A long, long time ago in a land far away, lived giant beasts stomping, scurrying and soaring over the earth. No, not a fairy tale but an image of life on earth around 290 million years ago before the climate continued to change, oxygen levels decreased and animals shrunk in size and long before humans came along.
In the late Carboniferous and early Permian, fossil evidence shows that most insects were much bigger than they are today. For example, Meganeura, a carboniferous dragonfly, had a wingspan of 75 cm and Ramsdelepedion schusteri, a carboniferous silverfish, was 6 cm long! Today, the largest insects include the Goliath beetles (Goliathus), which measure 5–11 cm in length as adults, and can reach weights of up to 80-100 g in the larval stage, Titan beetles (Titanus giganteus), which grow up to 16.7 cm in length (or 21 cm including antennae), the Titan stick insect (Acrophylla titan) with a body length of up to 50 cm and some butterflies and moths which have wingspans up to 28 cm. But why did the insects shrink?
There are a few evolutionary hypotheses for the decrease in size of the insects:
- climate change – as oxygen levels decreased, it became more difficult to get enough oxygen to all the tissues via spiracles and diffusion alone.
- the weight of an insect’s body at moulting might be greater than the soft cuticle could bear, and, being vulnerable to predators, they would probably have to hide.
- the appearance and rise of vertebrate predators, e.g. reptiles made it an evolutionary advantage to get smaller to avoid predation and radiate into all the available empty niches.
A recent paper published in PNAS by Kaiser et al. lends extra support to the climate change hypothesis…