Springwatch

[Pauline]

Great programme with some fantastic footage. Last night CP said that during the Carboniferous period dragonflies were massive compared to today and the reason he gave for this was that oxygen was 35% of the air as opposed to 21% as of now (combined with the structure of the insect for absorbing air). It made me wonder how a butterfly breathes, how long they have been around and whether they too were huge. Does anybody know?

[Padfield]

In answer to how old they are, see http://www.crystalinks.com/fossilbutterfly.html. I like the idea that they fluttered around dinosaurs' feet as they trundled (or zoomed) through the vegetation.

Butterfly 'breathing', like that of dragonflies, is very different from ours. Oxygen reaches our tissues by an active system of breathing and blood circulation, meaning that oxygenation is not a limiting factor to how large we can grow (in land mammals, it is leg strength and thickness which limit size, since as you scale up a creature its weight goes up as the cube of the scale factor but leg strength only as the square). In insects there is no active breathing and only a primitive blood circulation system, which is not designed for oxygen transport. Spiracles, or holes, in the cuticle allow air to enter a system of tubes or tunnels which surround the innards. Oxygen essentially reaches the tissues by diffusion - that is, natural, random movement down a concentration gradient - aided by the general movement of the insect. But diffusion is a slow, passive process, so a limit is imposed on the size an insect can grow to. If it were too big, there would be a build up of stale air and carbon dioxide in and around the tissues. In days when the oxygen concentration was higher, the size limit would also have been higher, as diffusion would have been an efficient method of delivering oxygen over slightly greater distances.

How big butterflies grew to in those times I have no idea!

Guy

[Pauline]

Wow! That's amazing! Thank you for sending me this Guy. Just imagine how exciting that must have been to find those fossils. The actual butterfly seems 'normal' size tho it is difficult to judge the size of the amber. I would love to think there were huge butterflies on the wing in those days. Is there anything you don't know Guy? Thanks again.

[Philzoid]

Thanks fpor that information Guy

I must admit I didn't think the oxygenation in insects was an entirely passive event and thought that some abdominal 'pumping; in insects such as 'seen in' dragonflies contributed to the exchange of gasses?

Phil

[Padfield]

You're right Phil - there is some active aeration. If I understand it aright, what gives mammals, reptiles &c. the real edge in terms of the ability to grow large is the transport of oxygen via haemoglobin in the blood, which coupled with the capillary system means no tissues are far from a (dissolved) gas exchange surface. Blood is incompressible and so can be circulated much more efficiently and rapidly than air. A giant insect would meet great technical problems ensuring sufficient airflow throughout a system of air channels that came close enough to all parts of its body.

Going off at a tangent, there was some interesting research published recently suggesting another reason why the giant dinosaurs could not compete with mammals. In short, it was that in growing to such great sizes, over such a long time, they had to be successful competitors in a great range of size niches, whereas mammals specialised in their own size niche. It wasn't entirely convincing as the Torygraph reported it but I'm sure the original paper was far more joined up than the journalist's version and it did seem worth thinking about.

Guy

[David M]

In my own clueless way, I've always pontificated upon why dinosaurs routinely grew to such gargantuan sizes, yet mammals (at least those that are land based) have never even come close to such dimensions.

There must be some kind of critical factor that naturally precludes mammals from thriving at the same huge size that lizards once did.

[Philzoid]

Thanks Guy.
(Even more of a tangent)

By the same token if we were able to take a T-Rex from the past and place it into the present (as in Jurassic Park), would the most ferocious dinosaur known to man just simply keel over and die due to insufficient oxygen

OK I'll get my coat.

[Debbie]

How lovely to see butterflies featured on springwatch , but only for a couple of minutes.

I am really looking forward to the day when I can see a butterfly emerge.

Debbie