What caused the size of plants and insects to change?

In my Environmental Science class we learned that millions of years ago insects, dragonflies etc. were gigantic compared to what we know them as today and why. When and why was that true for plants?

Drought Smart Plants reply:
This is not something I’m trained to answer, but I do have a theory why plants have changed in size compared to millions of years ago.

Many fossils that can be identified as similar to some of our plants today tell us that plants now are much smaller.


This could be due to climate change and other environmental changes, such as a much colder climate than there used to be, as well as different make up of gases in the atmosphere. Plants simply grew faster in the environment of the Jurassic period.

Don’t forget, in the interim, there were several ice ages, so only the smaller species of plants survived; the larger, more tender types would have succumbed to the cold. This is the effect of evolution that Charles Darwin was so intrigued by.

As to the exact dates that all this took place, our minds simply can’t comprehend the sheer number of years. Based on fossil evidence, millions of years ago is the best guess.

I hope this helps answer your question.