Maddie Deacon, of Balclutha Primary School, asks :-

Why are insects so small?

David Jarriault, an insect neurobiologist at the Department of Zoology University of Otago, responded.

Insects have not always been so small. Imagine yourself for an instant walking close to a river and encountering on your way a giant dragonfly with the wingspan of a sea gull, or a 2-meter-long millipede crawling past you. These giant insect species lived about 300 million years ago at a time during which the oxygen content of Earth’s atmosphere was much higher.

Insect size is indeed limited by the way oxygen diffuses in their body. They do not have a circulatory system with blood vessels, heart and lungs, as vertebrates do, to pump oxygen in the body and circulate it to all the organs. Although they do have blood, it cannot carry oxygen. Instead, the surrounding air enters the insect through thin tubes called tracheas that run through their body forming a dense and branched network that connect to the organs. Gases (oxygen and carbon dioxide) can then diffuse from or to the cells.

This system presents two limitations. First, air is not pumped inside in the same way as when we breathe but rather diffuses passively. Only in some large insects, body motion and compression can help moving the air. Secondly, remember your math classes, if you consider the insect body as a sphere, when you double its diameter, its volume increases 8-fold. All the cells in that volume need to be oxygenated. To do so, the insect’s body would have to be completely filled with tracheas leaving no room for the guts, blood, nerves and gonads.

The current composition of Earth’s atmosphere makes it easier for vertebrates to develop bigger sizes thanks to our circulatory system but as we, humans, work hard to alter the fragile gas equilibrium of our atmosphere things could change again in the future.

Even the current largest beetle in the world (Titanus giganteus; 15-17cm) would be very small in comparison to the prehistoric dragonfly (Meganeura; 70 cm-wingspans). The smallest insect known is the fairy fly (Mymar), which can be less than 0.2mm-long.