outside through tiny holes called spiracles.
The spiracles are visible on the side of the caterpillar, butterfly or
chrysalis when using a magnifying glass.
Some insects have air sacs, balloon-like structures that can store a small supply of air. In dry environments, such as deserts, this temporary air supply allows an insect to conserve water by closing its spiracles hot periods during the day. The supply of air also allows aquatic insects to move underwater and even to regulate their buoyancy. Air sacs also play a part during moulting where they inflate and help the insect push out of its old exoskeleton.
This method of exchanging gases is not as efficient as our systems with
lungs and blood vessels. Considering the size of the insects, this system
works well. For this reason, insects can not grow too big, unlike the
giant insects shown in some science fiction movies. If insects were the
size of a small dog their system of exchanging gases would be inadequate
and they would suffocate.
Insects have no blood vessels to carry blood to their internal organs.
Instead their internal organs are bathed in green coloured blood, which
is kept moving around the body by the heart. The heart is often a simple
tube open at both ends which squeezes blood through itself, as pictured on the right. The circulatory
system of the insect is a very simple system indeed, not as complex as
our own. Getting food to the cells and removing waste largely depends
on the temperature of the insect’s blood. The temperature of the
blood determines how much of any substance can dissolve in it. At low
temperatures the blood can not act as an efficient transport mechanism,
removing waste and bringing in nutrients. At temperatures around 25 oC substances
dissolve readily and move with increased speed within the insect’s
body fluids.
1) Why is oxygen necessary for the survival of the insect?
2) Explain how the human and insect respiratory systems differ.
3) What is the function of spiracles?
4) Which statement best explains how oxygen is brought to all the active cells of an insect's body?
5) Explain the importance of air sacs found within the insect's body
6) Give two reasons why the size of insects must remain relatively small.
7) How does the insect distribute nutrients to the cells that need it?
8) What are the differences and similarities between the mammalian heart and the insect heart?
Attempt the thinking question below.
Why is an insect inactive during periods
of low temperatures? Explain in terms of solubility.
Try to dissolve an equal amount of sugar in 100ml
of cold water and 100ml of warm water (50C). Can you dissolve more in
the warm water? Explain.
