Fossils reveal giants from the past

Insects are a highly successful and incredibly diverse group of organisms.  Their success is due to a number of factors, one of which is the exoskeleton.  This adaptation provides support, protection, and allows for the rapid and efficient intake of oxygen. 

Although the exoskeleton provides several benefits, it also limits how large an insect can grow.  The molting process leaves it vulnerable and subject to bodily collapse until the new exoskeleton hardens.

Cicada Molting.  Source: T. Nathan Mundhenk (Own work)
GFDL (http://www.gnu.org/copyleft/fdl.html), via Wikimedia Commons

As a result, insects are relatively small in size, which has led to their success at exploiting a great variety of ecological niches and resources.

But, insects haven’t always been small.

Fossil evidence from the Paleozoic Era shows that most insects were similar in size to their modern counterparts.  However, several early groups were able to reach “gigantic” proportions.

Cast of Meganeuridae on display at the Evolution Gallery of the
Museum des Sciences Naturelles, Brussels. The upper right corner shows the
relative size of a modern dragonfly.  Source:  Prehistoric Insects - ASU

Meganeuropsis permiana was the largest insect ever to have lived.  This early relative of modern dragonflies and damselflies lived during from the late Carboniferous Period to the Late Permian (320-247 MYA).

 A life-size model of Meganeuropsis permiana at the Technische Universitaet Clausthal
in Germany. Source: http://www.windsofkansas.com/meganeuropsiskraus.jpg

Sometimes mistakenly called a “giant dragonfly”, the impressive predator was actually a “griffenfly”.  Fossils have shown the wingspan of M. permiana could reach up to 28 inches (71 cm).  In comparison, the largest extant damselfly is Megaloprepus caerulatus 7.5 inches.

Source:  Prehistoric Insects - ASU

Fossils also show that several other invertebrate groups reached surprising proportions, including early mayflies, myriapods, and scorpions.

There are a number of proposed explanations as to why large insects were able to survive in the distant past, as well as why they disappeared.

The atmosphere’s oxygen concentration was much higher during the late Paleozoic and Early Mesozoic, which corresponds with the existence of many giant insects.  

It may be that the hyperoxic conditions allowed an increase in size without a decrease in respiration efficiency.  The energetic demands of flight of insects the size of Meganeuropsis permiana would not have been possible without extra oxygen.  Over time, oxygen levels declined, along with the prevalence of large insects.

There are other explanations for the loss of the ancient giants.  In 2012, researchers proposed that the evolution of birds capable of agile flight corresponded with the timing of a decrease in insect size, indicating the importance of competition and predation in species evolution and diversity (Clapham and Karr 2012). 

Photo by Johnny Wee. Bird Ecology Study Group. www.besgroup.org

Despite ongoing research and fossil discoveries, it's clear there are still many unanswered questions remaining about the evolution and success of these ancient and fascinating organisms!

References:

Grimaldi, D., & Engel, M. S. (2005). Evolution of the Insects. Cambridge University Press.

Clapham, M. E., & Karr, J. A. (2012). Environmental and biotic controls on the evolutionary history of insect body size. Proceedings of the National Academy of Sciences of the United States of America, 109(27), 10927–10930. doi:10.1073/pnas.1204026109

Prehistoric Insects | ASU - Ask A Biologist: https://askabiologist.asu.edu/explore/big-big-bugs