December 5, 2020

I, Science

The science magazine of Imperial College

You’ve probably seen this picture already as it’s been around for a week, but I just can’t get the plump little lady out of my head. I’ve never seen an insect like it. It’s a giant weta, and this carrot-munching monster is the largest so far found. At over 70g it weighs more than a sparrow, or roughly three mice, which seem to have become common units of mass since this image went viral.

Plucked from a tree on Little Barrier Island in New Zealand – which is the only place these insects are now found, having been eradicated from the mainland by rats originally introduced by Europeans – it gorged on that carrot until it had to be put back for its own good. “She would have finished the carrot very quickly”, said Mark Moffet, who found her, “but this is an extremely endangered species and we didn’t want to risk indigestion”.

I sent this image to a friend who lives in New Zealand and he somewhat burst my bubble. “I’m surprised wetas are the biggest insects”, he said. “I’d have thought there would be some really big bugs, like crab-sized or something”. And he has a point. Why aren’t there?

The limitation on size is probably mainly due to the way insects breathe. Since their respiratory system pulls in air through their skin, without lungs, there’s an upper limit to their surface-area-to-body-volume ratio. Or, as Steven Jay Gould put it in 1974, “Insects breathe through invaginations of the external surface. Since these invaginations must be more numerous and convoluted in larger bodies, they impose a size limit upon insect design: at the size of even a small mammal, an insect would be ‘all invagination’ and have no room for internal parts.”

There would also be a problem with the exoskeleton being able to support larger bodies. Too large and an insect would require more energy than it had available (see those issues with respiration again). They also shed those exoskeletons as they grow. In the brief time between shedding one shell and hardening up the next insects are soft and unsupported. Again, it’s not good being too large in that situation.

That’s all very well, I guess, but what about those fossils of dragonflies with 70cm wingspans and centipedes 2m long? Fascinatingly, there’s evidence that during these giant insects’ heyday in the Carboniferous period there was an oxygen spike in the atmosphere (Berner, “Atmospheric oxygen over Phanerozoic time”, PNAS, 1999). Significantly higher oxygen levels would have made insect-breathing much more of an efficient process. So, while we may be able to clone a mammoth, we won’t be cloning giant bugs any time soon without first radically altering the atmosphere.

Thanks to Dharshani Weerasekera for first showing me the picture!