Why cannot insects grow bigger still?
The way that insects breathe, that is the supplying of oxygen and the elimination of waste gases, prevents them from growing over a certain weight. The Goliath beetle seems to be the maximum size they can attain.
Which is the biggest insect of all?
The biggest of all insects is the Goliath beetle. This animal belongs to the same group as cockchafers and ladybirds.
Like all these insects it has a hard chitinous shell on its head and thorax. When the beetle is at rest, the fine membranous wings with which it flies are folded under thick shields which form a protective sheath.
The Goliath feeds off the flowers found at the top of the tall trees of Central Africa. It makes a great deal of noise as it flies, and its grub is a big six-legged white worm like the cockchafer's.
The biggest Goliath beetles weigh about three and a half ounces and are five inches long and three inches wide. This is the largest size that an insect can reach. Insects have breathing and circulatory systems very different from our own. Holes in the shell are linked to small ducts which then go as far as the muscles. In this way the air is channeled straight to the muscles.
Those parts of the muscles which are near the respiratory ducts are well oxygenated, but those parts further away are less well supplied.
All of an insect's organs are steeped in the bloodstream and are fed by it. A little oxygen passes through the walls of the respiratory ducts and into the blood (which is not quite like ours), but not much of it can be absorbed.
The distribution of the air is sufficient for a small organism, but would be insufficient for a bigger animal.
A very long time before Man appeared on the Earth, there existed a dragon-fly whose wings had a span of two feet. Although it was bigger than the Goliath, it had a much slimmer body.