What limits the growth of insects

Why are there no giant insects?

It hums and buzzes, it crawls and flies: insects have conquered almost every corner of our earth as a habitat. They come in the most bizarre forms and they also differ significantly in size. But only up to a certain point: Gigantic insects only exist in films and computer games - there are no ants, beetles and the like in large format. But why not? Andrea K. made us aware of this topic - thank you very much!

Thomas Wagner from the Institute for Biology at the University of Koblenz-Landau knows the answer. “There are two reasons for this: the exoskeleton of insects and their respiratory system limit growth,” says the zoologist. In contrast to vertebrates, insects do not have a bony skeleton that gives their body support from within. With them, the body mass is supported from the outside. When an insect grows, it has to shed its skin because the relatively rigid body shell can only be stretched to a very limited extent. "If the body mass was too large, the insect's body would, so to speak, melt like pulp under its own weight," explains Wagner. This effect would occur precisely during moulting, when the new body shell is still soft and cannot provide an adequate support function.

Exoskeleton and tracheal system

The other reason for the limited size of insects is their so-called tracheal system. This is a network of fine air tubes that runs through the body and supplies all parts directly with air oxygen. The larger the insect, the more space these tracheas need: "The diameter of the trachea does not increase linearly, but has to grow disproportionately in order to supply the body with sufficient oxygen," explains Wagner. The trachea cannot grow indefinitely, especially in the filigree legs of the insects.

At a size of around 17 centimeters, that's the end of it today - the largest insect in the world, the South American giant longhorn beetle (Titanus giganteus), reaches this size. However, this limit did not always apply, as fossils show: around 300 million years ago, for example, there were dragonflies that whizzed through the air with a wingspan of up to 70 centimeters, and some other spectacular arthropod representatives. "This size was probably only possible because the oxygen content of the atmosphere was much higher then than it is today," explains Wagner. Nowadays, however, such giant insects would simply put their feet to sleep.

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June 16, 2017