Have you ever considered why there are no giants today?

Many scientists have different ideas why giant insects lived on earth about 300 Million years ago, in an age known as Carboniferous.

One suggested - and compelling - explanation (in science we call that a Hypothesis) is that insects could grow to large sizes,
because they had enough oxygen to do so. It is called the Oxygen Limitation Hypothesis.

What has the size of the tracheal system to do with oxygen supply?

Animals (and of course insects) need oxygen to stay alive. The body needs oxygen to produce the energy (in form of adenosyne triphosphate or ATP) to maintain all vital functions. ATP is produced in the mitochondria, which are embedded in the cells. Cells are embedded in tissues and some tissues are far from the oxygen source - our air.

The function of the tracheal system of insects is to maintain the balance between oxygen supply (respiration) and oxygen consumption (mitochondrial metabolism), no matter how big the insect is. The ability of the tracheal system to conduct oxygen to the mitochondria (Mito in the image) is called Conductance. The conductance is a function of the cross-sectional area and the length of the tracheal tubes. The larger the cross-sectional area, the higher the conductance. The longer the tube, the smaller the conductance.

In large insects, some tissues are far away from the oxygen source and if the tracheal system would only grow in length and stay as thin as in a small insect, the conductance of the tracheal system decreases and all the oxygen that enters the tracheal tubes would be depleted by tissues that are closer to the spiracles. Mitochondria that are far away from the source might receive no oxygen at all.

To maintain the balance between oxygen supply and oxygen consumption, the conductance of the tracheal system of large insects has to be increased. Using thicker tubes or adding more thin tubes has the same effect. We call this HYPERMETRY of the tracheal system.

What has that to do with size?

Comparing the last two images, it is obvious that there is less space for the tissues! When the tracheal system takes up too much room in the body of an insect, it competes with other tissues that are vital to maintain the organism. Tracheal investment (the amount of space it fills in the body) has a limit. Once this limit is reached, tracheal conductance is at its limits. And when the conductance is maxed out, the insect could not get any bigger.
Studies from our lab have demonstrated increased tracheal investment in developing grasshoppers, in adult grasshoppers and in darkling beetles (Tenebrionidae).