I grew up loving minerals and geology.  My dad was a rather obsessive mineral collector when I was a kid and is still as passionate about the subject as I am about entomology.  He never really bothered with fossils though, so it wasn’t until grad school that I became interested in them.  My advisor adores insect fossils and even worked at an amazing fossil insect deposit in Germany, so I heard all sorts of fascinating stories.  When he offered a fossil insect class, I jumped at the chance to take it.  (What’s not to love about combining my interest in minerals with my interest in insects?!)  It quickly became my favorite grad-level class and I formed a deep and lasting appreciation for insect fossils during the class.

For the next two Monday posts, I’m going to share some of the insect fossil love! Today I’m going to cover the giant insects of the Carboniferous and Permian (358-289 and 290-248 million years ago respectively) and my favorite theory suggesting how these giant insects were able to develop.  Next week I’ll discuss a recent paper.  Let’s jump right into the giant insects, shall we?

Goliath beetle. Image from Wikipedia.

There are arguments over which living insect species is actually the largest insect on the planet.  Most of the debate centers around one problem: how do you define “largest?”  If you go by weight, the heaviest adult insect on record is the giant weta in New Zealand.  It’s thought that some of the African Goliath beetles might actually be heavier, but fewer of them have been weighed.  The heaviest immature insect on record is a Goliath beetle, so they’re definitely up there near the top.  If we go by length, the longest insect is (not surprisingly) a stick insect from Borneo.  There’s often a biggest species identified for each insect order too.  For example, the largest true bug is a giant water bug from South America that tops out at nearly 5 inches long.  All of these insects are big, the giants of the living insects.  However, they all pale in comparison to the largest insects ever discovered on Earth!

Griffenfly. Image from Wikipedia.

The largest insect was a member of the Meganisoptera, an extinct order of insects called the griffenflies or “giant dragonflies.”  As you can see, griffenflies superficially resemble dragonflies and have similar wing and body shapes, so they are commonly confused with the Odonata.  If you’ve ever heard that the largest insect ever was a dragonfly, this is why, but it’s not quite correct.  They were not true dragonflies, rather the precursors to the modern dragonflies.  And they were BIG!  REALLY BIG!  The largest insect ever discovered was a griffenfly called Meganeuropsis permiana, a giant with a wingspan of nearly 28 inches (71 cm) and a body length of almost 17 inches (43 cm).  Can you imagine an insect with a two foot wingspan buzzing around your head?!  Still, as amazing as the griffenfly fossils are, there’s still very little known about them.  Most fossils contain only wings fragments with no body attached.  The immatures remain unknown.  No one has any idea what these things ate, but given their relationship and similar appearance to the dragonflies, it is assumed they hunted flying animals just like their modern odonate relatives do.  Ultimately, as cool as fossils are, they leave you longing for more information.  The griffenflies have been extinct for well over 200 million years, so we might never learn much about them.

Paleodictyoptera. Image from http://www.geocities.co.jp/NatureLand/5218/ pareodhikuthioputera.JPG

The griffenflies were the biggest insects ever, but they weren’t the only big insects around during their time.  Giant mayflies and an extinct group called the Paleodictyoptera (at right) were also roaming the planet at the time.  Some enormous  scorpions and myriapods (like centipedes and millipedes) were also present, as were giant amphibians.  (How cool would it be to see a giant proto-frog eating a giant proto-dragonfly?)  That’s not to say all arthropods were giant during the late Carboniferous.  Most were similar in size to the insects we see today, with a few amazing exceptions that absolutely dwarfed their relatives.   But why did they get so big?  And why are none of these truly giant insects alive today?

Diagram of a simple insect tracheal system.

Because I work with insect respiration, my favorite theory of how insect gigantism came about has to do with how insects breathe.  If you recall from my post on insect respiration, insects depend on tubes called tracheae and tracheoles to exchange gasses with the environment.  The system works because there is less oxygen within the insect than outside the insect, so oxygen tends to flow down the tubes in an attempt to create an equilibrium.  It’s thought that insect size is limited by this system and that insects like the Goliath beetle and the giant weta are about as big as modern insects can be and still get all the oxygen they need.  So how was it possible for a 17 inch long griffenfly able to survive if the biggest insects today are as big as they can get?

Happily, there is evidence that oxygen levels on Earth have changed dramatically over time.  In fact, life on our planet began when there was very little or no oxygen on the planet.  By the late Carboniferous, 280 million years ago, there was so much oxygen on Earth that it made up about 35% of the gasses in the atmosphere.  This high level of oxygen could have in turn led to increased flow of oxygen into the insect respiratory system, at least compared to what we see at our current oxygen level of 21%.  Increased flow of oxygen into the tracheal system meant that the size limits the respiratory system imposed on insects also increased and insects were able to get bigger.

And it looks like they did!  The biggest of the giant insects happened to be flying around about the same time the planet’s oxygen levels were the highest they’ve ever been, suggesting that respiration played a role.  The giant insects then disappeared during the Permian, right about the time the oxygen levels dropped to a low 15%.  And when the oxygen levels rose again during the mid-Jurassic?  You guessed it!  Giant insects popped back up for a while, only to disappear when the oxygen levels dropped to the modern 21%.

This is only a theory of course and it’s unlikely we’ll ever know for sure whether this was really how it all worked, but the hypothesis certainly fits the fossil and climatological data well.  It has also been well received by entomologists, so the hypothesis is likely to hold its own for some time.  Several researchers have even pursued experiments in an attempt to support the validity of the high oxygen – giant insect correlation and gotten some interesting results.  Next week, I’ll discuss one such recent paper that deals with an oxygen study performed on stonefly nymphs.  It makes some interesting points regarding ancient insect gigantism, so I hope you’ll check back!

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References:

Grimaldi, D and Engel, MS.  2005.  Evolution of the Insects.  Cambridge University Press, 755 pp.

Graham, JB, Dudley, R, Aguilar, NM, and Gans, C.  1995.  Implications of the late Palaeozoic oxygen pulse for physiology and evolution.  Nature 375: 117-120.

Dudley, R.  1998.  Atmospheric oxygen, giant Paleozoic insects and the evolution o aerial locomotor performance.  Journal of Experimental Biology 201: 1043-1050.

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