Why are there no whale-sized insects?

01 August 2013

Question

Really enjoy your programme.

Why don't we see insects as large as whales or cows? Why is the ratio of largest to smallest so much bigger for mammals (blue whale vs kitti's hog nosed bat) than it is for insects (goliath beetle vs fairy fly)?

Thanks,

Jim Schaefer
Ann Arbor, Michigan, USA

Answer

Ian Burgess - Okay, right. Well, an insect depends for its structural integrity on having its external skeleton. Of course, we have an internal skeleton which can be relatively robust and therefore, relatively light. If you had a suit of armour on the outside, in order to get the same sort of robustness, it needs to be relatively thick and is actually quite heavy. If you've ever tried a suit of armour on, or even something very similar, you'll know how difficult it was fighting 400 years ago. Now, for an insect to carry that around, it needs a huge amount of muscle and a lot of energy expenditure. So, it requires particular conditions in order to grow bigger and the biggest insects that we have at the moment are things like goliath beetles which just about will cover the palm of your hand or the biggest ones will. Most of them are a bit smaller than that. But back in the Carboniferous which is what, 400 million years ago, the insects had more oxygen, although oxygen is an interesting thing as regards to insects because it can be toxic if they have too much. So, they were able to grow bigger because the conditions enabled them to actually have the energy that they could use and they could grow bigger and support in the heavier atmosphere.

Dave - So, isn't this because, they don't have lungs so they can't get the oxygen to the - if you had something in the size of cow, the middle of an insect just wouldn't be able to breathe.

Ian - No, I don't think that's really the issue. It's the physical structure of supporting all of that external skeleton. So, the things that did grow big then were relatively lightly constructed - things like dragonflies and if you look at a dragonfly, it's sort of spindly. It's like one of the wonderful designs of second World War aircraft. It looks as though it was only just held together. The other insects that grew moderately big, you got some big cockroaches and there was another insect, a big spider at that time as big as a dinner plate, but that's about as far as they could go. Now, if you wanted to get much bigger than that, then you'd have to find some way of supporting then other than necessarily the - just muscles and a big thick armour. So, if we had an insect the size of a whale, it would probably have about a meter thickness of external skeleton just to keep it together.

Chris - That's quite a skeleton isn't it?

Ian - So, unless it manages to find some way of - because of course we're dealing with biological materials rather than sort of a sheet metal, so in order to have it that big and support itself, so it has to find some way of getting bouyancy as well. The only reason why it grows as enormous as it does is because it has the water to support it. If you put a whale on land, even a little tiny whale, it suffocates because it can't even support its own breathing.

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